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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 115 capture, and cycle slip. This signal processing approach also can be combined with the inertial aiding techniques above, whereby correlator data, as well as accelerometer and gyroscope data are combined in an optimal fashion. The development and operational use of GPS receivers with improved integration of signal processing and navigation functions for enhanced performance in jamming and spoofing should be accelerated. Improved L2 Ionospheric Correction In a hostile environment where the L1 signal is jammed, PPS receivers will have access only to L2 signals, thus eliminating their ability to perform dual-frequency ionospheric corrections. In such situations, military users must rely on an ionospheric model such as that contained in the broadcast navigation message or on a single- frequency correction process such as DRVID (Difference Range Versus Integrated Doppler). 57 As the GPS signal travels through the ionosphere, the modulating codes (Y and C/A) are delayed by an amount proportional to the inverse square of the frequency. To first order, the carrier phase is advanced by the same amount, producing an effect termed "code-carrier divergence." DRVID is a technique that exploits this effect to compute the change in ionospheric delay over time. In order to determine the total ionospheric delay, an initial delay value must be known. This would work well in a scenario in which a receiver is initialized in a clear environment, that is, outside the region of L1 jamming, and then tracks signals into the jammed region using DRVID to make ionospheric corrections. The primary disadvantage of DRVID is that it relies on continuous carrier tracking, which is not likely to be possible in a high-jamming and possibly high-blockage environment. Currently, single-frequency receivers employ the 8-parameter Klobuchar model that is contained in the broadcast navigation message. This model is considered to be effective in eliminating approximately 50 percent of the total ionospheric delay with a day-to-day variability of 20 percent to 30 percent. It is suggested that enhancements to this model could improve the performance to the 70 percent to 80 percent level.58 Furthermore, based on the current performance of local-area DGPS, the NRC committee believes that local-area estimates of ionospheric conditions made just outside the jamming region could provide even greater improvements. 57 P.F. MacDoran, "A First-Principles Derivation of the Difference Range Versus Integrated Doppler (DRVID) Charged- Particle Calibration Method," JPL Space Programs Summary 37-62 II, 31 March 1970. 58 JA. Klobuchar, "Potential Improvements to the GPS Ionospheric Algorithm." Presentation at the GPS/PAWG Meeting, 14 July 1993, Peterson Air Force Base, Colorado.
