GNSS signals reach the antenna at a level 100 times lower than background noise. Literally, only a few hundred photons per 1 Hz bandwidth. However, thanks to code division technology, this is sufficient for stable reception.
Things get worse when the interference level is millions of times higher than the GNSS signal. This can happen when interference comes from nearby or powerful transmitters. Since this interference is not at GNSS frequencies, it is called out-of-band interference. Such interference overloads the amplifier, causing it to operate with significant distortion. If the amplifier has AGC (automatic gain control), it reduces the gain, and the weak GNSS signal simply does not reach the receiver's analyzers.
Out-of-band interference is combated using surface acoustic wave (SAW) filters that allow the necessary portion of the radio signal spectrum to pass through while attenuating the unwanted portion by a thousand times. Since the filter also attenuates the useful signal several times, a low-noise amplifier (LNA) is often used in conjunction with it. If the interference level is millions of times higher, one filter is not enough, and two filters should be used – both in the active antenna and in the receiver.
"We wanted to insert a WiFi dongle (USB dongle) into the receiver board. The antenna was on the roof, and we were in the car... However, until we moved the WiFi 30 centimeters away, the reception was almost nonexistent. When we moved it 60 centimeters away, the interference stopped. The interference affected not the antenna (which was separated by a metal roof), not the cable (which was coaxial), not the receiver itself (which was in a metal case), but a small section on the board – from the receiver to the antenna connector." - This is a clear example of why an antenna filter is not enough and why having a filter in the receiver is a good thing.
So if you have both a GNSS receiver and a transmitter nearby, first of all, try to keep them as far apart as possible and separate them with grounded metal plates. For example, place the GNSS antenna on top of a drone, with a ground plane beneath it, and then the transmitting antenna for communication with the ground below. Also, use boards where the receiver and surrounding circuitry are enclosed in a metal case, and SAW filters are present in both the antenna and the receiver.
Sources of interference can include not only WiFi but also mobile communications, drone radio control systems, and so on. Moreover, even distant powerful sources can have an impact, such as television towers or large space communication antennas.
© Eltehs SIA 2023
