Multipath-rejecting GPS antennas

Multipath interference limits the speed and accuracy of determining position by differential Global Positioning System (DGPS) techniques. A geodetic surveyor, for example, requires multipath interference rejection of about 36 + 20 log(10) sin epsilon dB, where epsilon is the elevation angle of the satellite being observed. Signal processing in a GPS receiver cannot satisfy this requirement. A receiving antenna is required that can sufficiently reject signals arriving from below the horizon. Available antennas have inadequate rejection, and brute-force methods of improving them, e.g., by enlarging their ground-planes, are impractical. A compact, ground-planeless, dual-band GPS antenna with improved multipath rejection has been designed and field-tested. This antenna resembles a vertical post rather than a horizontal platter; within its 0.1-m diameter 0.4-m tall radome is a vertical array of turnstile elements. In field tests, a three-element army antenna rejected multipath better than a 0.5-m diameter ground-plane antenna by an average of 5 dB. A five-element array antenna appears superior to a 0.9-m diameter ground-plane antenna.