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GPS APPLICATIONS AND REQUIREMENTS 30 Challenges to Full Utilization of GPS Selective Availability and Anti-Spoofing When SA dithering of the GPS signals is employed, the DGPS corrections required to circumvent the resulting accuracy degradation must keep up with the dithering rate. This is not a problem for local-area DGPS, since the local correction broadcast usually has a sufficient data rate to provide timely corrections. The space- based WAAS, however, broadcasts its differential corrections as part of the navigation message data carried by a GPS-like L1 signal. SA has a negative effect on this signal format; the high correction data rate necessary to keep up with the SA dither rate constrains the flexibility of providing additional information on this navigation message. SA also decreases navigation availability and integrity monitoring availability for SPS users because the ranging errors it introduces require better satellite geometry for the specified 100-meter level of navigation accuracy. This sometimes rules out the operational use of GPS, especially when there are failed satellites present, and significantly reduces the effectiveness of RAIM.22 The employment of A-S, which overlays the Y-code on L2 rather than the P-code, denies the second frequency needed for real-time ionospheric correction to all but authorized PPS users. Without dual-frequency receivers on board aircraft, the WAAS needs to employ a large network of ground sites to collect ionospheric data, that will be interpolated by the user to estimate the ionospheric delay in the pseudorange measurements. The disadvantages of this constraint are a decrease in the vertical positioning accuracy of wide-area DGPS, and an increase in the size, complexity, and cost of the WAAS ground network. Resistance to Radio Frequency Interference A-S also limits an SPS receiver's ability to deal with RF interference from known sources such as the third harmonic of some UHF (ultra-high frequency) television channels and airborne VHF (very-high frequency) transmitters. Solutions to the potential problem of RF interference must be found if GPS is to become the primary navigation and surveillance system for aviation, and organizations such as the RTCA are actively studying the issue. Resistance to interference can be greatly improved through the use of dual-frequency receivers that can track the code on both L1 and L2 because it is unlikely that interference from a single source will simultaneously affect both frequencies. As discussed in Appendix G, access to the wider bandwidth of the P- code, which is approximately 20 MHz (versus 2 MHz for the C/A-code), also would increase resistance to interference and reduce vulnerability to multipath. 22 An analysis of the effects of SA on RAIM was conducted for this study by the MITRE Corporation. The results are presented in the next chapter, and the full analysis can be found in Appendix F.
