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GPS APPLICATIONS AND REQUIREMENTS 51 alone, unless SA is eliminated and other improvements are made to increase the accuracy of the SPS. Geodynamics Despite improved post-processing software and the use of differential GPS, the effects of A-S and SA degrade the results by 50 percent or more, primarily through the loss of the signal-to-noise ratio using dual- frequency codeless receivers. The loss can be partially recovered by replacing existing receivers that are a few years old with newer equipment. Significant savings in time and costs would occur, however, if this was not necessary. Airborne Geophysics SA has little effect on airborne geophysical applications when differential GPS and post-processing are utilized. As with geodynamic applications, however, the presence of A-S greatly reduces the signal-to-noise ratio available to dual-frequency receivers. The dynamic, high-multipath environment that exists for GPS receivers on aircraft makes codeless receivers especially vulnerable to losing lock on the L2 signal and requires a lengthy reacquisition time. In lieu of code-tracking capability on L2 or an alternative L-band signal, improvements to the tracking loops in codeless receivers could improve this situation.39 Findings Using post-processed GPS orbits provided by the IGS network of differential reference stations, the effects of SA can be eliminated for most Earth science applications, and with the use of dual-frequency "codeless" receivers, centimeter-level positioning accuracies can be achieved. The availability of a second GPS frequency for civil use with unencrypted code would greatly enhance many Earth science applications that require high-precision accuracy. Dynamic, high-multipath applications, such as airborne geophysics, would benefit from faster acquisition and more robust tracking. Applications such as remote atmospheric sensing require submillimeter precision in the carrier-phase observables, which may be achievable using a second unencrypted signal. 39 The effects of SA and A-S on the use of GPS in airborne geophysics are discussed in more detail in the NRC report Airborne Geophysics and Precise Positioning: Scientific Issues and Future Directions, Appendix A: Effects of Selective Availability and Anti-spoofing.
