Research on GPS antennas at Mitre

This paper presents the results of various investigations related to GPS adaptive nulling antennas that were conducted in 1997 under a research program sponsored by The Mitre Corporation. Under this program, MITRE has demonstrated the feasibility of using fundamental and higher order modes within a single microstrip patch radiator to create a relatively simple, low cost adaptive antenna capable of forming a null in the vicinity of the horizon where the jamming threat is the greatest. Resistivity tapered ground planes have been built and tested to reduce the high back lobes of GPS microstrip antennas caused by fields diffracted from the outer edges of the ground plane on which the antenna is mounted; a 10 to 20 dB reduction of these back lobes has been measured, thereby providing the antenna with greater immunity against jammers and external interference. Investigations have been performed to determine the performance limitations imposed by an airborne platform on jamming suppression of GPS adaptive antennas through scale model measurements and analysis. Antenna pattern measurements have been made on a scaled, right hand circularly polarized GPS antenna placed on a 1/8 scale model of a typical tactical aircraft. An extensive electromagnetic analysis has also been conducted to investigate the effects of platform interaction of a GPS microstrip antenna installed on a cruise missile. The accuracy of the simulation has been validated by comparison with measurements. These investigations will help to quantify the effect on the nulling performance of these airborne arrays by electromagnetic effects such as finite size and curvature of the platform, multipath, creeping waves and mutual coupling between the main and the auxiliary elements in the adaptive array.