Spectracom - Essential Ingenuity

Antenna Polarization - Circular vs Linear for testing GPS


What type of transmitting antenna should I use with GSG for over-the-air testing?


GNSS systems use Right Hand Circular Polarization to combat multipath.  When a RHCP wave is bounced off a surface, the reflection is in the opposite direction, Left Hand CP.  A LHCP wave is strongly rejected by a RHCP antenna.  The second bounce (and the fourth, etc.) will be back to RHCP, but by then, the reflective loss from two or more bounces makes these multipath signals very weak.
A circular polarized signal can be represented by two time varying linear polarized waves – horizontal and vertical.
 If transmitting with a linear polarized antenna to a circular polarized receiving antenna,  only get half the energy (either horizontal or vertical, depending on which way the horn is oriented) is received. Every reflected linear wave is also linear so it couples into the antenna at the same level (minus the reflective loss) as the direct wave does. 

The standard antenna we offer is RHCP and just a wideband, general purpose antenna.  We offer it for customers needing to do OTA testing for receivers with integrated antenna and cannot separate out the receiver from the antenna.  Being a wideband antenna, the VSWR performance is compromised, but GNSS are very low power signals and therefore not a big concern.
All of our customers that are interested in testing antenna radiation patterns or finding the phase center of the antenna typically work with the antenna separately from the receiver. Using the spread spectrum GPS signal for testing antenna patterns is a bit unusual.  Most use a sweep frequency generator as a test signal. 
Note that for position accuracy testing from a simulator, the path between the GSG signal generator and the receiver’s antenna is not a factor.  All of the satellites are simulated together and their relative delays are simulated internally in the GSG.  Any additional delays from the OTA path will be added equally to all satellites.  Since GPS uses Time Difference of Arrival calculations, the position accuracy will always be the simulated position (or trajectory) specified in the scenario, accurate to 1 mm.  The simulator’s antenna pattern will only affect the amplitude of the received signal.  So if one wants to test parameters such as receiver sensitivity over the air, then a horn is a better choice.  But an even better way of measuring sensitivity, is directly connecting with a coax cable.
For all other measurements, the performance of the simulator’s antenna is not much of a factor.  It just needs to transfer the signal over the air to the receiver.
Be careful if attempting to use a high-end GPS receiver antenna as the GSG’s antenna.  Most of these have a Low Noise Amplifier (LNA) internally, which is in the wrong direction for using it as a transmit antenna.

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