GPS World, February 2018

ANTENNAS behavior similar to that of a single feed antenna An in house developed passive antenna with two feeds An in house developed passive four feed antenna All antennas were properly tuned to obtain optimal gain and minimum reflection losses input reflection coefficient 10 dB at L1 E1 central frequency The measured RHCP pattern for the various antennas is shown in FIGURE 5 The UIGD for these antennas is 09 07 and 02 ns respectively while the UIG is 466 385 and 90 Differential group delay was calculated from the measured phase values and is shown in FIGURE 6 see next page The results are similar to those obtained from simulation and clearly show the improved flatness of the DGD for the four feed case Moreover if the measured phase data are fed into an ideal GNSS receiver able to provide the tracking biases occurring in the pseudorange code measurement for all elevations and azimuths antenna effects only as weighted by the signal characteristics will be visible as in this case neither multipath nor receiver or satellite imperfections are included in the ideal receiver The results are shown in FIGURE 7 A substantial decrease in the antenna induced error is evident as expected when the four feed antenna is used The differences in performance among different antenna technologies shown here provide valuable insight in the choice of the antenna technology for a specific application thanks to the better understanding of the impact of the antenna characteristics on the error at pseudorange level Moreover they can support the evaluation and definition of antenna requirements and connect them to the expected GNSS pseudorange error such as during the process of MOPS definition as currently occurring for DFMC systems CONCLUSIONS After investigating the effects of pattern uniformity on antenna induced errors group delay behavior over aspect angle and frequency has been shown comprehensively for different antenna feeding technologies for the first time Minimal error in pseudorange measurements is obtained when the antenna has a smooth pattern with no abrupt variations or nulls sidelobes both in aspect angle and frequency Different antenna feeding technologies currently in use for circularly polarized radiation have been evaluated and the best performing one has been FEBRUARY 2018 WWW GPSWORLD COM GPS WORLD 31