GPS satellites are placed in medium Earth orbit to provide optimal coverage and signal stability
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Retrieved technical references indicate that global navigation satellite systems utilize medium Earth orbit to deliver position, navigation, and timing services across constellations.
Currently, the Global Navigation Satellite System (GNSS) mainly uses the satellites in Medium Earth Orbit (MEO) to provide position, navigation, and timing (PNT) service. The weak navigation signals limit its usage in deep attenuation environments, and make it easy to interference and counterfeit by jammers or spoofers. Moreover, being far away to the Earth results in relatively slow motion of the satellites in the sky and geometric change, making long time needed for achieved centimeter positioning accuracy. By using the satellites in Lower Earth Orbit (LEO) as the navigation satellites, these disadvantages can be addressed. In this contribution, the advantages of navigation from LEO constellation has been investigated and analyzed theoretically. The space segment of global Chinese BeiDou Navigation Satellite System consisting of three GEO, three IGSO, and 24 MEO satellites has been simulated with a LEO constellation with 120 satellites in 10 orbit planes with inclination of 55 degrees in a nearly circular orbit (eccentricity about 0.000001) at an approximate altitude of 975 km. With simulated data, the performance of LEO constellation to augment the global Chinese BeiDou Navigation Satellite System (BeiDou-3) has been assessed, as one of the example to show the promising of using LEO as navigation system. The results demonstrate that the satellite visibility and position dilution of precision have been significantly improved, particularly in mid-latitude region of Asia-Pacific region, once the LEO data were combined with BeiDou-3 for navigation. Most importantly, the convergence time for Precise Point Positioning (PPP) can be shorted from about 30 min to 1 min, which is essential and promising for real-time PPP application. Considering there are a plenty of commercial LEO communication constellation with hundreds or thousands of satellites, navigation from LEO will be an economic and promising way to change the heavily relay on GNSS systems.
Currently, the Global Navigation Satellite System (GNSS) mainly uses the satellites in Medium Earth Orbit (MEO) to provide position, navigation, and timing (PNT) service. The weak navigation signals limit its usage in deep attenuation environments, and make it easy to interference and counterfeit by jammers or spoofers. Moreover, being far away to the Earth results in relatively slow motion of the satellites in the sky and geometric change, making long time needed for achieved centimeter positioning accuracy. By using the satellites in Lower Earth Orbit (LEO) as the navigation satellites, these disadvantages can be addressed.
A GNSS receiver employs trilateration to determine its position on or near the Earth’s surface by timing signals from four or more GNSS satellites. Currently, GNSS is well recognized as the major enabler of ‘precision’, and widely used for the precise timing and positioning. There are two fully operational GNSS systems at present: the United States’ Global Positioning System (GPS), and the Russian Federation’s Global Navigation Satellite System (GLONASS). Two systems under development, the Chinese Beidou Navigation Satellite System (BeiDou), and the European Union’s Galileo system, which are both expected to achieve full global coverage capability by 2020.
In addition, the Japanese Quasi-Zenith Satellite System (QZSS) and Indian Navigation with Indian Constellation (NAVIC) are two regional systems to provide the augmentation service. The Multi-GNSS constellations provide the orbit as well as frequency diversity to strengthen the observation geometry and reduce the transmitting errors, resulting in improvement of positioning accuracy and stability [ 2 ]. The above-mentioned GNSS systems are consisted of the satellites at Geostationary orbit (GEO), Inclined Geostationary orbit (IGSO) or Medium Earth Orbit (MEO), and all of them are 20×~30× further from the Earth compared to the LEO.
By using the satellites at Lower Earth Orbiter (LEO) as the navigation satellites, these disadvantages can be improved [ 4 , 5 , 6 ]. LEO satellites (with altitude from 160 km to 2000 km) are 20 times closer to the Earth than GNSS at MEO, leading to stronger radio signals on the ground for navigation because of the lower path
The other advantages of LEO used as navigation satellites include whitening multipath and effective Doppler positioning, as the rapid motion of LEO makes the reflected signals are no longer effectively static over short averaging times and strengthens the Doppler shift [ 7 ]. However, being them too close to the Earth, the ground converge of LEO satellite is less than one-tenth of that of MEO. Hence, the hundreds of LEO satellites are needed to match the coverage of GNSS [ 3 ]. It is extremely expensive to deploy such a massive number of LEO satellites to only provide navigation service.
The simulation results of all these three stations are listed in Table 8 . This clearly demonstrate the advantage of LEO used as the navigation system. 5. Conclusions Currently, the Global Navigation Satellite System (GNSS) mainly use the satellites in Medium Earth Orbit (MEO) to provide position, navigation, and timing (PNT) service. The weak navigation signals limit its usage in deep attenuation environments, and make it easy to interference and counterfeit by jammers or spoofers. Moreover, being far away to the Earth results in relative slow motion of the satellites in the sky and geometric change, making long time needed for achieved centimeter positioning accuracy.
By using the satellites at lower earth orbit (LEO) as the navigation satellites, these disadvantages can be addressed. LEO satellites (with altitudes from 160 km to 2000 km) are 20 times closer to the Earth than GNSS at MEO, leading to stronger radio signals on the ground for navigation. The swift motion of LEO provides the rapid geometric change, which resulting quicker PDOP change and rapid convergence for integer ambiguity. We compared and analyze this theoretically.
Future autonomous transportation is one of the most demanding application areas in terms of connectivity, as it has to simultaneously meet stringent criteria that do not typically go hand in hand, such as high throughput, low latency, high coverage/availability, high positioning and sensing accuracies, high security and robustness to interferences, etc. In order to meet the future demands of challenging applications, such as applications relying on autonomous vehicles, terrestrial networks are no longer sufficient and are to be augmented in the future with satellite-based networks. Among the emerging satellite networks, Low Earth Orbit (LEO) networks are able to provide advantages over traditional Medium Earth Orbit (MEO) and Geo-Stationary Earth Orbit (GEO) networks in terms of signal latency, cost, and performance. Nevertheless, several challenges exist in LEO system design, which have not been fully addressed in the existing literature. In particular, the problem of LEO-system optimization of design parameters is a multi-dimensional problem with many aspects to be considered. This paper offers a comprehensive survey of the LEO-system design parameters, of the challenges in LEO system design process, and of the optimization methods for satellite communication, positioning, and sensing applications, as well as a summarizing discussion on the design considerations for LEO-based networks to support future autonomous transportation.
In order to meet the future demands of challenging applications, such as applications relying on autonomous vehicles, terrestrial networks are no longer sufficient and are to be augmented in the future with satellite-based networks. Among the emerging satellite networks, Low Earth Orbit (LEO) networks are able to provide advantages over traditional Medium Earth Orbit (MEO) and Geo-Stationary Earth Orbit (GEO) networks in terms of signal latency, cost, and performance. Nevertheless, several challenges exist in LEO system design, which have not been fully addressed in the existing literature.
Such expansion is needed in order to increase the coverage areas and the end-user ubiquitous access to wireless services and to provide equal accessibility worldwide. LEO orbits have altitudes ranging between about 200km to 2000km above the Earth’s surface (below the Van-Allen radiation belts), which makes LEO satellites cheaper to build and launch in comparison with satellites launched to MEO and GEO orbits. The lower costs of building and launching LEO satellites (compared to MEO and GEO ones) have also enabled better commercial viability of autonomous-transportation services.
In LEO constellations, these satellites can carry omnidirectional or directional (beamforming) antennas. The latter case is the one most encountered in LEO mega-constellations nowadays, and it is the one illustrated in Figure 1 , where each satellite beam can serve a certain end user. The ground segment hosts the GS satellite network and is complemented by a number of GSs, placed all over the Earth, with the main tasks of monitoring, managing, and controlling the platforms and the signals sent by the satellites. The ground segment typically does not interact with the user segment, but only with the space segment.
The coverage equation is modeled by (1) C = cos θ + ε = cos ε 1 + h R E where C is the coverage parameter, ε is the elevation angle of the viewing cone of the satellite, h is the satellite altitude, R E is the Earth radius, and θ is the central angle of coverage [ 87 ]. An illustrative example of these parameters is provided in Figure 4 . 6.2. Constellation Topology The major constellation topologies are the following four topologies: Street of Coverage Constellations: Multiple satellites placed in circular or near circular orbital planes with the same altitude and inclination and phase separation creates a Street of Coverage constellation.
[ 97 ] and 3D lattice flower by Davis et. al [ 98 ] models and 2D necklace flower [ 99 ] and 3D necklace flower constellation [ 100 ]. Walker and flower constellation configurations provide global coverage [ 101 ]. Table 6 presents the mathematical modeling of the two most encountered constellation topologies, namely Walker and Flower constellations. In Table 6 , i is the inclination of orbit, N T is the total number of satellites, N P is the number of orbital-planes of the constellation, F is the relative phasing parameter between adjacent orbital planes, and h is the Satellite (Sat) altitude.
(12) C / N 0 d B − H z = 10 log 10 ( P c a r r i e r P n o i s e ) Minimizing the costs is a logical objective for any commercial service; for a straightforward example, using a minimal number
Even though both constellations use a similar carrier frequency, in Ku-band (e.g., 12 GHz), their orbital altitude and orbital configuration is different. While Oneweb satellites are to be at altitudes of about 1200 km, Starlink satellites are to be distributed in orbits closer to the Earth, between 300 km and 600 km, thus offering a better link budget than those placed at higher altitudes. This is the main reason for the difference between C / N 0 levels; nevertheless, the C / N 0 distributions, as seen from the histograms in Figure 11 , are very similar, and they resemble an exponential distribution.
Figure 8 Example of an Optimization Problem from the User-Segment Perspective: The satellite-selection problem, i.e., the total number of satellites in view is to be reduced to an optimal subset according to application-specific optimization metrics. Figure 9 Example of Coverage Maps for Two Selected LEO Constellations: Oneweb ( top ) and Starlink ( bottom ), as number of satellites in view per Earth point for two different elevation masks: 10 o in the left-side plots and 50 o in the right-side plots. Figure 10 DOP metrics comparison for Oneweb and Starlink LEO constellations with two different elevation masks: ( a ) 10 ∘ elevation mask and ( b ) 50 ∘ elevation mask.
Receiver, Handheld, GPS, "Trailblazer XL" | National Air and Space Museum Skip to main content Entry to Museum in DC changed for Aug. 21-23. Plan ahead . Search Search Receiver, Handheld, GPS, "Trailblazer XL" 5 Images Gallery slides Slide 1 of 5 Download Image Slide 2 of 5 Download Image Slide 3 of 5 Download Image Slide 4 of 5 Download Image Slide 5 of 5 Download Image Gallery thumbnails Slide 1 of 5 Slide 2 of 5 Slide 3 of 5 Slide 4 of 5 Slide 5 of 5 This is one of the first early handheld Global Positioning System (GPS) receivers intended for civilian use.
GPS is a navigational system that provides accurate and instantaneous position information to those equipped with receivers such as this one. The system relies on a set of 24 satellites placed in orbit approximately 18,000 km above the earth. Each satellite carries atomic clocks on board, and broadcasts a signal that is accurate to within 3 billionths of a second. GPS units are tuned to receive signals from these satellites, and if the receiver can lock on to at least four of them, it can determine its position and altitude on Earth. This unit was donated to NASM by its manufacturer, the Magellan Corporation. It shows positional data in traditional latitude and longitude coordinates.
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