GPS World, February 2018

ANTENNAS a b c FIGURE 1 3D RHCP patterns at f 1575 MHz for a single feed antenna b dual feed antenna c four feed antenna FEBRUARY 2018 WWW GPSWORLD COM GPS WORLD 27 1 with φ f Az El being respectively the antenna phase frequency azimuth and elevation The DGD variation with respect to frequency and aspect angle that is elevation and azimuth actually poses a problem in precision applications as a matter of fact if the group delay were constant for all frequencies and all angles of arrival of the signal no additional error would be introduced in the position calculation because the group delay term common to all satellites would be encapsulated at the receiver into a user clock offset However group delay can change significantly with respect to aspect angle and frequency contributing in a different manner for each satellite due to different angles and for different signals due to the different spectral components of each signal therefore finally producing errors in the pseudorange estimation The influence of the DGD on pseudorange measurement error has already been studied in the past and is also taken into consideration in the antenna Minimum Operational Performance Standards MOPS for avionic antennas Empirical studies on the combined effect of antenna group delay and multipath effect on board commercial airplanes have been published recently However to our knowledge the correlation between the antenna intrinsic characteristics such as gain and phase patterns and smoothness and group delay behavior has not yet been properly analyzed leaving a gap in the full understanding of the antenna design impact on the final GNSS receiver performance GNSS antennas can be divided into families according to their geometry and the related radiation mechanisms for instance spiral helix and microstrip patch antennas are quite common in GNSS applications They differ in achievable bandwidth size and ease of manufacturing Even antennas of the same family can provide different performance mainly because of the number of feeding points which are the points where the signal is fed into the antenna In order to analyze the relationship between the group delay performance and the antenna properties we will take into consideration three GNSS antennas of the same family microstrip patch having all about halfeffective wavelength size with the effective wavelength considering the dielectric properties of the substrate material on which the patch antenna is positioned but with a different number of feeding points The antennas will be denominated respectively single feed double feed and four feed antennas The single feed antenna is a square patch with truncated corners to achieve circular polarization On the other hand the double and four feed antennas are square patches having feeds positioned along their x and y axis The feeds are fed progressively that is with same amplitude and 0 90 phases for the double feed and 0 90 180 270 phases for the four feed Single feed antennas are representative of lower cost antennas used in mass market applications due to their extreme simplicity allowing for low cost production However their performance exhibits strong cross polarization levels and non uniform patterns over the azimuth Dual and four feed antennas are more complicated to manufacture and need further hybrid circuits to properly distribute the signal between the different feeding points However an increase in the feeding points leads to more uniformity in the radiation