Using a VNA to Design Antennas for GPS Interference Detection
GPS interference can come in many forms. The most well-known is intentional jamming or spoofing, denying GPS for a variety of (typically nefarious) reasons. It’s common knowledge that parts of the Middle East are denied GPS service, evidenced by cases of ships in the Black Sea suddenly reporting being located 25 nautical miles away, inland, at an airport.
There are also numerous recorded cases of unintentional interference from multiple sources. Real-life examples include a worn cable in a car shark-fin antenna causing the entire roof to radiate; a waterlogged antenna on a building re-radiating and jamming nearby GPS antennas; and even the output of a GNSS simulator accidentally left unterminated after an exhibition demonstration! Being able to quickly determine the presence of GPS interference, and its direction of arrival, is vital in both civilian and defense applications.
GPS signals are very weak—below the thermal noise floor. A GPS receiver needs to boost the signal-to-noise ratio (SNR) to acceptable levels using process gain—i.e., by knowing in advance the precise sequence it’s looking for, a receiver can distinguish the signal from the noise.
The process gain is sufficient to identify a signal in a typical environment’s noise level. However, if the noise level is higher than normal, then the process gain can be insufficient, at which point the receiver is jammed. Fortunately, the GPS L1 frequency (a 20-MHz band centered on 1575.42 MHz) is a protected frequency and radiation in this band is tightly controlled.