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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 113 Antenna Subsystem Improvements The jamming analysis shown in Appendix L assessed three candidate techniques for improved jamming resistance: aiding of tracking loops with inertial sensors, increased processing gain via wider signal bandwidth, and nulling/directive antenna systems. Nulling antennas were found to provide the single biggest improvement in jamming performance on the order of 25-35 dB. Both military and civilian users have deployed multi-element antenna structures for several years. In the late 1970s, work on the multi-element military AE-1 began. This antenna system was designed to effectively null a single jammer. In both this unit and its derivative design, the AE-1A, element phase shifting and combining is carried out in the radio frequency analog domain. These units have been deployed on a number of aircraft, but have not yet been widely utilized on other military weapons systems, primarily due to cost and size considerations. Recently, more effective techniques have been developed wherein element phase shifting and amplitude weighting is done after spread-spectrum signal correlation, eliminating the RF phase shift components in favor of lower-cost correlator ASIC's and signal processing at baseband. With processing gain applied before nulling and beam forming algorithms, much improved jamming-to-signal margins are available.55 These and future developments aimed at reducing the size and cost of antenna structures should be actively pursued. Nulling antennas and antenna electronics should be employed whenever feasible and cost effective. Research and development focused on reducing the size and cost of this hardware should actively be supported. Inertial Aiding Improvements Aiding can be defined as the usage of any non-GPS-derived user dynamics and clock information in GPS receiver signal-tracking and navigation functions. The availability of such data can have a profound impact on GPS signal acquisition time, code and carrier tracking thresholds, interference and jamming resistance, anti- spoofing capability, and receiver integrity. Aiding works by providing auxiliary observations, which sense a vehicle's motion parameters. Inertial aiding is especially effective because of its invulnerability to electromagnetic interference and because its error characteristics are complementary to those of radionavigation systems, that is, inertial noise errors are low frequency and GPS signal tracking errors are high frequency. Since the earliest days of GPS, the military has exploited synergism, at first with loosely-integrated inertial navigation systems (INS)/GPS built around existing aircraft INS mechanization, and more recently with "embedded" INS in which the inertial sensor and 55 These concepts have been privately developed and patented by the Magnavox Electronic Systems Company (MESC), Patent 4734701. MESC has been continuing to enhance these concepts since inception.
