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SATNAV . . . That’s right – satellite navigation signals, including those from the Global Positioning System (GPS), can be picked up in space and used to determine the receiver’s position. It’s a bit tricky since signals from these satellites were only intended to be used within the Earth’s atmosphere. But with some intelligent engineering and calculations, it can be done. There is even the possibility that our Moon might get its own navigation satellites! W ith the likely forthcoming return to the Moon (possibly as early as 2024), and ongoing space exploration, it is vital to have reliable and accurate means to navigate in space. Of particular interest for lunar exploration are ice deposits in craters near the south pole of the Moon, which could be used for drinking water and also turned into hydrogen and oxygen for rocket fuel and breathing. We have GPS and other satellite navigation systems here on Earth, as described in detail in the November 2019 issue (siliconchip.com.au/Article/12083). Those systems were designed for determining location in the terrestrial, atmospheric and the near-Earth space environment. But could those same signals be used in space or on the Moon? GPS and other GNSS satellites orbit at an altitude of around 20,000km so, in principle, any vehicle below that altitude should be able to ‘see’ the satellites and make a position fix. Since the antennas look down, one might think it’s not possible to get a signal above the orbit of a GPS satellite, but that is not the case. 10 Silicon Chip According to NASA, GPS signals can be received and used in space in the same manner as on Earth, up to an altitude of 3000km. NASA calls the space between the surface of the Earth and an altitude of 3000km the “Terrestrial Service Volume” (see Fig.1). In this volume, GPS works normally according to the GPS Standard Positioning Service (SPS) Performance Standard (www.gps.gov/ technical/ps/). The volume at altitudes between 3000km and 36,000km (geosynchronous satellite orbit) is defined by NASA as the “Space Service Volume”. In this volume, which is subdivided into two parts, performance is not guaranteed to be as good as in the Terrestial Service Volume. As 36,000km is well above the 20,000km altitude of the GPS satellite constellation, you might think that the signals could not be received because the GPS antennas are pointing down toward Earth and not up. But there is another way the GPS signal can be received. Instead of receiving a signal from a satellite above you, you could receive a signal from a satellite on the opposite side of the Earth (see Fig.2). Its antenna is pointing Australia’s electronics magazine siliconchip.com.au . IN SPACE! by Dr David Maddison Fig.1: the “service volumes” for GPS, with the Terrestrial Service Volume being everything below 3000km altitude. The GPS satellite and geosynchronous orbit altitudes are also shown for comparison. down toward Earth, but some of the signal would reach your receiver. A high Earth orbit (HEO) satellite and its trajectory, which varies in altitude, is shown in Fig.2. Its path extends from the Terrestrial Service Volume, below 3000km, to beyond the geosynchronous orbit altitude of 35,887km (rounded to 36,000km) which is beyond even the Space Service Volume. The signal from one GPS satellite is shown, along with the first side lobes (off-axis antenna radiation pattern) for the L1 GPS frequency of 1575.42MHz. Fig.3 shows this radiation pattern in more detail. The receiving satellite can obtain a GPS signal from the satellite shown from either the main lobes or the first side lobes, or the signal may be entirely blocked by the Earth. Around 97% of radio energy is located in the main lobe and just 3% in the side lobes, so a sensitive receiver is needed. Only one GPS satellite is shown for simplicity; in reality, other satellites will be visible and not blocked by the Earth. As with terrestrial GPS, four satellites are required for an accurate position fix. siliconchip.com.au Fig.2: this shows how GPS signals are received in space, even when the receiving spacecraft is above the orbit of the GPS satellites. The dark green circle is the Earth, while the lighter green shaded area is the umbra or shadow of the Earth, where the satellite signals are blocked. The receiving satellite is in an elliptical orbit encompassing all possible volumes of space accessible with GPS. Australia’s electronics magazine October 2020  11 Fig.3: a simplified generic diagram showing the radiation pattern from GPS or similar antennas. The main lobe of a GPS satellite is generally not available in space as it is blocked by the Earth, but the first side lobe may be available. The other side lobes and back lobe would be too weak to be usable. Source: NASA. Earlier versions of GPS satellites did not consider performance in the Space Service Volume and performance was variable due to different side lobe radiation patterns and power levels. This was addressed by NASA and the US Department of Defense by writing specifications for performance levels for the Space Service Volume during 2003-2005. These specifications were implemented on Block III, SV 11+ (Space Vehicle 11) and subsequent GPS satellites. It doesn’t matter where the receiver is located; if the signals from four GPS satellites can be received, then you can identify your position in space. This should even work on the surface of the Moon. However, additional calculations would be needed to establish the relationship between the Fig.4: GPSPAC was the first attempt to pick up GPS signals in space. It was launched aboard LANDSAT 4 in 1982. Source: USGS. location of the Moon and the Earth to establish one’s position on the surface of the Moon. Positional accuracy on the Moon will be less than on Earth due to the much greater distances involved, resulting in more significant timing and thus distance errors. The distance from the centre of the Earth to the centre of the Moon averages 385,000km. But it varies by over 50,000km, and it can change as rapidly as 75m/s (270km/h). These are important factors to keep in mind when using GPS on the Moon, and they need to be incorporated in the relevant calculations. Based on an Earth radius of 6371km, a Moon radius of 1737km and a GPS satellite altitude of 20,183km, the closest a GPS receiver on the Moon could be to a GPS satellite Figs.5&6: command and telemetry boards carried by TEAMSAT. This gives you an idea of the relatively basic electronics used in the late 90s. Interestingly, both boards seem to be centred around FPGAs (field-programmable gate arrays). 12 Silicon Chip Australia’s electronics magazine siliconchip.com.au is 356,709km. That’s more than 17 times further than the same receiver on Earth. However, to receive a GPS signal on the Moon, that signal would have to come from a satellite on the far side of the Earth, over 409,817km away. That’s 20 times further away than the nearest a GPS receiver could be to a satellite on Earth. Hence, timing and distance errors will be around 20 times greater than on Earth (as rough figures), assuming the accuracy of the receiver clock is the same in both locations. Note that GPS is already routinely used in the near-Earth environment with vehicles such as low Earth orbit satellites and the International Space Station and its Crew Return Vehicle, as they are all well below the altitude of the GPS satellites. The limits of GPS Currently, the formal altitude limit of GPS is that of the outer limits of the Space Service Volume of 36,000km; but the real practical limits are not yet known. Limits are imposed by the available signal strength, signal availability as determined by geometric limitations imposed by satellite antenna main and sidelobe patterns, and the occultation (blocking) of GPS satellite signals by the Earth. Uses for high orbital altitude GPS The ability for satellites and other space vehicles to use GPS at high orbital altitude confers many advantages due to better knowledge of space vehicle location. These include: • better satellites station-keeping • improved space vehicle rendezvous and docking • geosynchronous satellite servicing possibilities • better Earth science measurements including atmospheric, ionospheric, geodesy and geodynamics • better navigation by uncrewed launch vehicles • formation flying of constellations of satellites such as MMS (magnetospheric multiscale mission; see below) • improved weather satellites • improved space weather observations • improved astrophysical observations due to better navigation by orbiting telescopes • better navigation en-route to the Moon and on the Moon • closer spacing of satellites in geostationary orbit due to better location fixes • use of GNSS for time synchronisation of science experiments and space vehicle clock. High orbital altitude GPS experiments It had long been speculated that GPS could be used above the maximum orbital altitude of the constellation. Many GPS receivers were launched into space from 1982, and especially from 1991 onwards, mainly in the Terrestrial Service Volume (below 3000km). For a complete list up to 2003 see http://gauss.gge.unb. ca/grads/sunil/missions.htm Note that GPS became available to civilians in 1983. Significant early experiments with high altitude GPS use were as follows: • The first time GPS was installed on a satellite was LANDSAT 4 in 1982 (Fig.4). It carried a package known as GPSPAC. Three more GPSPAC units were also launched on LANDSAT 5 in 1984 and US Department of Defense vesiliconchip.com.au Fig.7: TEAMSAT, launched in 1997, carried YES (Young Engineers’ Satellite). Its primary purpose was to study GPS reception at altitudes above the GPS constellation (20,183km). Source: ESA. hicles in 1983 and 1984. The GPS constellation was not fully operational at that time, and four satellites were in view for just a few hours per day. The GPSPACs provided essential data that was used in the development of the rest of the Global Positioning System. • Falcon Gold was an experiment of the US Air Force Academy in 1997 to use a GPS receiver above the altitude of GPS satellites. The GPS signal was received up to an altitude of 33,000km. The experiment confirmed the possibility of using GPS in locations above the orbit of the GPS satellite constellation, plus the ability to use GPS sidelobe signals for navigation, previously a matter of debate. • YES (Young Engineers’ Satellite) was launched in 1997 as a sub-satellite of TEAMSAT (Figs.5-7), which itself was part of MaqSAT H. An orbit of 531 × 26,746 km was achieved, with its primary purpose to study GPS reception at altitudes above the GPS constellation. • Also in 1997, a GPS receiver was flown in the high Earth orbit satellite Equator-S (Fig.8), above the altitude of the GPS satellites. No navigation solution was possible because the required four satellites could not be simultaneously seen; however, useful signals were received at an altitude up to 61,000km. Australia’s electronics magazine October 2020  13  Fig.8: Equator-S was also launched in 1997 and carried a GPS receiver. It was not able to get a location fix, but it was determined that useful signals could be picked up at altitudes of up to 61,000km. Fig.9: the AMSAT (OSCAR-40) amateur satellite was launched in 2000. In 2001, its onboard GPS receiver picked up valid signals to the satellite’s maximum altitude of 60,000km, and mapped the main and sidelobe signals. • In 2000, Kronman et al. were able to perform orbit determination of a geosynchronous satellite which received GPS signals from the far side of the Earth and then retransmitted them to a ground-based receiver where all data processing was performed, to determine the satellite’s orbit (see Fig.10). The use of a satellite just to relay signals is known as “bent pipe architecture”. No suitable receiver was available off the shelf, so one had to be made. According to Kronman, the following features were required but not commercially available at the time in one unit: The ability to navigate off the Earth (for acquisition), a second-order tracking loop to accommodate anomalous Doppler, the ability to accept commands to track specific PRNs (Pseudorandom Noise code), the availability of individual PRN pseudorange data referenced to a precise local time source, Selective Availability correction without P(Y)-code capability (military encryption). • In 2000, the AMSAT Phase 3-D (OSCAR-40) amateur satellite (Fig.9) was launched with NASA-sponsored GPS experiments onboard, using existing receiver technology. The actual GPS experiment was done in 2001. It received signals up to the satellite’s maximum altitude of 60,000km, and mapped main and sidelobe signals. As with the previous experiments, actual GPS locations were computed on the ground rather than the satellite, and not in real time. Based on the results, it was determined that navigation considerably above 60,000km could be performed with a suitable receiver and antenna. • Also in 2000, two STRV-1 (Space Technology Research Vehicle) missions were launched, the STRV-1c and STRV1d spacecraft (Fig.11). They had a 615 x 39,269km orbit. They were equipped with GPS receivers which mapped GPS signals to geosynchronous orbit, approximately 36,000km up. • GIOVE-A (Galileo In-Orbit Validation Element-A) was a European Space Agency (ESA) satellite launched in 2005 and retired in 2012 (Fig.12). Its purpose was to test aspects of Europe’s GNSS navigation system, Galileo. According to the ESA, its primary objective was to “secure vital frequency filings, generate the first Galileo navigation signals in space, characterise a prototype rubidium atomic clock, and model the radiation environment of Medium Earth Orbit (MEO) for future Galileo spacecraft”. The satellite was equipped with a GPS receiver. In 2006, the receiver was activated for 90 minutes, and it was confirmed that it could receive GPS data and it downloaded a full almanac. After its retirement, it was moved to a “graveyard” orbit 100km above the Galileo constellation altitude of 23,222km. That is beyond the 20,183km altitude of the GPS constellation. In the retirement phase, in 2013, new software was up- 600 nmi (1.5 SCD at GEO) GPS 26 ,5 60 km Nominal Visibility Region 12.2° GEO 42,200km Fig.10: the relative geometry of a GPS satellite, geosynchronous satellite (GEO) and Earth for the Kronman et al. experiment in 2000. It is the sidelobes of the GPS satellite transmissions that are being received. The GEO satellite receives signals in the shaded zone from 1.5 to 3.5 degrees above the limb of the Earth. EARTH 14 Silicon Chip Australia’s electronics magazine siliconchip.com.au Fig.11: STRV-1c and STRV-1d (Space Technology Research Vehicle), launched in the year 2000. They were equipped with GPS receivers which mapped GPS signals to geosynchronous orbit, approximately 36,000km up. loaded to the GPS receiver on the satellite, and more extensive tests were made. Particular emphasis was made on measuring properties of the GPS satellite sidelobe signals. Current civilian missions using high-altitude GPS In 2015, NASA launched the MMS (Magnetospheric Multiscale) mission (Phase 1) which is a four-satellite constellation which flies in a tetrahedral formation 7.2km apart, to study aspects of the Earth’s magnetic field (Fig.13). Each was equipped with a highly sensitive high-altituderated GPS receiver called Navigator for real-time position x Fig.12: GIOVE-A (Galileo In-Orbit Validation Element-A) was launched in 2005, to test aspects of Europe’s GNSS navigation system, Galileo. measurements (see Fig.14). In 2016 and 2019, the highest altitude GPS fixes to date were obtained at 70,006km and 187,167km respectively. The Navigator GPS receiver is designed for fast and weak GPS signal acquisition, and it is the highest operational GPS receiver to date, at a distance of around halfway to the Moon. It is designed to work in a variety of space regimes such as low Earth orbit (LEO), geosynchronous orbit (GEO), high Earth orbit (HEO), up to and beyond 12 Earth radii (76,452km+), at launch and re-entry. Pseudorange is the distance measured between the GPS Other means of navigating the Moon There is no significant magnetic field on the Moon, so a compass cannot be used. Also, the lack of atmosphere makes it hard for astronauts to judge distances. The Apollo 14 crew missed a crater they had intended to visit by only 30m because of these difficulties. When Neil Armstrong landed the LEM on the Moon in 1969, he used his eyes and maps to find the appropriate place to land (the famous Apollo Guidance Computer was not intended to locate the exact landing place). In space it is always good to have a backup plan, so apart from NASA developing lunar GPS, they are also developing “terrain relative navigation” (see below). This is similar to what Neil Armstrong did, but instead of using eyes to compare lunar terrain to a map, a computer compares the lunar terrain (imaged with a camera) to maps in the computer’s memory. Apart from terrain relative navigation, returning astronauts will also use GPS, navigation Doppler lidar and hazard detection lidar. Other methods that will be used to navigate on the Moon include: • radiometric methods utilising the existing Deep Space Network to measure range and speed (updated to allow for lunar tracking). • lunar orbiting spacecraft such as the LRS (see separate panel). and lunar surface stations such as the LCT (same panel). • inertial navigation. • optical techniques such as viewing stars relative to lunar surface features. Images from a test of NASA’s terrain relative navigation in the Mojave Desert. The live image is on the left, and a reconstructed image is on the right. It identifies and matches known features in the images to determine the current position. siliconchip.com.au Australia’s electronics magazine October 2020  15 The Lunar Relay Satellite (LRS) and Lunar Communications Terminal (LCT) Apart from navigation on the Moon via GPS, for effective communications (especially if people are living on the Moon’s surface), it will be desirable or even necessary to have lunar relay satellites along with a Lunar Communications Terminal (LCT). NASA has proposed a system of two satellites to relay communications between the surface of the Moon and the Earth, as well as crewed lunar vehicles, all part of the Artemis program. These vehicles include Orion, to launch from Earth and orbit the Moon, and the Altair lunar lander, to take the crew from Orion to the surface of the Moon. The orbit will be a “12 hour frozen elliptical lunar orbit”. This is a special type of highly elliptical stable orbit. It is required because above about 1200km altitude, Moon orbits are usually unstable and short-lived (tens of days) due to the ‘tug-of-war’ with the Earth’s gravity. Below 1200km, the inherent ‘lumpiness’ of the Moon and thus variations in gravity cause orbits to be unstable and shortlived as well. The proposed LRS satellites will have a service life of 7-10 years, a data bandwidth of 100Mbps from lunar habitats and the LCT, and 50Mbps from elsewhere on the lunar surface. The LCT will be a communications node for rovers, crew, habitats, science experiments etc. It will provide some navigational support, 802.16 wireless LAN and line of sight communications to 6km and have a 1m Ka-band antenna. Navigation support will be in the form of one- and two-way ranging to determine the range of a vehicle to the LCT, Doppler satellite and the receiving satellite, and differs somewhat from the true range due to several physical effects. Its measurement precision depends on the signal strength received (see Fig.15), but simulations show that the pseudorange with strong signals is better than ±1.5m. The pseudorange with weak signals is better than ±13m, and for measurements when a strong carrier phase signal is present, precision is better than ±1mm. The receiver has been tested at velocities up to 10km/s. An artist’s rendering of NASA’s proposed Lunar Relay Satellite (LRS) along with the Moon based Lunar Communications Terminal (LCT). tracking for measurement of the range from space vehicles to the LCT and beacon signals. There are no official Internet top-level domains (TLDs) currently assigned to the Moon but, .ln, .le (lunar embassy) and .lunar have been unofficially proposed. However, they are not currently supported by the root servers. It has also tracked as many as 12 GPS satellites simultaneously, many more than expected. GOES-16 or Geostationary Operational Environmental Satellite was launched in 2016; it is a weather satellite in geostationary orbit. It is the first civilian geostationary satellite to use GPS for orbit determination. This will be used, along with other equipment, to maintain an orbital position within a 100m radius. Extending GPS to the Moon High-altitude GPS research has the ultimate objective of extending GPS for use on the Moon, and NASA plans to use existing GPS infrastructure to do this. The GPS receiver that Fig.13: an artist’s concept of the MMS satellite constellation examining so-called “magnetic reconnection” phenomena in the Earth’s magnetic field (represented by blue lines). The exact satellite locations must be known to create accurate magnetic field maps, hence the use of GPS. Source: NASA. 16 Silicon Chip Fig.14: the Navigator GPS receiver, as used on MMS mission satellites for high-altitude GPS fixes. Australia’s electronics magazine siliconchip.com.au Perigee Apogee Perigee Hz Strong (main lobe) signals Apogee: most signals in side lobes Weak (side lobe) signals Fig.15: measurements of signal strengths vs position in orbit for MMS mission satellites. Strong main lobe signals are shown above the dotted line, while weaker side lobe signals (the majority) are below. This shows the importance of sidelobe signals for satellites orbiting above the GPS constellation. Apogee is the point of an orbit farthest from Earth and perigee is closest to Earth. Source: NASA. will be used for this is based on the Navigator described above, and the NavCube which we will soon discuss. For use beyond its current orbit of almost halfway to the Moon, the Navigator GPS will be enhanced with a higher-gain antenna (up to 14dB of gain), antenna steering to keep the antenna pointed towards Earth and the GPS constellation, a more accurate clock and various other updated electronics. While NASA is intending to leverage existing GPS infrastructure for Lunar use, it is not a perfect solution and will also not work on the dark (far) side. It will be augmented by other methods. The idea of building a mini GPS-like system around the Moon called LunaNet is also still under consideration for the much longer term (see Fig.16). Apart from Fig.16: an artist’s concept of LunaNet, providing navigation, communications and other services on the Moon. siliconchip.com.au navigation, it would provide many other services, such as communications. NavCube NavCube (Fig.17) is a combination of two NASA technologies. One is SpaceCube, which is a reconfigurable and fast flight computing platform, and the other is the Navigator GPS receiver used in the MMS mentioned above. For high-altitude and near- or on-Moon real-time GPS fixes, a powerful computer is needed for data processing. The NavCube combines both the GPS receiver and the com- Fig.17: NASA’s NavCube. It uses a Navigator GPS receiver and has substantial computing abilities for processing GPS signals in lunar orbits and on the surface of the Moon. It measures 25 x 20 x 15cm and weighs around 5kg. Australia’s electronics magazine October 2020  17 puter. NavCube can also provide precise timing signals for another experiment using X-rays for communications (XCOM). A NavCube was recently placed on the International Space Station for testing. Estimates of the accuracy of GPS on the Moon with NavCube vary. The worst accuracy is considered to be around 1km, which is useful enough but not ideal. With a highly accurate atomic clock onboard, or accurate time signals beamed from the proposed Lunar Gateway (see Figs.18 & 19), it could be improved to around 100m. The Lunar Gateway is a mini space station proposed to orbit the Moon in 2024 as a communications hub, labo- 18 Silicon Chip ratory, habitation module and a holding station for lunar equipment. Cheung, Lee et al. have estimated an accuracy figure of 200-300m based on modelling. Meanwhile, Winternitz, Bamford et al. came up with several estimates depending on whether the Lunar Gateway is crewed or uncrewed, as the presence of crew causes perturbations which affect accuracy. For GPS in conjunction with an onboard rubidium atomic frequency and an uncrewed vessel, the lateral position accuracy is 31m, and the range accuracy is 9m; for a crewed vessel, the figures are 77m lateral and 21m in range. With ground tracking from the Earth using the Deep Space Net- Australia’s electronics magazine siliconchip.com.au work (no GPS), uncrewed accuracy is 468m lateral and 33m in range; crewed is 8144m lateral and 451m in range. The first demonstration of lunar GPS could be in November 2021, with the launch of an uncrewed Orion capsule on the Artemis 1 mission (to be launched with the Space Launch System). Orion will record GPS signals throughout the mission to determine the usefulness, and measure signal characteristics of GPS around the Moon. Problems with using GPS in space The speed of space vehicles requires fast signal acquisition. There is also the problem of much lower signal strength due to having to rely on side lobe signals, and also the long ranges from the GPS satellites. Additional problems include large dynamic ranges between “weak” and “strong” satellites with wide signal gain variability; high Doppler and Doppler rates of change of GPS signals; fewer GPS satellite signals visible; mission antenna placement causing visibility problems; multipath reflections and radiation on very dynamic platforms. Table 1 expands on these problems and their solutions. How much accuracy does Lunar GPS require? Terrestrial GPS can achieve accuracies of around one Fig.18: the Lunar Gateway “lunar space station” concept, showing an Orion spacecraft docking. The Orion will carry GPS and test it in the lunar environment as early as November 2021. The Lunar Gateway, when placed into lunar orbit in 2024, will also carry GPS with signals augmented by a very accurate onboard atomic clock. siliconchip.com.au Australia’s electronics magazine October 2020  19 Fig.19: an artist’s rendering of the Lunar Gateway. It could help provide navigation services on the Moon by transmitting a highly accurate timing signal to improve the accuracy of lunar-based GPS. Fig.20: an artist’s concept of a mining operation on the Moon. Accurate navigation will be necessary for such activities. Note the mirrors used to illuminate the area. metre or better. Lunar GPS will be somewhat less accurate; however, there are no roads to locate on the Moon, and any target location such as a crater, mining site or base will be visually apparent. So accuracies of even a few hundred metres will be adequate. For autonomous vehicles or other applications requiring greater navigational accuracy, this could be achieved by augmentation with beacons and machine vision, plus artificial intelligence (AI) to avoid obstacles or locate targets in outer space.. Regime Altitude Problems Terrestrial Service Under 3000km High Doppler rates, Volume fast signal rise and set, accurate ephemeris upload required, signal strength and availability comparable to Earth use Lower Space 3000-8000km Service Volume More GPS signals available than for terrestrial service volume; very high Doppler rates Upper Space 8000-36,000km Earth shadow Service Volume significantly reduces main lobe signal; significant periods with fewer than four satellites available; weak signal strength Beyond Space 36,000-360,000km Very weak signals Service Volume (Moon) and very poor signal geometry Mitigation Development of In widespread use purpose-built space receivers; fast acquisition eliminates the need for ephemeris upload (data for estimated position of GPS receiver relative to satellites) Improved antennas; receivers must be able to process higher Doppler rates Silicon Chip In use by the USA and others Higher gain antennas, In use by the USA more sensitive receivers, and others use of GPS side lobe transmissions, algorithms such as in GEONS software to navigate with fewer than four satellites Higher gain antennas and receivers; accept degraded performance; use other signals of opportunity if possible, eg, beacons, perhaps from LCT or LRS (see panel) Table 1 – Problems and solutions for spaceborne GPS. Based on J.J.K. Parker, NASA. 20 Status Australia’s electronics magazine In use to 187,000km by MMS (USA); will be extended to lunar orbit on Artemis 1 mission in 2021 SC siliconchip.com.au