The Global Positioning System: A Shared National Asset (1995)
Chapter: Signal Visibility
Challenges to Full GPS Utilization
Orbit Determination and Orbital Positioning
The GPS SPS currently delivers sufficient accuracy for real-time orbit determination requirements. The well-known orbital dynamics of most spacecraft allow filtering of the data, which helps to mitigate the effects of SA. A-S does not have a significant effect on real-time orbit determination because most satellites are above the densest part of the ionosphere and can probably ignore the ionospheric delay contribution.
For precise post-flight requirements SA does not pose a problem, but the presence of A-S reduces the orbit determination accuracy of spacecraft missions which rely on codeless L2 measurements. Topex/Poseidon, for example, relies on the use of dual-frequency data, but does not carry a receiver capable of tracking the Y-code. The processing of raw data from a large network of stations will still be required, however, even if the basic GPS accuracy is improved. Nevertheless, all improvements to the basic system will aid in the search for the last centimeter or millimeter of precision.
Attitude Determination
Because GPS attitude determination techniques use differential carrier-phase measurements, SA has little or no effect on the accuracy achievable. A-S may have some effect in that it prevents the use of differential P-Code measurements for coarse attitude determination and makes the use of dual frequency differential carrier-phase measurements more difficult. As mentioned previously, however, the accuracy of GPS for this application is limited primarily by design parameters related to receiver electronics and antenna structure.
Signal Visibility
Satellites in orbit near or above the GPS constellation are only able to track GPS signals that pass beyond the limb of the Earth. On the current Block II/IIA satellites there is sufficient antenna beamwidth to allow orbit determination to be performed at geosynchronous altitudes using GPS and a significant amount of dynamic modeling.61 The Block IIR and IIF satellites, however, may not have the same antenna beamwidth, and the L-band signals broadcast from these antennas may no longer pass beyond the limb of the Earth. This could eliminate the ability of a geosynchronous satellite to receive GPS signals, precluding a potentially important GPS application.
