Global Positioning System (GPS)

In the last few years the Global Positioning System (GPS) has been used in a variety of applications for which new and more restrictive requirements in the design of the receiving antenna have been introduced. In particular, for high-precision GPS applications, such as differential GPS, GPS-based spacecraft attitude determination or geodetic surveying, a receiving antenna with superior rejection to multipath signals is required.

Multipath arises when the GPS transmitted signal takes different paths to the receiving antenna and, being the signals from these paths added with different phases, this results in a significant amplitude and phase distortion. As the GPS signal is right-hand circularly polarised (RHCP), odd reflections are left-hand circularly polarised (LHCP). Hence, use of antennas with a good rejection of LHCP signals can potentially eliminate multipath effects arising from direct reflections. Effects due to double reflections remain but they are normally much weaker. Additionally, considering that reflections often impinge on the antenna at low elevations, the multipath rejection performance can be improved by shaping the antenna gain pattern to reject low-elevation signals while guaranteeing an adequate hemispherical coverage.

Furthermore, in order to satisfy the demanded precision and reliability, a high-performance GPS antenna must be capable of operation at the two GPS frequencies (L1: 1.57542GHz, L2: 1.2276GHz). As a solution an antenna array has been proposed . Unfortunately, this solution, although it may easily provide the desired radiation characteristics, becomes impractical due to the operational requirements on size and weight.