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Chapter 8System Architecture: Satellite-Based Navigation

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Title: a case study analysis for designing a lunar navigation satellite system with time-transfer from earth-gps.

Abstract: Recently, there has been a growing interest in the use of a SmallSat platform for the future Lunar Navigation Satellite System (LNSS) to allow for cost-effectiveness and rapid deployment. However, many design choices are yet to be finalized for the SmallSat-based LNSS, including the onboard clock and the orbit type. As compared to the legacy Earth-GPS, designing an LNSS poses unique challenges: (a) restricted Size, Weight, and Power (SWaP) of the onboard clock, which limits the timing stability; (b) limited lunar ground monitoring stations, which engenders a greater preference toward stable LNSS satellite orbits. In this current work, we analyze the trade-off between different design considerations related to the onboard clock and the lunar orbit type for designing an LNSS with time-transfer from Earth-GPS. Our proposed time-transfer architecture combines the intermittently available Earth-GPS signals in a timing filter to alleviate the cost and SWaP requirements of the onboard clocks. Specifically, we conduct multiple case studies with different grades of low-SWaP clocks and various previously studied lunar orbit types. We estimate the lunar User Equivalent Range Error (UERE) metric to characterize the ranging accuracy of signals transmitted from an LNSS satellite. Using the Systems Tool Kit (STK)-based simulation setup from Analytical Graphics, Inc. (AGI), we evaluate the lunar UERE across various case studies of the LNSS design to demonstrate comparable performance as that of the legacy Earth-GPS, even while using a low-SWaP onboard clock. We further perform sensitivity analysis to investigate the variation in the lunar UERE metric across different case studies as the Earth-GPS measurement update rates are varied.

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• Published: 03 August 2020

Status, perspectives and trends of satellite navigation

• Guenter W. Hein   ORCID: orcid.org/0000-0002-5672-816X 1 , 2  

Satellite Navigation volume  1 , Article number:  22 ( 2020 ) Cite this article

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This paper reviews the status of satellite navigation (as per 11 May 2020)—without claim for completeness—and discusses the various global navigation satellite systems, regional satellite navigation systems and satellite-based augmentation systems. Problems and challenges for delivering nowadays a safe and reliable navigation are discussed. New opportunities, perspectives and megatrends of satellite navigation are outlined. Some remarks are closing this paper emphasizing the great value of satellite navigation at present and in future.

Introduction

The Global Positioning System (GPS) project was started by the U.S. Department of Defense in 1973, with the first prototype spacecraft of Block 1 launched in 1978. The first of nine satellites in the initial Block II series was launched 1989 and the full constellation of 24 satellites became operational (Full Operational Capability—FOC) in 1993.

Shortly after (1982) the Russian GLONASS system was built up with a first Final Operational Capability (FOC) in 1996. However, due to the short life of the satellites the constellation dropped down in 2002 to as few as seven satellites. In 2011 FOC was achieved again with 24 satellites after having launched approximately 140 satellites in total (Langley 2017 ).

In the first step, China developed an active positioning system called BeiDou Navigation Demonstration System (BDS-1), which started in 1994 and consisted of two in-orbit Geostationary Earth Orbit (GEO) satellites launched in 2000 and a third one launched in 2003. In the second step, the passive positioning system BeiDou Navigation Satellite (Regional) System (BDS-2) followed between 2004 and 2012 with a total of 14 satellites, including 5 GEO, 5 Inclined Geosynchronous Orbits (IGSO) and 4 Medium Earth Orbit (MEO) satellites serving the Asia–Pacific region. The third step, BeiDou Navigation Satellite System with Global Coverage (BDS-3), is developed between 2004 and 2020. It will be comprehensively completed with the launching of 30 satellites (China Satellite Navigation Office 2019 ).

The European Union (EU) launched the first In-Orbit Validation (IOV) satellites GIOVE-A and GIOVE-B of the Galileo system 2011. Galileo will be completed end of 2020/beginning of 2021.

It took obviously 20 years to build up the first Global Navigation Satellite System (GNSS), namely GPS, the last one, BDS-3 required only 3 years.

Regional navigation satellite systems (Japan, India) were developed for the sake of joining this new high-tech space world and gaining exclusive access to a system for governmental and military reasons.

Since GPS was first considered as a military system (now it is dual-use), civilian aviation was hesitating to use it for aircraft navigation and landings. Satellite-Based Augmentation Systems (SBAS) were developed all over the world delivering the required integrity of the system via GEO satellites to the user.

We can show now over 47 years of modern satellite navigation: four global systems, two regional systems and a large number of SBAS are available. However, it is only less than one decade that the user started to take advantage. The number of possible GNSS applications is not limited by technology rather than by our imagination. And the development of satellite navigation is not finished. New opportunities are coming up, however, also new threats for a safe and reliable navigation appear.

This paper reviews the state-of-art of the global, regional and augmentation systems. Problems and challenges are discussed and new opportunities, perspectives and megatrends of satellite navigation are outlined.

Satellite navigation systems

Global navigation satellite systems (gnss).

Four GNSSs are available for the users, two already fully available, one of them to be finished first half of 2020 (BDS-3) and the other to become fully operational end of 2020/beginning 2021 (Galileo Navigation Satellite System (Galileo)). Assuming an unobstructed view, 35 GNSS satellites could be used on 11 May 2020 in Munich considering a mask angle of 10° (Fig.  1 ). 5 SBAS satellites are available. A GNSS System of Systems has been built up (Fig.  2 ).

GNSS Satellites in View in Munich on 11 May 2020. Reference: https://qzss.go.jp/en/technical/gnssview/index.html

GNSS: a system of systems. (The South Korean System KPS is not considered since developments did not start yet)

Three-dimensional positioning and navigation require four satellites in minimum. Thus, even in urban areas we may see some redundancy in satellites. The so-called interoperability (in its most strict sense assuming same center frequencies but different codes) achieved between almost all of the four GNSS may have the drawback that the internal noise floor is increasing causing eventually problems in signal acquisition of the receivers (Hein 2010 ). According to this, the benefit of various GNSS when the number of its satellites is, let’s say, higher than 24 is questionable since all receivers are anyway of type multi-GNSS ones (at least in the civilian world), no more receivers considering only one system are built and sold, and the civilian user is applying a multi-GNSS positioning. Similar arguments can be found also for the regional satellite navigation systems.

Their main purpose can be only military and/or to follow the high-tech developments in that satellite field. For example, the two regional systems of Japan (QZSS) and of South Korea (KPS) are located so near together that both will be visible for the users in the corresponding countries. As mentioned above, 35–40 satellites (depending on the mask angle) are visible for the user in areas with no obstructions. Do we require so many satellites, do we need four global systems? Four satellites are necessary for three-dimensional positioning and when combining several GNSS, up to three additional satellites for solving the time-offsets in between. What are we doing with that high redundancy in satellite observations? Multiple Receiver Autonomous Integrity Monitoring (RAIM) can be applied in order to control the different satellite observations/systems and advanced multipath mitigation are becoming possible, just to mention two applications. However, there are more possibilities which are not yet really explored!

Global Positioning System (GPS)

The first two satellites of the next generation GPS III were launched 23 December 2018 and 22 August 2019 respectively and have successfully completed the in-orbit check. The main new features of the GPS III satellites include increased accuracy and transmission power, inherent signal integrity, the new L1C civil signal and a longer life of 15 years. The launch of the third GPS III satellite is planned for July 2020. Currently (21 April 2020) there are 11 Block IIR, 7 Block IIR-M, 12 Block IIF and 1 Block III satellites operational. The next generation Operational Control System (OCX) is the future version of the GPS control segment which will command all modernized and legacy GPS satellites, manage all civil and military navigation signals, and provide improved cybersecurity and resilience for the next generation of GPS operations. OCX will be ready for transitions to operations mid of 2022 ( http://www.gps.gov ).

GLObal NAvigation Satellite System (GLONASS)

The last GLONASS-M launch took place on 16 March 2020. A new generation of GLONASS-K satellites is under development, with two initial spacecraft already in orbit. Further GLONASS-K launches are expected next year via Soyuz and Proton-M rockets. Main recent changes of the GLONASS system are the introduction of Code Division Multiple Access (CDMA) signals while keeping the Frequency Division Multiple Access (FDMA) signals and the improvement of the on-board clock stability. The future addition of an IGSO regional part (GLONASS-B)—similar to BeiDou—and a better world-wide geographically distributed control network (currently Russia only) are planned. Meanwhile on-board cross-links are used for orbit and clock updates outside the current ground control visibility ( http://www.glonass-iac.ru/en/ ).

Galileo Navigation Satellite System (Galileo)

The next two satellites to be launched with Soyuz spacecraft are planned for end of 2020 or beginning of 2021, upgrading the constellation to 24 operational satellites (including three In-Orbit Validation (IOV) satellites), see e.g. http://www.gsc-europa.eu/system-service-status/constellation-information , http://www.gsa.europa.eu/european-gnss/galileo/galileo-european-global-satellite-based-navigation-system , (Chatre and Benedicto 2019 ). Based on this, the European Union may declare then “full operational capability” depending on how this will be defined. Earlier EC statements were looking for 30 satellites. The Signal-in-Space Error (SISE) of about 0.25 m (95%) achieved in 2019 (Benedicto 2019 ) is smaller than that of GPS ( https://www.nstb.tc.faa.gov/reports/2019_Q4_SPS_PAN.pdf ), (Barnes 2019 ; Lavrakas 2020 ). However, these values depend on the update rate frequency of Galileo (100 min) versus GPS (12 h).

Galileo experienced an outage from 11 July to 17 July 2019. The 6-day service outage occurred during a system upgrade in the ground infrastructure due to a mishandling of a temporary equipment and follow-on events.

The contract for the first order (Batch 4) of Galileo transition to Galileo Second Generation (G2G) satellites is planned to be placed end of 2020. Batch-3 for in-orbit spares and replacements for the oldest Galileo (IOV) satellites (launched 2011/12) contained 12 satellites. Since then, the decision for a now free-of-charge “commercial service” has been taken and the old commercial service will be replaced by a High-Accuracy Service (HAS) and a Commercial Authentication Service (CAS), expected to become operational in 2020. The HAS will provide Precise Point Positioning (PPP) in E6B and achieve accuracies of 20–40 cm globally, with a 5-min convergence. Additional corrections broadcast regionally in Europe will have target convergence within 100 s.

BeiDou Navigation Satellite System (BDS)

Since November 2017, there were 18 successive launches within 2 years. 28 BDS-3 satellites and BDS-2 backup satellites have been successfully injected the last launch on March 9, 2020 carried the 54th BDS satellite and 29th BDS-3 satellite into the designated geosynchronous orbit while the BDS-3 construction has entered the final stage. One more GEO satellite will be launched probably in May 2020 which completes the BDS-3 system about half a year ahead of the scheduled target. The nominal BDS-3 constellation consists of 24 MEO, 3 IGSO and 3 GEO satellites. BeiDou has intersatellite links and provides also a PPP service (Li et al. 2014 , 2020 ; Ruan et al. 2020 ; Viet et al. 2020 ; Yang et al. 2020 ; Zhang et al. 2019 ; Zhu et al. 2018 ), ( www.en.beidou.gov.cn ).

The orbital constellations of the GNSS (as per 11 May 2020) can be found in Table  1 .

Regional Navigation Satellite Systems (RNSS)

Indian regional navigation satellite system irnss/navic.

Figure  3 shows the IRNSS/NavIC. The independent Indian satellite-based positioning system for critical national applications has the main objective to provide reliable position, navigation and timing services over India and about 1500 km around India. It has been (re-)named Navigation with Indian Constellation (NavIC) recently. It consists currently of three GEO and five IGSO satellites. In January 2017 a complete failure of IRNSS 1A occurred when all 3 atomic clocks failed. One launch (IRNSS-1H, on 3 August 2017) was unsuccessful the satellite could not reach orbit.

The Indian Regional Navigation Satellite System (IRNSS/NavIC)—Reference: http://www.isro.gov.in

Japanese Quasi-Zenith Satellite System (QZSS)

Figure  4 shows the QZSS, which is a regional satellite navigation system complement to GPS. It will take over after 2020/23 also the transmission of the Japanese (multifunctional) Satellite-Based Augmentation Systems (SBAS) called Multifunctional Transport Satellites (MTSAT) (or The MTSAT Satellite Augmentation System (MSAS)) serving currently mainly aviation. Three other satellites will be added after 2023 extending the current QZSS with four IGSO satellites.

The Japanese Quasi-Zenith Satellite System (QZSS)—Reference: http://qzss.go.jp/en/

Regional South Korean Positioning System (KPS)

In its 3rd Basic Plan for Space Development the South Korean government has decided in February 2018 to plan its own regional satellite navigation system of three GEOs and four elliptical IGSOs (Fig.  5 ), similar to NavIC and QZSS, covering South Korea and about 1000 km of its surrounding area.

The Regional South Korean Positioning System (KPS)—Constellation and Target Area. Reference: Moonbeom ( 2019 )

Satellite-Based Augmentation Systems (SBAS)

SBAS has the two main purposes: to provide integrity for civil aviation and to transmit differential GNSS and ionospheric corrections. This is achieved by geostationary satellites (in general two to three per SBAS) which are transmitting the so-called integrity message and the corrections. A corresponding ground network covering the SBAS area under consideration determines the integrity of GPS, the differential and ionospheric corrections and uplinks it to the GEOs. Europe is currently developing European Geostationary Navigation Overlay Service (EGNOS) V3, the first world-wide dual-frequency (L1/E1, L5/E5a) dual-system (GPS and Galileo) SBAS, to go into operation around 2026 when Full Operational Capability (FOC) of GPS L5 is available. Figure  6 shows global SBAS realised and under development.

Existing and satellite-based augmentation systems (SBAS) under development. Reference: https://www.gsa.europa.eu/sites/default/files/brochure_o_2017_v6.pdf (figure expanded)

Problems and challenges

Compatibility. Footnote 1 There are good reasons that the L-band was mainly taken for satellite navigation systems (all-weather-system). However, the frequencies are all heavily occupied. In the past smart signal processing methods allowed the co-existence of several navigation signals within a certain small neglecting interference level (e.g. < 0.2 dB). There all still intentions to put even more signals on the L-band and GPS/GNSS frequencies. The communication signals of the 5G Ligado broadband network were approved (with the condition “Broadcast till you break it”—the GPS interference Standard) end of April 2020 by the U. S. Federal Communication Commission (FCC) despite the concern of the Department of Defense, the Department of Transportation and many others, see https://www.gpsworld.com/fcc-approves-ligado-broadband-network-dod-and-gps-industry-react/ .

For all satellite navigation system providers, it is a serious question how any future evolution on signals may be realized. The possible use of smart signal processing techniques as mentioned above is coming to a limit—no other signals in the L-band are possible. The S-band is already crowded, and using the C-band (investigated already by the U. S. in the 1960s when developing GPS) has more serious drawbacks (more required signal power on the satellite and/or active antennae, influence of rain and snow, larger antennae and higher costs for a receiver) than appreciating the value of a possible increase of accuracy due to smaller wavelengths. Flexibilities in signal generation and transmission at satellite level and user reception may contribute to solve the congestions of the frequency bands by giving (partly) up the backward compatibility in coming evolutions of satellite navigation systems.

Un - intended and intended interferences (jamming and spoofing) are increasing with every day making it more and more complicated or even impossible to fully protect the safety-of-life and authorized/military signals. GNSS jamming devices can be easily bought by everybody, in particular in the internet. In many countries these may be purchased legally though their use is not permitted. All kinds of intentional and unintentional interferences in the GNSS band can be expected to increase. In addition, spoofing devices are nowadays readily available which in the past were available for military use in NAVigational WARfare (NAVWAR) only. There are some measures undertaken to monitor interferences but these are more on a local and regional scale. GNSS signal authentication is a powerful counterpart to GNSS spoofing. Most of the GNSS receivers are neither equipped with interference and spoofing detection nor mitigation software for those effects. GNSS satellites of the past were not prepared for cyber-attacks.

However, all those developments may have a crucial impact on safety-related applications. Providing secure and trustworthy satellite navigation will be one of the main future challenges (Kaplan 2019 ; Simsky 2019 ), ( https://www.maritimeglobalsecurity.org/media/1043/2019-jamming-spoofing-of-gnss.pdf ).

On the system side, one possibility would be to apply advanced frequency hopping spread spectrum techniques where the signal is rapidly switching transmitting signals as well as appropriate anti-jamming methods (see e.g. Gao et al. 2018 ). Of course, increasing the transmitting satellite signal power by a large number of dBs (see e.g. GLONASS-K2 power capacity 4370 W) would be the best possible anti-jamming method, however, it requires larger satellites and violates the ITU (International Telecommunication Union) conventions and rules when using the high power.

From the above we may conclude that we have to meet challenges in future in improving not only the receivers but also the satellites with respect to anti-jamming, anti-spoofing and other cyber-attacks (Harrison et al. 2020 ; Wang et al. 2020 ).

Interoperability Footnote 2 in its most strict sense assuming same center frequencies for signals (H/W) but allowing different codes (S/W) and a different reference system both in time and coordinates has the great user advantage to have a simple receiver for tracking signals from several satellite navigation systems. However, also here we may come to a limit. We are increasing the internal satellite noise floor to a level where we could get problems in acquiring signals with a normal receiver (Hein 2010 ). Interoperability has another advantage for the user: it forces the systems to take up improvements coming from the other ones. The market will only consider a system in a navigation chip if it is comparable in quality to the other ones. Otherwise it would disregard systems which cannot possibly contribute in a combination approach. The progressive digitalization of both, the satellite navigation payload and the user receiver including the front-end may change the strict hardware requirements for interoperability in near future, in particular assuming identical center frequencies.

New opportunities and perspectives

Advanced receiver autonomous integrity monitoring (araim) and sbas.

It is generally recognized that ARAIM has a great potential for SBAS (EU-US Cooperation on Satellite navigation 2015 ; EU-US Cooperation on Satellite navigation 2016 ; Fernández et al. 2019 ). Horizontal ARAIM is expected to be available around 2023 and vertical ARAIM following a few years later. SBAS systems are guaranteed until 2035, especially for aviation ( http://www.faa.gov ). But what happens after 2035? Will SBAS systems become obsolete?

Potential of 5G wireless networks

Introduction of 5G wireless networks is expected after 2020 (Fig.  7 ). The standardisation process for the first release incorporating 5G capabilities was completed in June 2018 with 3GPP Release 15. Phase 2 is about to be completed. 5G technology with its many new mission-critical services and positioning applications may represent a new mobile revolution in the wireless landscape. The main targets include the Internet of Things (IoT) and ultrafast enhanced mobile broadband using millimeter wave bands and small cells. The standardized positioning levels of 3GPP can be found in Prieto-Cerdeira et al. ( 2019 , Table 2). A competitor of our GNSS? Or will the number of GNSS applications decrease? Or, most likely, a hybridization/fusion GNSS/5G will start to develop for certain applications.

5G wireless networks applications—Reference Prieto-Cerdeira et al. ( 2018 )

In the following there will be a short discussion what the role of GNSS and the one of 5G most likely will be in future (Cozzens 2019 ; Kishiyama et al. 2017 ; Prieto-Cerdeira et al. 2018 , 2019 ) (Fig. 8 ).

Applications of 5G wireless networks and GNSS

5G Timing by GNSS The high-performance mobile services delivered over the 5G networks are extremely dependent on precise time from GNSS so they can synchronize radios, enable new applications and minimize interference.

GNSS in areas with scarce population The high accuracy of 5G networks can be only realized using many dense base stations. Due the commercial character of the operating companies, this will be only the case where the population is high—certainly not in the country side.

Dedicated 5G networks for large companies and production In order to become independent by the telecom operators and aiming for the highest 5G positioning cm/mm accuracy in- and outdoor for their production, large factories intend to install and operate their own very local 5G network with dense base stations. Here GNSS may be replaced by 5G (except GNSS timing).

Fusion of GNSS and 5G in urban areas Due to the fact that GNSS may have a downgraded accuracy in urban canyons caused by limited satellite availability, unfavorable satellite geometry and multiple multipath, a fusion of the 5G cm-wave with GNSS might result in higher positioning accuracies (Peral-Rosado et al. 2018 ). Therefore, compatibility and interoperability of 5G and GNSS is necessary.

Satellite navigation and new space (Hein 2018 ; Reid et al. 2018 )

In the last years, a move in space technology came up, called “New Space”. Although there is no unique definition, it is certainly a movement and new philosophy, encompassing a globally emerging, private spaceflight and aerospace industry which is more socio-economically-oriented. In other words, working commercially and independent of governmental-funded (political) space programs with a faster, cheaper and better access to space.

In a wider definition of New Space, new business models and new manufacturing processes building up on alternative methods are considered in addition (ESA Space 4.0).

Examples of New Space systems might be the Low Earth Orbit (LEO) systems with many hundreds or even thousands of mini-satellites mainly dedicated for communication and internet. OneWeb ( https://onewebsatellites.com ) which has been aiming to launch at least 648 satellites to deliver global broadband connectivity, has 74 satellites in orbit. Footnote 3 SpaceX Starlink ( https://www.spacex.com/webcast ) is currently being built-up. SpaceX’s deployed 60 Starlink satellites in orbit after a successful launch on April 22, 2020 bringing the broadband internet project to more than 420 satellites. The first phase of the Starlink network will include 1584 satellites orbiting about 550 km above Earth in planes inclined 53 degrees to the equator. That part of the constellation SpaceX intends to launch through the end of 2020. ( https://www.nzz.ch/wissenschaft/starlink-so-funktioniert-das-satelliteninternet-von-elon-musk-ld.1493375 ).

Amazon’s project Kuiper ( https://www.geekwire.com/2019/amazon-project-kuiper-broadband-satellite ) will move in 2020 to a permanent research and development headquarter with state-of-the-art facilities for the design and testing of its planned mega-constellation of 3236 LEO satellites in altitudes of 590/609/629 km for low-latency, high-speed broadband. Telesat Canada ( https://www.telesat.com/news-events ) has similar plans for broadband communications scheduled to start operations from their LEO satellites (first Phase 1 LEO satellites were launched in 2018).

But, can those LEO systems be used for satellite positioning and navigation?

Some quick considerations: GPS signals broadcast at 27 Watts which are received at 158 × 10 −18 Watts on Earth. LEO signals of Starlink are 1000 × (30 dB) stronger compared to MEO (GNSS). But it takes 7 LEOs to match the coverage of 1 MEO.

200 + LEOs are needed for similar coverage—no problem, all mentioned LEO systems have significantly more than 200 satellites. Consequently, the geometry (Dilution of Precision—DOP values) is three times better than that of present GNSS. Considering further that a positioning error is approximately Signal-in-Space (SIS) User Range Error (URE) x geometry, it becomes clear that the LEO system’s geometry is three times better and relaxes the URE. A constellation like SpaceX Starlink could have three times worse URE and still reaches a positioning performance comparable to GPS (about 3 m horizontally, 4–5 m vertically).

The chip-scale atomic clocks (low power < 120mW, small size 17 cc volume, low-cost < 1000 USD … 300 USD) in the LEO satellites are approximately 100 × worse at one day compared to GPS atomic clocks. However, we may get comparable performance if they were updated once per LEO orbit (approx. every 100 min) instead of once per 12 h (GPS). Simple computations of LEO orbits by ground stations indicate that it is possible to achieve 3 m RMS, if using in addition cross-links even approximately 1.5 m.

What about costs? No taxpayer’s money has to be provided by governments…?

One can only speculate whether or not all LEO systems for satellite communications and internet mentioned above will be actually realised. As a result, tremendous competition for market shares would ensue between the companies, also affecting terrestrial communication, in particular 5G Wireless Networks. Also, I would not expect the various companies to modify their payloads to include satellite navigation as discussed above.

However, the Beijing Future Technology Company (Su et al. 2019 ; Yang 2019 ) is planning, developing and will operate a LEO satellite-based augmentation system to the MEO GNSS, called Centispace-1 (Fig.  9 ). Small satellites with a weight of approx. 100 kg in a Walker constellation 120/12/0, an altitude of 975 km and an inclination of 55° should receive GNSS from the MEO satellites and transmit in GNSS L1/L5 interoperable frequencies. High-speed crosslinks between the satellites are designed. The launch of a first experimental satellite happened already 2018, five experimental satellites will follow in 2020. Between 2021 and 2023 120 operational satellites will be launched and the ground segment finalised. Centispace-1 will deliver high accuracy and service of the order of 50 cm and an integrity service with an alarm time < 3 s and 99.99% global availability. In the combined processing with MEO GNSS data a point positioning < 10 cm with a significantly smaller convergence time of less than 1 min (due to the high doppler of the LEO satellites) is expected.

The Centispace-1 LEO satellite-based augmentation system. Reference: Yang ( 2019 )

However, these will be not the latest developments over the next years. The Cubesat technology and many low-cost low power miniaturized sensors fitting on them will enable many new IoT applications as well as the LEO augmentation of various MEO GNSS.

Megatrends in satellite navigation

Global navigation satellite systems As mentioned above, all of the four GNSS will be fully operational available by the end of 2020/beginning of 2021. The Chinese BDS, also the last one which started with the developments, is the most advanced: It is currently the only one which has a regional part with IGSO satellites (which will be also used for the transmission of SBAS messages) and it will be already extended by a LEO component called Centispace in the next years which significantly improves the convergence time of high-precision absolute positioning.

GPS III will improve its robustness over the next years whereas Galileo still has to prove it (in particular after the long outage in 2019). ESA has studied a regional aspect of IGSO satellites over Europe with regard to the evolution of the system. However, it is not yet decided whether it will be realised with the second generation of Galileo after 2025. The Russian GLONASS system has similar plans (GLONASS-B). What is even more needed, however, is a globally distributed ground GLONASS control system.

Regional navigation satellite systems The South Korean KPS will be developed over the next decade—overlapping the Japanese QZSS system which will be further expanded to 7 satellites.

Satellite - Based Augmentation Systems (SBAS) It is expected that after the first dual-frequency dual-system EGNOS V3, also Russia and China will incorporate in their SBAS their own GNSS (GLONASS and BDS, respectively) in addition to GPS. Whereas the SBAS in South Korea, in Russia, Australia and China are still being developed, and a guarantee of the availability of SBAS for civil aviation is guaranteed till 2035, is ARAIM showing already its large potential for providing Cat-I integrity similar to SBAS. Horizontal ARAIM will be available in the next 3–4 years and vertical ARAIM might come by the end of this decade. Will it replace then the SBAS after 2035?

CubeSats, mini - and nano - satellites The potential of CubeSats and the availability of miniaturized, low-power and low-cost sensors for those mini- or nano-satellites in LEO is increasing with every day, see e.g. https://www.nanosats.eu , https://www.esa.int/Enabling_Support/Space_Engineering_Technology/Technology_CubeSats , https://www.nasa.gov/mission_pages/cubesats/index.html . Thus, many IoT and other Earth observation applications become possible on a regional scale with a relatively small budget. CubeSats have passed the time where they were only considered as an educational tool for universities. The expensive space hardening of the payload is replaced by cheaper smart (redundancy) techniques. CubeSats will form space augmentations in LEO to the present GNSS over the next years. However, also exploration to Moon, Mars and other planets will take advantage of it. Corresponding studies are already running. We will see soon GNSS beyond the Earth up to the moon and further in space ( https://www.esa.int/Enabling_Support/Space_Engineering_Technology/Winning_plans_for_CubeSats_to_the_Moon ).

Digitalisation will be considered in GNSS payloads enabling on-orbit reprogramming of GPS signals and transmissions and artificial intelligence in space traffic management.

Quantum communications will contribute to a more reliable and trustworthy satellite navigation. Quantum communication takes advantage of the laws of quantum physics to protect data. These laws allow particles—typically photons of light for transmitting data—to take on a state of superposition, which means they can represent multiple combinations of 1 and 0 simultaneously. The beauty from a cybersecurity perspective is that the transmission of highly sensitive data by quantum communication is ultra-secure.

In the next years we will see many projects addressing one of the main challenges of satellite navigation: GNSS safety and security (space cyber security) . In the past years, our society and economy have become largely dependent on GNSS, computer networks and Internet of Things (IoT) solutions. This has led to a significant growth of cyber-attacks. Big data, virtual and augmented reality and artificial intelligence will even create more cyber risks. This evolving environment presents new opportunities for the space industry to come up with new commercial cyber security solutions.

GNSS receiver Although the H/W and S/W tools, like the inertial navigation system on a chip, the chip-scale atomic clock, the phased array antenna, detection/mitigation techniques for interferences are developed and jamming and spoofing may be happening, is the consideration of those tools in the civilian receivers still rare. Smartphones have seen some progress, which are nowadays equipped with almost all GNSS and RNSS. Android phones provide the capability to use GNSS raw data and can use self-developed software for specific user applications. It is only to be expected that more and more sensors combining various navigation methods will be implemented over time.

5G wireless networks Assuming a dense network of base stations, wireless 5G is able to provide centimeter navigation—however, only on a local scale. Will it be substituting or complementing/locally augmenting the global GNSS—as predicted in Fig.  8 ? Interesting developments—to be carefully followed and monitored.

Fighting with Space Debris As mentioned above, thousands of satellites will be launched in the coming years. The International Space Station (ISS) had to change its course often in the past in order to avoid getting seriously damaged by space debris and other satellites. Therefore, space traffic management studies have started at ESA and will intensively continue over the next decade Navigation of satellites will play an important role ( https://www.esa.int/Safety_Security/Space_Debris ).

Some remarks

Although we had to think in longer timeframes considering the developments of GNSS (which took at the beginning almost two decades for a system) it is hard to predict the future of satellite navigation. Like computers GNSS receivers are depreciated over a time of three years. It is therefore understandable that a forecast for more than a few years is almost impossible.

If we look to the future of GNSS and RNSS, we have to accept:

The signal is weak… The signal is easily jammed…The signal can be spoofed… The signal is subject to atmospheric perturbations…The signal doesn’t penetrate buildings…The signal has problems with urban and natural obstructions…

But is there a real substitute or alternative to GNSS?

Back-up by eLoran? Iridium NEXT?

Chip-scale atomic clocks, other terrestrial systems?

Map matching, radar, lidar, vision?

Com cell-id, 5G, INS, WiFi?

However, none of the above are also all-weather systems, have excellent accuracy, global coverage, high reliability, low cost, low complexity, minimal infrastructure needs, versatility.

Satellite navigation systems are not like other space projects serving only small scientific communities and last only for a few years. They are serving every citizen with Positioning, Navigation and Time (PNT). PNT is never the primary product; it is an enabler for many value-added applications. The critical infrastructure of many states depends already on GNSS. After more than two decades of building up the satellite systems, satellite navigation will stay many decades….

To what extent there will be an impact of the worldwide coronavirus pandemic and the subsequent crisis in economy is currently (April 2020) unclear. So far, we have seen delays in satellite launches, space projects and OneWeb’s filing for bankruptcy.

Compatibility refers to the ability of space-based positioning, navigation, and timing services to be used separately or together without interfering with each individual service or signal, and without adversely affecting national security. (NSPD-39: U.S. Space-Based Position, Navigation, and Timing Policy, December 15, 2004).

Interoperability refers to the ability of civil space-based positioning, navigation, and timing services to be used together to provide better capabilities at the user level than would be achieved by relying solely on one service or signal. (NSPD-39: U.S. Space-Based Position, Navigation, and Timing Policy, December 15, 2004).

OneWeb filed for Chapter 11 bankruptcy on 28 March 2020. Other LEO systems like Iridium, Globalstar, Orbcomm and Teledesic all went bankrupt about two decades ago, though only Teledesic failed to emerge from bankruptcy and deploy a second-generation constellation (Henry 2020 ).

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A Case Study Analysis for Designing a Lunar Navigation Satellite System with Time Transfer from the Earth GPS

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There is growing interest in designing a future lunar navigation satellite system (LNSS) while utilizing a SmallSat platform. However, many design decisions, e.g., regarding the satellite clock and lunar orbit, are yet to be finalized. In our prior work, we developed an LNSS architecture that leverages intermittently available Earth-GPS signals to compute timing corrections, thereby alleviating the need for a higher-grade onboard clock. In this work, we formulate twenty case studies with different grades of clocks and lunar orbits to analyze the trade-offs in designing a SmallSat-based LNSS with time transfer from the Earth GPS. For each case study, the accuracy of ranging signals is assessed via the lunar user equivalent range error (UERE). Even with lower-grade clocks, the lunar UERE exhibits performance comparable to that of the Earth GPS. Furthermore, variations in the lunar UERE are also examined when the available Earth-GPS measurements are processed at different rates.

• case study analysis
• lunar navigation satellite system
• size weight and power (swap)
• time transfer
• 1 INTRODUCTION

Over a half century since the end of the Apollo program, the National Aeronautics and Space Administration (NASA) will return humans to the Moon. In the coming decade, through the Artemis missions, NASA will land the first woman and person of color on the lunar south pole ( Smith et al., 2020 ). Additionally, many space agencies are participating in an international effort to establish a sustainable human presence on the Moon, which serves as a crucial platform to support future deep space exploration ( Laurini & Gerstenmaier, 2014 ). Indeed, we are entering a second space race, with international space agencies planning over forty lunar missions in the next decade and with crucial involvement from commercial space industry companies, including SpaceX and Blue Origin. To support increasing plans for crewed and robotic activities, future lunar missions will require access to reliable and precise position, navigation, and timing (PNT) services everywhere on the Moon.

Recently, the NASA Goddard Space Flight Center and the European Space Agency (ESA) conceptualized global positioning system (GPS)-like satellite constellations around the Moon, named LunaNet ( Israel et al., 2020 ) and Moonlight ( Cozzens, 2021 ), respectively. Furthermore, there has been an emerging interest in the use of a SmallSat platform for these PNT constellations to allow for cost-effectiveness and rapid deployment ( Israel et al., 2020 ). According to Mabrouk, 2015 , a SmallSat is about the size of a large kitchen fridge and weighs < 180 kg. These lunar PNT constellations by NASA and the ESA will assist in the overarching effort of establishing a sustainable human presence on the Moon by providing global PNT and communication services to lunar users. In particular, in the next decade, these initiatives will seek to satisfy needs expressed by the global exploration community, with a targeted position accuracy of less than 50 m for lunar users ( InsideGNSS, 2021 ).

While the lunar positioning accuracy of a lunar navigation satellite system (LNSS) depends on a variety of different factors, a few key factors are as follows: a) the lunar user equivalent ranging error (UERE), which determines the ranging accuracy of transmitted satellite signals, b) the minimum received power, which affects the signal acquisition and tracking performance, c) the constellation size, which ensures the visibility of a minimum number of LNSS satellites for any lunar user at any time, d) the geometric dilution of precision, which evaluates the effect of measurement error on the estimated position covariance, and e) the overall cost, which depends on the costs of launches from Earth, the costs of injection into a stable orbit around the Moon, and onboard equipment and maintenance costs. To provide a systematic approach, the current work assesses the LNSS design in terms of lunar UERE, while assessments for other factors will be explored in future works.

Given that the lunar PNT constellation initiatives are in the preliminary design phases, the LNSS design involves finalizing many key design considerations, including the following:

Lunar satellite orbit. Several types of lunar orbits that have previously been investigated include low lunar orbits (LLOs), prograde circular orbits (PCOs), near-rectilinear halo orbits (NRHOs), and elliptical lunar frozen orbits (ELFOs). In particular, ELFOs refer to a specific category of frozen orbits providing a greater coverage of the lunar poles, wherein frozen orbits represent those orbits that maintain nearly constant orbital parameters for extensive periods of time, without requiring station-keeping ( Folta & Quinn, 2006 ; Whitley & Martinez, 2016 ). Although PCOs are not frozen, they maintain multi-year stability with orbital parameters exhibiting predictable, repeatable behavior ( Whitley & Martinez, 2016 ). Due to the low orbiting altitude, LLOs have shorter orbital periods around the Moon, and there exist a few inclinations at which LLOs are also considered to be frozen or quasi-frozen ( Folta & Quinn, 2006 ). While NRHOs are less stable than the above orbits, thereby requiring more frequent station-keeping maneuvers, these orbits are highly elliptical, with nearly constant visibility of Earth and the lunar poles ( Schonfeldt et al., 2020 ).

Onboard satellite clock. The choice of onboard clock is critical for designing a navigation system, as its grade (which depends on timing stability) directly affects the ranging precision offered to lunar users. Among various clock choices is the commercial chip-scale atomic clock (CSAC) with its radiation tolerance and low size, weight, and power (SWaP), which has been specifically developed for space applications ( Schmittberger & Scherer, 2020 ). Another potential clock choice is the deep space atomic clock (DSAC), which has been recently designed by NASA to provide greater long-term timing stability and to assist in spacecraft radio navigation (T. Ely et al., 2022 ; T. A. Ely et al., 2018 ; Seubert et al., 2022 ).

Furthermore, designing a SmallSat-based LNSS involves unique challenges as compared with the legacy Earth GPS, which lead to the following additional design limitations: a) Limited size of LNSS satellites. A SmallSat platform limits its payload capacity, including the SWaP of the onboard clock. Given that lower-SWaP clocks tend to have worse timing stabilities ( Schmittberger & Scherer, 2020 ), the SWaP limitation on the clock directly affects the timing stability. b) Limited ability to monitor LNSS satellites. Given that a limited number of ground monitoring stations can be established on the Moon and that resources on Earth for monitoring the lunar constellation are limited, it is desirable for the LNSS satellites to require less maintenance, including fewer station-keeping maneuvers and clock correction updates. c) Increased orbital perturbations in the lunar environment. Because the Moon has a highly nonuniform distribution of mass ( Melosh et al., 2013 ), its gravitation field is more anisotropic than that of Earth. In addition, Earth’s gravity can significantly impact satellites in high-altitude orbits around the Moon, thereby limiting the set of feasible and stable lunar orbits.

Given these challenges for designing a PNT constellation in the lunar environment, one may consider the potential of leveraging the existing Earth’s legacy GPS, which is equipped with higher-grade atomic clocks and an extensive ground monitoring network. At lunar distances of approximately 385000 km, the Earth-GPS signal is significantly attenuated, and the Earth-GPS satellites directed toward Earth are largely occluded by Earth and often the Moon. This limits the Earth-GPS signal availability at lunar distances, with signals coming only the Earth-GPS transmitting antenna’s side lobes and the small, unoccluded parts of the main lobe. NASA’s Magnetospheric Multiscale Mission (MMS) has used these largely attenuated and intermittently available Earth-GPS signals to successfully compute position estimates in space (L. B. Winternitz et al., 2017 ). In fact, the MMS broke the Guinness World Record for the highest altitude for achieving an Earth-GPS fix in 2016 at distances of approximately one-fifth of the distance to the Moon ( Johnson-Groh, 2016 ; L. B. Winternitz et al., 2017 ) and then surpassed its previous record in 2019 by obtaining a fix at approximately half of the distance to the Moon ( Baird, 2019 ). Several simulation works have also demonstrated the feasibility of using the Earth GPS at lunar distances ( Cheung et al., 2020 ; Schonfeldt et al., 2020 ; L. B. Winternitz et al., 2019 ). Through its GPS Antenna Characterization Experiment (ACE) study, NASA has characterized GPS antenna gain patterns at high elevation angles from boresight for space users ( Donaldson et al., 2020 ). NASA has also developed a spaceborne Earth-GPS receiver (L. Winternitz et al., 2004 ), which will be tested on the lunar surface for the first time in 2023, as a part of the Lunar GNSS Receiver Experiment (LuGRE) ( InsideGNSS, 2021 ; Kraft, 2020 ). Additionally, in 2023, the ESA will launch the Lunar Pathfinder communication satellite to the Moon, which will utilize a spaceborne, high-sensitivity Earth-GPS receiver to provide a position fix for the first time in lunar orbit ( Cozzens, 2021 ).

In our prior work ( Bhamidipati et al., 2021 , 2022a ), we designed an LNSS architecture that harnessed the legacy Earth GPS to provide precise timing corrections to the onboard clock, as depicted in Figure 1 . The time-transfer technique leverages intermittently available Earth-GPS signals to alleviate the SWaP requirements of the onboard clocks and to mitigate the need for an extensive ground monitoring infrastructure on the Moon. We also devised a mathematical formulation of a lunar UERE metric, which is proportional to the root mean square (RMS) timing error, to analyze the ranging accuracy of an LNSS satellite. This proposed method achieved a low lunar UERE of less than 10 m while using a low-SWaP CSAC for an LNSS satellite in an ELFO.

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Architecture of the proposed time transfer from the Earth GPS ( Bhamidipati et al., 2021 ), which utilizes intermittently available Earth-GPS signals to correct the lower-grade clocks onboard the LNSS satellite

Given that many design choices, including the grade of the onboard clock and the orbit type, still need to be finalized for the future SmallSat-based LNSS, in this study, we extend upon prior work ( Bhamidipati et al., 2021 , 2022a ) to analyze the LNSS performance using the proposed time-transfer architecture (in Figure 1 ) from the Earth GPS under various case studies. Specifically, an LNSS satellite is simulated in various lunar orbit types, including an ELFO, LLO, PCO, and NRHO, and equipped with different grades of onboard clocks. For a given LNSS satellite orbit, the Earth-GPS continual outage period (ECOP) metric is examined to analyze the visibility effects of the Earth GPS on the performance of onboard clock corrections. The lunar UERE metric is estimated to perform a comparison across different case studies. Through this analysis, a trade-off can be observed between the different design considerations of the onboard clock and orbit type for an LNSS design that leverages the Earth-GPS time transfer. Furthermore, a variation in the lunar UERE is observed for different rates of collecting available Earth-GPS measurements (see Section 4.4 for the results of this sensitivity analysis). This analysis provides insights into the extent of Earth-GPS signal tracking and processing required to provide sufficient ranging precision. In particular, less frequent use of Earth-GPS measurements would allow the LNSS satellite to continually switch off the onboard Earth-GPS receiver for longer periods of time in order to save power. Across the various case studies, the time-transfer architecture allows the LNSS to achieve a performance comparable to that of the legacy Earth GPS, even while using a low-SWaP onboard clock. This work is based on our recent 2022 Institute of Navigation International Technical Meeting conference paper ( Bhamidipati, Mina, & Gao, 2022b ).

1.1 Key Contributions

The key contributions of this paper are as follows:

We design various case studies related to the grade of onboard clocks and orbit types for analyzing the trade-offs in designing an LNSS with time transfer from the Earth GPS. In particular, five clock types are investigated, with diverse SWaP characteristics that range from a low-SWaP CSAC to a high-SWaP DSAC (T. A. Ely et al., 2018 ) developed by NASA. Additionally, four previously studied lunar orbit types are investigated, namely, the ELFO, NRHO, LLO, and PCO.

A comparison analysis is performed across various case studies by investigating the associated RMS timing errors. Note that the RMS timing errors are primarily governed by the duration for which no Earth-GPS satellites are visible (ECOP metric) and the geometric configuration between the Earth-GPS constellation and the LNSS satellite (occultations due to Earth and the Moon).

The lunar UERE metric is evaluated for each case study and demonstrates a measurement ranging accuracy comparable to that of the legacy Earth GPS, even for a low-SWaP onboard clock.

Additionally, the variation in the lunar UERE is examined for different rates of collecting Earth-GPS measurements. This analysis provides insight into the extent of Earth-GPS signal processing required and, correspondingly, the amount of power required to operate the onboard Earth-GPS receiver, in order to provide sufficient ranging precision.

The remainder of this paper is organized as follows. Section 2 summarizes the previously proposed time-transfer architecture from the Earth GPS to the LNSS ( Bhamidipati et al., 2021 ) and describes the modifications incorporated to conduct further analysis. Section 3 provides a high-level overview of various case studies and describes the high-fidelity lunar simulation setup used, which involves modeling the onboard clock and orbit for each case study. Section 4 discusses the implications of our case study analysis in designing an LNSS. Section 5 provides concluding remarks.

• 2 TIME TRANSFER FROM THE EARTH GPS TO THE LNSS

This section summarizes our prior work ( Bhamidipati et al., 2021 ) on time transfer from the Earth GPS, wherein an LNSS satellite is considered to be equipped with an Earth-GPS receiver and an onboard clock that can provide short-term timing stability. A timing Kalman filter ( Krawinkel & Schön, 2015 ; Zucca & Tavella, 2005 ) updates the LNSS satellite clock with the intermittently available Earth-GPS signals and formulates the lunar UERE metric to characterize the ranging accuracy of transmitted navigation signals. An overview is also provided on the aspects of further analysis conducted in this work. In particular, the sensitivity of the lunar UERE metric in different simulated case studies is analyzed, including modifications of the measurement update rate for the proposed timing Kalman filter.

To maintain the LNSS clock estimate, the timing Kalman filter performs a time update every T pred seconds, based on the clock error propagation model. For this, the associated process noise covariance Q is defined in terms of the power spectral density (PSD) coefficients h 0 , h −1 , h −2 from an Allan deviation plot for the clock ( Krawinkel & Schön, 2015 ). These PSD coefficients reflect the short-term and long-term stability of the onboard clock and can be heuristically computed from the Allan deviation plots of a clock based on equations derived in ( Van Dierendonck & McGraw, 1984 ). To perform time transfer from the Earth GPS, the filter first determines whether any Earth-GPS signals are visible by examining the received carrier-to-noise density ratio C / N 0 . Then, the timing Kalman filter conducts a measurement update for the available Earth-GPS measurements with sufficiently high C / N 0 .

During the measurement update step, the expected pseudorange and pseudorange rates are determined from the visible Earth-GPS satellites to form a measurement vector of residuals. In particular, the measurement residual vector is formulated by leveraging the LNSS satellite position and velocity information from the available ephemeris. Indeed, the LNSS satellites are expected to maintain real-time position and velocity estimations within a target accuracy, although the exact framework and assisting infrastructure (e.g., ground monitoring, lunar base stations, etc.) for doing so have not yet been finalized ( National Aeronautics and Space Administration, 2022 ). Relativistic effects between the Earth-GPS satellite transmitter and lunar satellite receiver are not simulated in this work, but are left for future work. However, with a relativistic correction model, the LNSS satellite can correspondingly apply this correction to the received Earth-GPS measurements to formulate the measurement residual vector.

Based on the RMS error in the filter estimate, we formulate a lunar UERE metric that characterizes the accuracy of the LNSS ranging signals for lunar users. On the Moon, any atmospheric delays are minimal. Moreover, multipath effects experienced by users on the lunar surface are considered to be negligible due to the lack of building infrastructure and foliage. As a result, the final lunar UERE can be computed in terms of the four most significant error components as follows:

where the errors due to the differential group delay σ gd, LNSS and receiver noise σ rec, LNSS will depend on the final LNSS signal structure and lunar user receiver. Note that because the timing filter uses the LNSS satellite position and velocity information from the ephemeris, the lunar ephemeris error component σ eph, LNSS directly impacts the pseudorange residual measurement received at the LNSS, which will thus also affect the LNSS clock error σ clk, LNSS .

In this work, the variation in the LNSS clock error component σ clk, LNSS is investigated for various grades of onboard clocks and various types of lunar orbits, and the corresponding impact on the overall lunar UERE is analyzed. Additionally, the impact on the lunar UERE is investigated for a potentially reduced measurement update rate when Earth-GPS signals are available, with a sampling period of T meas = mT pred , where m is a positive integer. Indeed, a larger choice of m corresponds to less frequent GPS measurement updates, which allows the spaceborne Earth-GPS receiver onboard an LNSS satellite to be switched off for longer durations of time to save power.

• 3 OVERVIEW OF CASE STUDIES ON CLOCKS AND ORBITS

An extensive case study analysis is performed to examine the trade-off between different choices of onboard clocks and orbit types that can be considered for designing an LNSS with time transfer from the Earth GPS. In particular, a high-fidelity simulation is developed for an LNSS satellite for each orbit type using the Systems Tool Kit (STK) software by Analytical Graphics, Inc. ( AGI, 2021 ). For each modeled orbit type, case studies are developed in MATLAB by simulating various grades of onboard clocks. For each case study, the start time epoch is 9 Nov 2025 00 : 00 : 00.000 UTC, and the experiment time duration is 2 months (equal to 61 days).

First, an overview is provided of the case studies related to the onboard clocks and lunar orbit types investigated in this work. Thereafter, the simulation steps are presented, as executed in the STK software and MATLAB for modeling the transmission and reception of Earth-GPS signals in each case study, which are based on the previous validation framework ( Bhamidipati et al., 2021 ).

As mentioned in Section 1 , many prior studies ( Delépaut et al., 2020 ; Schönfeldt et al., 2020 ) have investigated various types of lunar orbits, primarily based on their stability and the duration for which their stability can be ensured. The lunar orbit types for this work include the ELFO, NRHO, LLO, and PCO, whose coverage and stability characteristics are discussed in Section 1 . Realistic simulations are created for an LNSS satellite in different lunar orbits by leveraging the high-precision orbit propagator (HPOP) in the STK software ( AGI, 2021 ). The HPOP generates and propagates accurate position and velocity solutions of the LNSS satellite by accounting for precise force models of the Earth, Sun, and Moon. Note that, in this paper, all of the orbits are naturally propagated using the HPOP, and no station-keeping is involved.

Three orbit types are modeled, namely, the ELFO, LLO and PCO, in the STK software using classical orbit mechanics ( Montenbruck et al., 2002 ). With this approach, objects orbiting in space require six elements (six Keplerian parameters) to fully characterize their position and velocity at any point in time. Specifically, prior literature ( Delépaut et al., 2020 ; T. A. Ely & Lieb, 2006 ; Whitley & Martinez, 2016 ) on ELFOs, LLOs, and PCOs defines their corresponding six Keplerian parameters at the start time epoch, including the semi-major axis, eccentricity, inclination, argument of perigee, right ascension of the ascending node (RAAN), and mean anomaly. Table 1 lists the associated Keplerian parameters of the three lunar orbit types, while Figure 2 shows associated illustrations in the Moon’s inertial frame.

Illustration of three (ELFO, LLO, and PCO) of the four lunar orbit types considered in this case study analysis

All of these orbits are visualized in the Moon’s inertial frame. The semi-major axis is 9750.5 km for the ELFO (orange), 1837.4 km for the LLO (magenta), and 4737.4 km for the PCO (blue). The HPOP tool in STK is used to propagate the initial conditions defined by the Keplerian parameters of these orbits in Table 1 .

• View inline

Keplerian Parameters for Three (ELFO, LLO, and PCO) of the Four Lunar Orbit Types Considered in This Case Study Analysis

In contrast, to model the fourth lunar orbit type in this case study analysis, namely, the L2 south NRHO, the initial position [ r x , r y , r z ], and velocity [ v x , v y , v z ] are specified in the Moon-centered Earth–Moon rotating frame. Defining the initial conditions in an Earth–Moon rotating frame is particularly useful when discussing halo orbits in the Earth–Moon system ( Williams et al., 2017 ). In the Earth–Moon rotating frame, the x-axis is along the instantaneous Earth–Moon position vector, the z-axis is along the instantaneous angular momentum vector of the Moon’s orbit around the Earth, and the y-axis completes the orthogonal system. Specifically, this approach is based on prior literature ( Williams et al., 2017 ) that solves for an NRHO in the ephemeris model (L2, radius of perigee of 4500 km, south family) using a forward/backward shooting process to provide the following initial state vector at an initial time epoch of 8 Nov 2025 23 : 22 : 07.10353 TDB: r x = −125.952 km, r y = 120.961 km, r z = 4357.681 km, v x = −0.042 km/s, v y =1.468 km/s, and v z = −0.003 km/s. The orbital period for this NRHO orbit is approximately 171.5 hr. Illustrations of the designed NRHO in both the Moon’s inertial frame (similar to that in Figure 2 ) and the Moon-centered Earth–Moon rotating frame are shown in Figures 3(a) and 3(b) , respectively.

The NRHO (L2, radius of perigee = 4500 km, south family) visualized in (a) the Moon’s inertial frame and (b) the Moon-centered Earth–Moon rotating frame

The x-axis of the Earth–Moon rotating frame is along the instantaneous Earth–Moon position vector, the z-axis is along the instantaneous angular momentum vector of the Moon’s orbit around the Earth, and the y-axis completes the orthogonal system. The HPOP tool in STK is used to propagate the NRHO orbit from its initial state vector defined in ( Williams et al., 2017 ).

Given the interest in the SmallSat platform for the future LNSS, various case studies of onboard clock types are chosen while keeping in mind that the limited payload capacity restricts the SWaP of the onboard clock. Based on prior literature ( Schmittberger & Scherer, 2020 ), the clock types for this work include Microchip’s CSAC, Microchip’s micro atomic clock (MAC), the Stanford Research Systems (SRS) PRS10, Excelitas’ rubidium atomic frequency standard (RAFS), and NASA’s DSAC (T. A. Ely et al., 2018 ). The specifications of these clock types are listed in Table 2 , which have been arranged in increasing order of their SWaP for convenience. Note that the Microchip MAC and SRS PRS10 are not space-qualified clocks and have been considered in this case study analysis as a way to incorporate options for increasing the magnitude of timing stabilities from low-cost CSAC to high-SWaP DSAC. Moreover, DSAC is included in order to provide a benchmark in terms of the current state-of-the-art timing stability that can be attained in deep space. For each clock type, the true clock error model is simulated in MATLAB to have a constant drift in the clock bias, and thereafter, the clock states are propagated forward in time using a first-order state transition matrix. The true clock drift (constant value) is assigned based on the known specifications of time deviation (TDEV) observed at the end of a day, which are reported in Table 2 . Note that for any clock type, TDEV refers to the expected error in reported time after a certain holdover time, which essentially depends on the Allan deviation and frequency drift. For the time update in the timing filter described in Section 2 , the PSD coefficients listed in Table 2 are used to model the corresponding process noise covariance matrix Q .

SWaP Characteristics, Time Deviation, and PSD Coefficients of the Five Clock Types Considered in This Case Study Analysis

SWaP and TDEV values were taken from ( Schmittberger & Scherer, 2020 ) while PSD coefficients were computed from their respective Allan deviation plots. The Allan deviation plot for CSAC was taken from Lutwak, 2011 . For the higher-grade RAFS clock and DSAC, the Allan deviation plots found in the literature ( Almat, 2020 ; T. A. Ely et al., 2018 ) do not capture the effects of noise components related to the h −1 and h −2 coefficients. As a result, after a confirmation based on heuristic analysis, the process noise covariance Q terms depending on h −1 and h −2 are considered to be negligible for the RAFS and DSAC.

For the chosen LNSS satellite clock and orbit type in each case study, the simulation scenario is modeled in the STK software and MATLAB to compute C / N 0 and the measurement residual vector for visible Earth-GPS satellites, which are later given as input to our timing filter. The key modeling aspects are summarized below, while a more detailed explanation has been provided in our earlier work ( Bhamidipati et al., 2021 ).

First, the simulated Earth-GPS constellation consists of 31 satellites with 8 satellites from Block IIR, 7 from IIRM, 12 from IIF, and 4 from Block III. While the performance for a standalone Earth-GPS L1 C/A lunar receiver is examined in this work, this proposed time-transfer architecture can be applied to other terrestrial signals from other global navigation satellite system (GNSS) constellations, such as the GPS L5 and the Galileo E1 and E5 signals, which have also been considered in prior works ( Capuano, Basile, et al., 2015 ; Capuano, Botteron, et al., 2015 ). The additional terrestrial GNSS signals are expected to result in better lunar UERE values with the proposed time-transfer architecture, due to the corresponding increase in received measurements. The transmission antennas on Earth-GPS satellites are modeled by utilizing the transmission power and antenna gain patterns of the L1 C/A signals, which are available from the NASA GPS ACE study ( Donaldson et al., 2020 ). Next, a spaceborne Earth-GPS receiver is simulated with a steering antenna pointed toward the Earth so as to maximize the visibility of Earth-GPS signals at the LNSS satellite. Based on prior literature ( Capuano, Basile, et al., 2015 ; Delépaut et al., 2019 ), a high-gain antenna is considered with 14 dBi at an off-boresight angle of 0° and a 3-dB beamwidth of 12.2°. For details regarding the approximate sizing of the high-gain antennas and their gimbals/steering equipment, the reader is directed to the LuGRE mission, the details of which can be found in Table 2 of Parker et al., 2022 . Based on the specifications detailed in Parker et al., 2022 and past missions ( Wertz et al., 2011 ), the mass of an LNSS satellite is heuristically computed to be 133.3 kg in Bhamidipati, Mina, Sanchez, et al., 2022 . This finding ensures that the mass of the LNSS case studies discussed in this paper conforms to the SmallSat constraint, which was reported in Section 1 as < 180 kg. For more details, refer to Section 3.2 of Bhamidipati, Mina, Sanchez, et al., 2022 . An Earth-GPS satellite is considered to be visible when, for a continuous time duration of at least 40 s, the received C / N 0 value is greater than 15 dB-Hz, which is a conservative threshold for acquisition and tracking determined from prior works ( Capuano, Basile, et al., 2015 ; Capuano, Botteron, et al., 2015 ; Delépaut et al., 2020 ).

Finally, measurements received at the LNSS satellite are simulated by incorporating the true clock bias and drift in the true range and range rate between the visible Earth-GPS and LNSS satellites, respectively. Note that the C / N 0 , true range, and range rate values are extracted from the STK simulation, whereas the true clock bias and drift are obtained from the simulated clock error model in MATLAB. To formulate the residual vector given to the measurement update explained in Section 2 , stochastic errors are induced based on simulated uncertainties from the receiver tracking loops.

• 4 CASE STUDY ANALYSIS: RESULTS AND DISCUSSION

For designing an LNSS with time transfer from the Earth GPS, the trade-off across different case studies is analyzed with respect to the onboard clock and orbit type, as listed in Section 3 .

To characterize the lunar UERE discussed in Section 2 , the group delay and receiver noise error magnitudes are taken to be the same as those of the Earth GPS, i.e., σ gd,LNSS = 0.15 m and σ rec, LNSS = 0.1 m. Given that the future LNSS will have greater limitations on ground monitoring infrastructure than the Earth GPS, the error component due to the broadcast ephemeris is scaled in the lunar UERE as σ eph, LNSS = 3 m. This value is essentially one order of magnitude higher than that of the Earth GPS and aligns with the desired position requirements of lunar navigation satellites listed in National Aeronautics and Space Administration, 2022 , i.e., < 4 m, 1- σ RSS. Our prior work provides a sensitivity analysis ( Bhamidipati, Mina, & Gao, 2022a ), wherein the timing errors and lunar UERE values are analyzed as the ephemeris errors vary at 0.3 m, 3 m, 30 m, and 300 m. The timing filter’s time update step is executed every T pred =60 s. Details regarding the reduced update rate with a sampling period of T meas = mT pred for the measurement update will be discussed below.

4.1 Validation Metrics

To perform a comparison analysis across different case studies, the following four validation metrics are defined:

Satellite visibility, which indicates the percentage time of the entire experiment duration for which the number of visible Earth-GPS satellites is greater than a pre-specified threshold. Two satellite visibility parameters are examined:

a) the percentage time for which at least 1 Earth-GPS satellite is visible, as this is the minimum number required to estimate the clock bias and drift;

b) the percentage time for which at least 4 Earth-GPS satellites are visible, as this is the minimum number required to estimate the full state vector, which includes the position, clock bias, velocity, and clock drift.

Maximum ECOP to identify the region of maximum continuous time when no Earth-GPS satellites are visible.

RMS errors in clock estimates across the entire simulation time duration to analyze the performance of the Earth-GPS time transfer for a reduced measurement update rate with a sampling period of T meas = mT pred and m = 5.

Lunar UERE metric, which characterizes the ranging measurement accuracy of signals transmitted by an LNSS satellite. As explained in Section 2 , the lunar UERE metric depends on the RMS error in the clock bias.

A case study is considered to be desirable if it exhibits either some or all of the following: greatest satellite visibility, shortest maximum ECOP, and lowest lunar UERE. To perform a sensitivity analysis of the examined case studies, the lunar UERE metric is computed for different reduced measurement update rates with m = 1, 5, 30, 60. Note that m = 1 provides a baseline comparison (non-reduced rate), as it depicts the case in which the measurement update step is executed whenever Earth-GPS satellites are visible, corresponding to the original framework of the timing filter proposed in our prior work ( Bhamidipati et al., 2021 ).

4.2 Across Orbit Types: Variation in Satellite Visibility and Maximum ECOP

For the four orbit types considered in this case study, the number of visible Earth-GPS satellites is shown in blue in Figures 4(a) – 4(h) while the highlighted red vertical bars indicate the regions of ECOP. Based on Table 3 , the NRHO achieves the highest visibility for at least one satellite, which is 99.9% of the total time, and the highest visibility for at least four satellites, which is 92.9% of the total time. The NRHO also exhibits the shortest maximum ECOP of only 420 s, while the other orbit types experience an ECOP of at least 2880 s. These observations related to the NRHO seem reasonable, as an LNSS satellite in the NRHO operates at high altitudes of 4500–700000 km above the Moon’s surface and thus experiences fewer occultations from Earth and the Moon. By comparing the magnified plots in Figures 4(b) , 4(d) , 4(f) , and 4(h) , one can observe that the LLO, which has a low altitude of 100 km, exhibits the smallest time spacing between consecutive regions of ECOP. Furthermore, note that these magnified plots capture ECOPs experienced by different orbit types over a random two-day period and do not necessarily showcase the maximum ECOP occurrences.

Earth-GPS satellite visibility and maximum ECOP across different orbit types The blue dotted lines indicate the number of visible Earth-GPS satellites, and the red vertical bars indicate regions of ECOP. (a), (c), (e), and (g) show the satellite visibility for the entire time duration for the ELFO, NRHO, LLO, and PCO, respectively. (b), (d), (f), and (h) show the magnified satellite visibility for a shorter time segment of 2 days. The NRHO not only exhibits the shortest maximum ECOP of 420 s but also the greatest visibility for at least one satellite at 99.9%.

Comparison Analysis Across Different Orbit Types

4.3 Across Clock Types: Variation in RMS Timing Errors

Table 4 compares the RMS estimation error in clock bias and drift across different case studies. Intuitively, the RMS timing error provides insights regarding the component of the lunar UERE metric, which will be discussed in the next two subsections.

Comparison Analysis Across Different Onboard Clock and Orbit Types

As the clock SWaP increases (indicating a higher-grade clock), the timing stability is correspondingly less sensitive to the choice of orbit type.

For a given Microchip CSAC (17 cm 3 ·0.0035 kg·0.12 W), whose low SWaP characteristics are given in Table 2 , Figure 5 provides an illustration of the variation in timing errors across the four orbit types, namely, ELFO, NRHO, LLO, and PCO. In Figure 5 , the same color coding is used as that in Figures 2 and 3 to denote different orbit types, with ELFO indicated in orange, NRHO in green, LLO in magenta, and PCO in blue. As stated above in the description of validation metrics, the measurement update rate is reduced by setting the sampling period to T meas = mT pred , where m = 5.

Comparison of estimation errors in clock bias and drift across different orbit types with an onboard Microchip CSAC

ELFO is indicated in orange, NRHO in green, LLO in magenta, and PCO in blue. (a) and (c) demonstrate the errors in clock bias and clock drift for the entire experiment duration, respectively, while (b) and (d) present magnified errors in clock bias and clock drift for a smaller time segment of 2 days. Three of the orbit types, namely, the ELFO, NRHO, and PCO, demonstrate comparable RMS timing errors of < 0.0408 μ s in clock bias and < 0.0407 ns/s in clock drift, while the LLO exhibits a slightly higher RMS error in clock bias of 0.0679 μ s.

Three orbit types, namely, ELFO, NRHO, and PCO, demonstrate comparable RMS timing errors of < 4.08 × 10 −2 μ s in clock bias and < 4.07 × 10 −2 ns/s in clock drift, while the case study based on the LLO shows a higher RMS error in clock bias of < 6.79 × 10 −2 μ s. This observation implies that the lowest RMS timing error not only depends on the shortest maximum ECOP and the greatest satellite visibility but also on orbital parameters, namely, the eccentricity, inclination, and altitude, which govern the geometric configuration between the Earth GPS and LNSS. Note that there is only a small degree of correlation between the orbital viewing geometry for the least stable clock, i.e., CSAC. This trend occurs because the mean distance between the Earth GPS and the lunar navigation satellite is already quite large (approximately 385000 km); thus, altitude variations in the lunar orbit, which are on the order of only thousands, are not sufficient to induce a significant difference in timing error. This observation is interesting, as it ensures uniformity in the accuracy of onboard clock estimates, even if a hybrid constellation involving varied lunar orbit types is considered in the future LNSS.

Additionally, a larger RMS timing error for any case study can indicate a larger timing error in either of the two segments: a) when at least one Earth-GPS satellite is visible and b) at the end of the ECOP (during the ECOP, only a prediction update is executed; thus, the estimation errors continue to increase). For instance, the largest RMS error in clock bias (6.79 × 10 −2 μ s) for the LLO–CSAC among all case studies can be attributed to the following factors: a) the least stable clock (i.e., CSAC) among different clock types combined with a significant ECOP of 2880 s and b) a poor visibility for at least one Earth-GPS satellite of only 61.5% (as shown in Table 3 ). These factors cause the estimation errors both during and at the end of the ECOPs to be higher (as depicted by magenta spikes in Figure 5[a] ). Note that while the LLO trades unfavorably in terms of orbit stability, ECOP, and satellite visibility, its key advantage lies in its low orbiting altitude. From a SmallSats perspective, the smaller distance of the LLO from the lunar surface users enables the use of smaller antennas for transmitting lunar navigation signals. As shown in Table 4 , as the SWaP and timing stability of the onboard clock increase, the variation in RMS error across orbit types becomes less significant, wherein the RMS error in clock bias and drift for DSAC are < 0.87 × 10 −2 μ s and < 0.46 × 10 −3 ns/s, respectively. The RMS value for DSAC is significantly lower (approximately one order of magnitude) than those of the other clock types, namely, CSAC, MAC, PRS10, and RAFS. This result occurs because the TDEV per day and PSD coefficients of the DSAC, as presented in Table 2 , are also significantly lower than those of the other clock types, indicating a higher timing stability for NASA’s DSAC. A lower RMS timing error for DSAC also indicates smaller errors at the end of the ECOPs compared with other clock types.

4.4 I Across Case Studies and Earth-GPS Measurement Update Rates: Sensitivity Analysis of the Lunar UERE

The motivation behind the sensitivity analysis is to quantify the variation in the lunar UERE as the Earth-GPS measurement update rate is varied. Note that quantifying the power saved by the use of less frequent Earth-GPS measurements requires a more complex analysis, which is beyond the scope of this paper. Figure 6(a) demonstrates the variation in lunar UERE metric across the case studies examined in this work while considering no reduction in the measurement update rate, i.e., setting T meas = T pred =60 s or m = 1. The proposed time transfer achieves a low lunar UERE of < 10 m for most case studies, except for the case study involving the LLO and the Microchip CSAC, which exhibits a value of 18.4 m. These observations imply that if the desired lunar UERE to be maintained by an LNSS satellite is, for instance, < 10 m at all times, one can easily opt for an onboard clock that falls in the lower end of the SWaP spectrum, such as the Microchip CSAC (17 cm 3 ·0.0035 kg·0.12 W) or Microchip MAC (50 cm 3 ·0.0084 kg·0.5 W), instead of a high-SWaP clock, such as Excelitas’ RAFS or NASA’s DSAC. To maintain a desired lunar UERE, one can also wisely choose an orbit type that is easy to maintain and has a longer lifespan, such as the LLO, PCO, or ELFO, over the more complex NRHO, which requires constant station-keeping maneuvers to maintain stability. Furthermore, for future investigations, the future SmallSat-based lunar PNT constellation could potentially be heterogeneous, wherein the grade of the onboard clock is chosen based on the orbit of each LNSS satellite to satisfy a desired lunar UERE. For instance, to maintain a desired lunar UERE of < 5 m, Figure 6(a) demonstrates the potential of designing a heterogeneous LNSS constellation based on an ELFO or LLO, wherein the satellites in an LLO can be equipped with a PRS10 clock while the satellites in an ELFO can be equipped with a lower-SWaP Microchip MAC.

Sensitivity analysis of the lunar UERE metric across different case studies for reduced measurement update rates

(a) m = 1 is the baseline case with no reduction in the measurement update rate, i.e., T meas = T pred , (b) m = 5, (c) m = 30, and (d) m = 60.

Figures 6(b) – 6(d) show the variation in lunar UERE metric for three cases of reduced update rates, where T meas = mT pred with m = 5, m = 30, and m = 60 respectively. As the Earth-GPS measurement update rate increases for low-SWaP clocks with sampling periods from T meas =60 min ( m = 60) to T meas =1 min ( m = 1), an increased sensitivity of the lunar UERE metric is observed across lunar orbit types, i.e., the difference in value between the LLO and other orbit types increases. Additionally, the estimated lunar UERE is < 30 m for a reduced Earth-GPS measurement update rate with a sampling period of up to T meas =30 min, which is comparable in order of magnitude to that of the baseline case with m = 1 (in Figure 6[a] ) as well as the legacy Earth GPS. Thus, even at a reduced measurement update rate, the LNSS design, which utilizes time transfer from the Earth GPS, lowers the SWaP requirements of the onboard clock. Furthermore, one can re-observe the potential for a heterogeneous, SmallSat-based PNT constellation explained above, wherein not only the grade of the onboard clock is carefully chosen, but also the reduced update rate of the timing filter based on the LNSS satellite orbit so as to satisfy a desired lunar UERE.

• 5 CONCLUSION

We performed an exhaustive case study analysis for designing a SmallSat-based LNSS with time transfer from the Earth GPS, wherein trade-offs between different design considerations related to the onboard clock and lunar orbit type were investigated. In the proposed time-transfer approach, the SWaP requirements of the onboard clocks were alleviated by leveraging the intermittently available Earth-GPS signals to provide timing corrections. The lunar UERE metric was also designed to characterize the ranging accuracy of LNSS satellites.

Using high-fidelity simulations of an LNSS satellite in the STK software of Analytical Graphics, Inc., multiple case studies were designed comprising five onboard clocks and four lunar orbit types. The shortest maximum ECOP of only 420 s was observed for an NRHO because this orbit type experiences fewer occultations from Earth and the Moon given its high altitude. A low lunar UERE of < 30 m was demonstrated for low-SWaP onboard clocks (e.g., Microchip CSAC, SRS PRS10) even for reduced Earth-GPS measurement update rates with sampling periods of up to 30 min. Through a case study analysis of time transfer from the Earth GPS, lower-SWaP onboard clocks and easier-to-maintain lunar orbit types were shown to still achieve the desired lunar UERE across the entire LNSS constellation. In this context, the easier-to-maintain lunar orbit types are those with higher orbital stability and longer lifespans, such as the LLO, PCO, or ELFO, in contrast to the more complex NRHO, which requires constant station-keeping maneuvers. Similarly, the lower-SWAP onboard clocks are those that fall in the lower end of the SWaP spectrum, such as the Microchip CSAC or Microchip MAC, instead of a high-SWaP clock, such as Excelitas’s RAFS or NASA’s DSAC. Based on this single-satellite analysis of the time-transfer architecture, future work will include the design of a SmallSat-based LNSS constellation that provides reliable and precise PNT services to support upcoming lunar exploration missions. Preliminary work based on this concept has been recently published in Bhamidipati, Mina, Sanchez, et al., 2022.

• HOW TO CITE THIS ARTICLE

Bhamidipati, S., Mina, T., & Gao, G. (2023). A case study analysis for designing a lunar navigation satellite system with time transfer from the earth GPS. NAVIGATION, 70 (4). https://doi.org/10.33012/navi.599

• ACKNOWLEDGMENTS

We thank the Analytical Graphics, Inc. educational alliance program for providing us with the STK software license to perform this research. We also thank Keidai Ilyama for insightful discussions related to this work and Daniel Neamati for reviewing this paper.

This is an open access article under the terms of the Creative Commons Attribution License , which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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Global Positioning System

Lead Authors: Heidi Davidz, Richard Freeman, Alice Squires , Contributing Authors: Tom Hilburn, Brian White

The Global Positioning System (GPS) case study was developed by the United States Air Force Center for Systems Engineering (AF CSE) located at the Air Force Institute of Technology (AFIT). The GPS is a space-based radio-positioning system. A constellation of twenty-four satellites, including three spares, comprise the overall system which provides navigation and timing information to military and civilian users worldwide. GPS satellites, in one of six Earth orbits, circle the globe every twelve hours, emitting continuous navigation signals on two different L-band frequencies. The system consists of two other major segments: a world-wide satellite control network, and the GPS user equipment that can either be carried by a human user or integrated into host platforms such as ships, vehicles, or aircraft.

This case study discussion is based on the original source (O’Brien and Griffin 2007), which provides useful insights into what we might consider a "traditional" SE application. A second global positioning case study looks at the same case study from the perspectives of system of systems (sos) system of systems (sos) engineering and enterprise systems engineering (ese) enterprise systems engineering (ese) .

• 1 Domain Background
• 2 Case Study Background
• 3.1 Enabling Individuals
• 3.2 Configuration Management
• 3.3 Enabling the Organization
• 3.4 Risk Management
• 3.5 Life Cycle Management
• 3.6 Systems Thinking
• 5.1 Works Cited
• 5.2 Primary References
• 5.3 Additional References

Domain Background

When looking at the Global Positioning System (GPS), it would be difficult to imagine another system that relies so heavily upon such a wide range of domains, with the possible exception of the World Wide Web (WWW). Additionally, the various systems operating within these domains must all function together flawlessly to achieve success. It is evident from reading this case study that it directly relates to the following domains:

• communications; and
• transportation.

This is also an example of systems of systems systems of systems (SoS) and is considered an innovative technology.

The GPS case study includes a detailed discussion of the development of the GPS and its components, as well as other applicable areas. The reader of this study will gain an increased understanding of the effect that GPS has on military and commercial industries in the context of the systems engineering support required to achieve success.

Case Study Background

The United States Air Force Center for Systems Engineering (AF CSE), established in 2002 at the Air Force Institute of Technology (AFIT), was tasked to develop case studies focusing on the application of systems engineering principles within various aerospace programs. The GPS case study (O'Brien and Griffin 2007) was developed by AFIT in support of systems engineering graduate school instruction. The cases are structured using the Friedman-Sage framework (Friedman and Sage 2003; Friedman and Sage 2004, 84-96), which decomposes a case into contractor, government, and shared responsibilities in the following nine concept areas:

• Requirements Definition and Management
• Systems Architecture Development
• System/Subsystem Design
• Verification/Validation

Risk Management

• Systems Integration and Interfaces
• Life Cycle Support
• Deployment and Post Deployment
• System and Program Management

The Friedman-Sage framework (2004) is provided in Appendix A of the case study. This case study is an example where the government - specifically the JPO Systems Engineering Directorate - bore the responsibility for systems integration and configuration management. That is, the government played more than an oversight role in the systems engineering of the GPS system of systems. As mentioned in the case study, JPO developed the CONOPs, mission analysis, requirements and design analysis including security, and developed their own approach to the cryptology methodology. JPO coordinated the Configuration Control Board (CCB) chaired by the Program Director. JPO was also responsible for Level I ICDs and system design configurations; where the contractors were responsible for the system architecture and ICDs within their segment.

Case Study Description

The “Global Positioning System - Systems Engineering Case Study” describes the application of systems engineering during the concept validation, system design and development, and production phases of the GPS program (O'Brien and Griffin 2007). The case examines the applied systems engineering processes, as well as the interactions of the GPS joint program office (JPO), the prime contractors, and the plethora of government agencies that were associated with the program’s development and fielding. The systems engineering process is traced from the initiation of studies and the development of key technologies, which established the vision of a satellite navigation system in the 1960s, through to the multiphase joint program that resulted in a fully operational capability release in 1995. This case study does not cover system enhancements incorporated through Blocks IIM, IIF, and III.

The GPS case study derived four learning principles (LPs) that explain the more broadly applicable areas of systems engineering knowledge that are addressed by the case study. These four LPs relate strongly to the SEBoK in the following areas:

• enabling individuals (LP1);
• configuration management (LP2);
• enabling the organization (LP3); and
• risk management (LP4).

Additionally, the GPS case study contains a thorough overview of life cycle management and exemplifies systems thinking principles.

Enabling Individuals

Learning Principle 1 : Programs must strive to staff key positions with domain experts.

From the program management team, to the systems engineering, design, manufacturing, and operations teams, the individuals on the program were well-versed in their disciplines and all possessed a systems view of the program. While communications, working relationships, and organization were important, it was the ability of the whole team at all levels to understand the implications of their work on the system that was vital. Their knowledge-based approach for decision making had the effect of shortening the decision cycle because the information was understood and the base and alternative solutions were accurately presented.

Configuration Management

Learning Principle 2 : The systems integrator must rigorously maintain program baselines.

The joint program office (JPO) retained the role of managing and controlling the system specification and, therefore, the functional baseline. The JPO derived and constructed a mutually-agreed-to set of system requirements that became the program baseline in 1973. While conducting the development program, the GPS team was able to make performance, risk, cost, and trade analyses against the functional baseline to control both risk and cost. The JPO was fully cognizant of the implications of the functional requirements on the allocated baseline because they managed the interface control working group process. Managing that process gave them first-hand knowledge and insight into the risks at the lowest level. The individual with the system integrator role must rigorously maintain the system specification and functional baseline. There must be appropriate sharing of management and technical responsibilities between the prime contractor and their government counterparts to ensure success.

Enabling the Organization

Learning Principle 3 : Achieving consistent and continuous high-level support and advocacy helps funding stability, which impacts systems engineering stability.

Consistent, continuous high-level support provides the requirements and assists funding stability. In this role, the Office of the Secretary of Defense (OSD) provided advocacy and sourced the funding at critical times in the program, promoted coordination among the various services, and reviewed and approved the GPS JPO system requirements. The OSD played the central role in the establishment and survivability of the program. The GPS JPO had clear support from the Director of Defense Development, Research, and Engineering, Dr. Malcolm Currie, and program support from the Deputy Secretary of Defense, Dr. David Packard. Clearly, the armed services – particularly the Navy and the Air Force early on, and later the Army – were the primary users of GPS and the eventual customers. However, each armed service had initial needs for their individual programs, or for the then-current operational navigation systems. Additionally, the secretary of the Air Force provided programmatic support to supply manpower and facilities.

Learning Principle 4 : Disciplined and appropriate risk management must be applied throughout the life cycle.

The GPS program was structured to address risk in several different ways throughout the multiphase program. Where key risks were known up front, the contractor and/or the government utilized a classic risk management approach to identify and analyze risk, as well as develop and track mitigation actions. These design (or manufacturing/launch) risks were managed by the office who owned the risks. Identified technical risks were often tracked by technical performance measures (such as satellite weight and software lines of codes) and addressed at weekly chief engineer’s meetings.

Serving in the clear role of program integrator allowed the JPO to sponsor risk trade studies at the top level. The JPO would issue study requests for proposals to several bidders for developing concepts and/or preliminary designs. Then, one contractor would be down-selected and the process would continue. This approach provided innovative solutions through competition, as well as helped in defining a lower risk, more clearly defined development program for the fixed-price contracts approach that was being used for development and production.

As the system integrator, the JPO was also closely involved with technical development. To identify unforeseeable unique technical challenges, the JPO would fund studies to determine the optimal approaches to new issues. There were schedule risks associated with the first launch due to unforeseen Block II issues with respect to the space vehicle and control segments (software development). Although it was a catastrophic event, the Challenger accident actually provided much needed schedule relief. Using decision analysis methodology led the JPO to an alternative approach to develop the expendable launch vehicle for the Block II satellites.

Good communication, facilitated by cooperative working relationships, was a significantly positive (though intangible) factor in the success of the GPS program, regardless of whether it was between the contractors and the government (JPO or other agencies), or between contractors and sub-contractors. A true team environment also played a significant role in reducing risk, especially considering the plethora of government agencies and contractors that were involved in the effort.

Life Cycle Management

The GPS case study takes the reader through the initial concept of GPS (March 1942) all the way to the development, production, and operational capability of the system. The current GPS program traces its heritage to the early 1960s when Air Force Systems Command initiated satellite-based navigation systems analyses conducted by The Aerospace Corporation. The case study follows the execution of the GPS program from the inception of the idea to the full operational capability release on April 27th, 1995. The concentration of the case study is not limited to any particular period, and the learning principles come from various times throughout the program’s life.

Systems Thinking

The GPS case study highlights the need for systems thinking throughout. GPS satellites, in one of six Earth orbits, circle the globe every twelve hours. These satellites emit continuous navigation signals on two different L-band frequencies. The system consists of two other major segments: a world-wide satellite control network and the GPS user equipment that can either be carried by a human user, or integrated into host platforms such as ships, vehicles, or aircraft. The ability to conceive, develop, produce, field, and sustain the GPS demands the highest levels of systems thinking.

The GPS case study is useful for global systems engineering learning and provides a comprehensive perspective on the systems engineering life cycle. The study is applicable for detailed instruction in the following areas:

• enabling individuals ;
• configuration management ;
• enabling the organization ;
• risk management ;
• life cycle management ; and
• systems thinking .

The GPS case study revealed that key Department of Defense personnel maintained a clear and consistent vision for this unprecedented, space-based navigation capability. The case study also revealed that good fortune was enjoyed by the JPO as somewhat independent, yet critical, space technologies matured in a timely manner.

Although the GPS program required a large degree of integration, both within the system and external to the system, amongst a multitude of agencies and contractors, the necessary efforts were taken to achieve success.

Lastly, the reader of the GPS case study will gain an increased understanding of the effect that GPS has on the military and commercial industries in the context of the systems engineering support required to achieve success. The system was originally designed to help “drop five bombs in one hole” which defines the accuracy requirement in context-specific terms. The GPS signals needed to be consistent, repeatable, and accurate to a degree that, when used by munitions guidance systems, would result in the successful delivery of multiple, separately-guided munitions to virtually the identical location anywhere at any time across the planet. Forty to fifty years ago, very few outside of the military recognized the value of the proposed accuracy and most non-military uses of GPS were not recognized before 1990. GPS has increasingly grown in use and is now used every day.

Works Cited

Friedman, G.R. and A.P. Sage. 2003. Systems Engineering Concepts: Illustration Through Case Studies. Accessed September 2011. Available at: http://www.afit.edu/cse/docs/Friedman-Sage%20Framework.pdf .

Friedman, G. and A. Sage. 2004. "Case studies of systems engineering and management in systems acquisition." Systems Engineering. 7(1): p. 84-96.

O’Brien, Patrick J., and John M. Griffin. 4 October 2007. Global Positioning System. Systems Engineering Case Study. Air Force Center for Systems Engineering (AFIT/SY) Air Force Institute of Technology (AFIT). 2950 Hobson Way, Wright-Patterson AFB OH 45433-7765.

Primary References

O’Brien, Patrick J., and John M. Griffin. 4 October 2007. Global Positioning System. Systems Engineering Case Study . Air Force Center for Systems Engineering (AFIT/SY) Air Force Institute of Technology (AFIT). 2950 Hobson Way, Wright-Patterson AFB OH 45433-7765.

Additional References

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A case study of satellite navigation reliability: Bangladesh perspective

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International Conference in Communications, Signal Processing, and Systems

CSPS 2018: Communications, Signal Processing, and Systems pp 886–890 Cite as

The Satellite-Based M2M Services for Smart Grids: Case Study

• Ning Jiang 40 , 41 ,
• Chengjun Yang 40 ,
• Xiaobo Guo 41 ,
• Zhijian Zhang 40 &
• Jian Wang 40  
• Conference paper
• First Online: 04 May 2019

1266 Accesses

Part of the book series: Lecture Notes in Electrical Engineering ((LNEE,volume 515))

The smart grid (SG) is an intelligent evolution of the electricity network that depends on highly reliable and secure connectivity between various infrastructure components from utilities, in order to meet the requirements of smart metering communication. Machine-to-machine (M2M) technology is designed for automatic equipment when it exchanges data, which is suitable for SG; Wireless communication is a main mode of M2M, and satellite communication provides support for the coverage of SG communication network. In this paper, an overview of the satellite-based M2M service for the smart grid is provided, and a case system is discussed with Beidou-based M2M service for smart grid.

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1 Introduction

At present, the communication infrastructure of smart grid includes wired and wireless. Wireline-based solutions have available and developing, but wireless communication technologies will become increasingly important, especially in rural environments, scenes that need long-distance communication. All of that can greatly benefit from wireless or satellite-based system for the smart grid. Wireless automatic meter reading is of particular interest to utilities for both urban and suburban environments [ 1 ].

M2M has been applied more and more in smart grid [ 2 , 3 , 4 , 5 ]. M2M communication happens between machines, this practice often ignores direct manual intervention. Smart grid provides conditions for devices that require large numbers of automated operations.

Advanced Metering Infrastructure (AMI) is a significant component of the SG, designed to be Demand Response-enabled [ 6 ], the two-way communication between power companies and consumers enable consumers to monitor and adjust the use of energy as well. Figure  1 shows a schematic diagram of typical AMI architecture, it contains four components: Meter Database Management system (MDMS), Neighborhood Area Network (NAN), Wide Area Network (WAN), and Home Area Network (HAN). Increasingly, companies are planning to launch intelligent metering services by incorporating M2M devices in the meters.

Typical AMI architecture

In our proposed model, we focus on a case study of the satellite-based M2M service for smart grid: Beidou-based M2M service for the smart grid. Section  2 introduces the development and application of the satellite-based M2M service; Sect.  3 discusses the present situation of Beidou-based M2M service for smart grid, and our model.

2 The Satellite-Based M2M Service

The satellite-based M2M service provided connectivity over large areas. Since terrestrial networks may not service all locations on earth, satellite operators, low earth orbit satellites such as Iridium, Mid orbit satellites such as Inmarsat, and Beidou offer global connectivity services that can extend M2M communication to almost 100% of the earth. Hence, the delivery of relatively small quantities of data in almost all cases is supported by the satellite-based M2M service [ 1 ].

Over the past decade, M2M communication through satellite communications, especially with the development of satellite networks, have played an important role in many fields. A typical application is the sensor network [ 8 ], M2M sensor devices are grouped into clusters and communicate with satellite gateways. The characteristics of M2M communication system are a large number of equipment, low data rate, and large coverage area, which fits the demand of the smart grid. The satellite-based M2M service in the smart grid is similar to the satellite-based M2M sensor network, and the smart meter (concentrator) is grouped into clusters in the local and communicate with satellite gateway. It is a workable scheme to collect electrical information from the Beidou short message. At the end of 2012, the system was formally put into operation in the three functions in the Asia Pacific region, including Navigation and Position, Time Giving, and Short Message. The Beidou satellite communication system has a wide range of coverage, no communication blind district and enciphering of message [ 7 ].

Therefore, we take the application of M2M service in smart grid based on Beidou satellite as a case.

3 Case: The Beidou-Based M2M Service

3.1 demands for sg and characteristics of bd.

Beidou satellite communication has its limitations, so it needs to be perfect for the relevant measures to meet the needs of smart grid. The characteristics of Beidou (BD) satellite communication and the demand for smart grid communication are shown in Table  1 .

From the table, Beidou satellite communication system cannot meet all the needs of smart grid communication, we need to make some changes.

3.2 System Architecture

We need to make some improvements when Beidou satellite communication is added to WAN in the AMI system. As seen in Fig.  2 , the addition of a gateway between MDMS and WAN, between WAN and NAN, can ensure that the original network can be changed as little as possible, and solve the existing problems properly. The ARQ coding scheme can be used to solve the problem that the Beidou satellite communication cannot guarantee reliable transmission in the transmission of power data into the communication network; The long message segmentation method can be used for the problem of the lack of the capacity of the Beidou satellite communication, it breaks data into packets in this end and combines incoming message into a single message in other end; On the response speed problem, we use gateway to response the end, the new response strategy is specified between the gateway, the gateway converts the protocol as well.

Satellite-based M2M service for smart grid

But even so, the time needed to send a longer message is still too long, so the longer message can be compressed first, such as LZ77, which can reduce the length of the message to be sent, which will reduce the time cost.

The gateway structure is shown in Fig.  3 . The physical layer provides physical access for connection; the operating system loads the related drivers, following the corresponding protocol; encoding (long message enhancement, ARQ coding, LZ77 coding, etc.) and protocol conversion happen in the application layer.

Satellite gateway

4 Conclusion

We summarize the effect of M2M and satellite communication in smart grid, which is demand-focused communication network. The satellite-based M2M service has a good prospect in many fields. More and more studies have begun to be attempted in smart grid for characteristics of this system. The Beidou satellite communication is running well in the Asia Pacific region and has been widely used. Therefore, the Beidou-based M2M service is taken as a case to solve the problems, which exists in the fusion process of the satellite-based M2M service for smart grid.

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Acknowledgements

This work was supported by State Key Laboratory of Smart Grid Protection and Control of NARI Group Corporation, and the Postgraduate Research and Practice Innovation Program of Jiangsu Province (SJCX18_0004).

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Ning Jiang, Chengjun Yang, Zhijian Zhang & Jian Wang

Science and Technology on Information Transmission and Dissemination in Communication Networks Laboratory, The 54th Institute of CETC, Shijiazhuang, Hebei, China

Ning Jiang & Xiaobo Guo

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Correspondence to Jian Wang .

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Jiang, N., Yang, C., Guo, X., Zhang, Z., Wang, J. (2019). The Satellite-Based M2M Services for Smart Grids: Case Study. In: Liang, Q., Liu, X., Na, Z., Wang, W., Mu, J., Zhang, B. (eds) Communications, Signal Processing, and Systems. CSPS 2018. Lecture Notes in Electrical Engineering, vol 515. Springer, Singapore. https://doi.org/10.1007/978-981-13-6264-4_104

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