Navigation Performance of Global Navigation Satellite Systems in the Space Service Volume

D.A. Force

Abstract:	Besides providing position, navigation, and timing (PNT) for terrestrial users, the Global Positioning System (GPS) also provide PNT information for earth orbiting satellites. The first experimental use of GPS for refining the position, velocity, and timing of spacecraft was in the early eighties. Now GPS provides PNT services for spacecraft, as well as for land, maritime, and aerial applications. Spacecraft whether above and below the GPS constellation, now receive and process GPS signals either from above or using signals that spill over the limb of the earth. GPS has been demonstrated to provide decimeter level position accuracy in real-time for satellites in low Earth orbit (centimeter level in non-real-time applications). GPS is useful for satellites in low Earth orbit, satellites in geosynchronous orbit, and for satellites in highly elliptical orbits. The additional Global Navigation Satellite Systems (GNSS) becoming available can also provide these services, either individually or in combination. Depending on how many satellite signals are received, different levels of service are possible. Kinematic position requires four satellites, but because orbital motion is predictable, it is possible to build up knowledge of the orbital position gradually through time without a need for constant four-satellite reception. However, it is desirable to have four satellite reception when performing satellite maneuvers, since there can be significant changes in velocity, leading to large changes in orbit parameter, causing substantial divergence in position over time. The Space Service Volume is the volume between three thousand km altitude and geosynchronous altitude. Space below the altitude of three thousand km, the Earth’s surface and atmosphere are the Terrestrial Service Volume. Within the Terrestrial Service Volume GNSS services have very similar performance to that on the Earth’s surface, and satellites use a zenith-facing antenna and the information on the antenna patterns for earthbound uses. Above three thousand km the use of signals passing by the Earth’s limb becomes important, so it is desirable to have additional information on signal strength, phase delay, and group delay covering wider beam angles than are needed for terrestrial service. NASA has worked with other United States government agencies to ensure that GPS continues to support spacecraft navigation. Support of GPS space applications is now part of the system plan for GPS. NASA has proposed support of the Space Service Volume by GNSS providers to the UN International Committee on GNSS (ICG). The use of the Space Service Volume requires commitment to adequate beamwidth and providing information on signal strength, phase delay, and group delay within that beamwidth. In 2006, F. H. Bauer, et. al., defined the Space Service Volume in the paper “GPS in the Space Service Volume”, presented at ION’s 19th international Technical Meeting of the Satellite Division, and looked at GPS coverage for orbiting satellites. With GLONASS already operational, and the first satellites of the Galileo and Beidou/COMPASS constellations already in orbit, the coverage calculations were extended to include combined Global Navigations Satellite Services (GNSS) in “Combined Global Navigation Satellite Systems in the Space Service Volume” by Dale A. Force and James J. Miller; presented this paper at the ION International Technical Meeting this year. This paper extends the coverage calculation by calculating the navigation performance of the combined GNSS constellations in the Space Service Volume. The navigational performance calculations are performed in Satellite ToolKit (STK). Besides the medium earth orbit (MEO) satellites of the GNSS constellations, the Beidou and supporting satellites such as WAAS, EGNOS, and others at geosynchronous altitude are included in the analysis. To eliminate tropospheric effects and shadowing, a mask eliminates all signals passing with fifty kilometers of the geoid. The first step in the calculations is to determine which GNSS signals are received at each point, considering the transmitters location, pointing, beamwidth, and Earth masking. The signal strength is then evaluated for each signal. This provides the carrier strength for each of the signals. The second step is to calculate the carrier-to-(noise plus interference) ratio for each signal. This requires calculating receiver noise as well as the noise due to the Earth and Sun. Since the GNSS satellites use the same frequency bands, the signals from other GNSS satellites cause the interference with each satellites signal. The fifty-kilometer mask is not used in calculating interference. If the carrier-to-(noise plus interference) ratio is too low, the signal is eliminated from the analysis. Coding gain is included in determining the required carrier-to-(noise plus interference) ratio. The third and final step in the calculation determines navigation performance by including geometric dilution of precision effects due to the satellite configuration found in the first two steps of the analysis. The best configuration would have satellites widely distributed over the sky; however, in many cases some of the satellites will be near each other in the sky, causing the measurements to be redundant. The main results presented in this paper are the average navigation performance for various latitudes and altitudes. Variations with longitude average out over time. Of course, for a spacecraft in an inclined orbit some of the variation in latitude will also average out. As an additional result, this paper also reports the average and the worst-case carrier-to-(noise plus interference) ratios for the various altitudes and compare them to the carrier-to-noise ratios to evaluate the importance of interference.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3325 - 3328
Cite this article:	Force, D.A., "Navigation Performance of Global Navigation Satellite Systems in the Space Service Volume," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3325-3328.
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