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.
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).
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.
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
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1.
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.
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2.
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?
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Back-up by eLoran? Iridium NEXT?
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Chip-scale atomic clocks, other terrestrial systems?
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Map matching, radar, lidar, vision?
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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.
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3.
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….
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4.
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.