HE910 Hardware User Guide
1VV0300925 Rev. 32 Page 63 of 111 2019-11-25
can either attenuate the original signal, or if received in place of the original signal add
additional error in determining a solution because the distance to the particular GPS
satellite is actually longer than expected. It is this phenomena that makes GPS navigation
in urban canyons (narrow roads surround by high rise buildings) so challenging. In
general, the reflecting of the GPS signal causes the polarization to reverse. The
implications of this are covered in the next section.
7.3.2. GPS Antenna Polarization
The GPS signal as broadcast is a right hand circularly polarized signal. The best antenna
to receive the GPS signal is a right hand circularly (RHCP) polarized antenna.
Remember that IS-GPS-200E specifies the receive power level with a linearly polarized
antenna. A linearly polarized antenna will have 3 dB loss as compared to an RHCP
antenna assuming the same antenna gain (specified in dBi and dBic respectively).
An RHCP antenna is better at rejecting multipath than a linearly polarized antenna.
This is because the reflected signal changes polarization to LHCP, which would be
rejected by the RHCP antenna by typically 20 dB or so. If the multipath signal is
attenuating the line of sight signal, then the RHCP antenna would show a higher signal
level than a linearly polarized antenna because the interfering signal is rejected.
However, in the case where the multipath signal is replacing the line of sight signal, such
as in an urban canyon environment, then the number of satellites in view could drop below
that needed to determine a 3D solution. This is a case where a bad signal may be better
than no signal. The system designer needs to make tradeoffs in their application to
determine which is the better choice.
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