GNSS is changing. An increasing number of receivers are capable of accessing multiple constellations (GPS / GLONASS / BeiDou /Galileo). Yesterday’s single feed antennas were perfectly fine for single constellation / single frequency access, but in today’s world, Tallysman’s Accutenna® technology is essential to provide the precision you demand.
What is a Dual-feed Antenna?
Dual feed antennas are two orthogonally oriented dipoles. When the received signals of each dipole are summed, after a 90 degree phase shift of one of the signals, it perfectly replicates the circular response over the full bandwidth of the antenna. This greatly improves rejection of cross polarized (multi-path) signals and thus provides much higher precision than single feed antennas.
A single feed patch antenna is circular only at its single frequency of resonance. As carriers depart from this single frequency of resonance, they will appear increasingly more elliptical to this antenna. So when a single feed antenna is used to access two constellations, such as GPS (at 1575.42 MHz) and GLONASS (at 1602 MHz), it is tuned to the mid-point of the two frequencies; typically 1590 MHz. The result is the GPS and the GLONASS signals appear elliptical to this antenna. So when the single feed antenna receives the GPS and the GLONASSsignals, it also receives cross-polarized (multi-path) signals which are also elliptical. The end result is statistically very poor precision.
Figure 2 illustrates this point well. At the tuned frequency of 1590 MHz, the single feed antenna has a cross polarized signal rejection of about 25 dB but only about 5 dB of rejection at the frequencies of interest; 1575.42 MHz and 1602 MHz.
GNSS receivers are at the mercy of the quality of the signal presented by the antenna. No receiver can fully mitigate the effects of a poor antenna.