Detection and the Cause

Interference Detection

By the time GNSS signals reach the Earth’s surface, they are weak and below the thermal noise floor. As such, they are indistinguishable from noise by using ordinary signal detection techniques; and to see those signals a finely tuned antenna and radio frequency (RF) front end are needed—which is what distinguishes a GNSS receiver from a signal analyzer.

So it is possible to use the GNSS receiver as a sensor to look at all electromagnetic energy in the frequencies of interest. However, this use of the receiver as a special sensor needs much more than the standard receiver output. A designer and
manufacturer of GNSS receivers, such as NovAtel, is able to examine and use the raw signals before they are constructed into navigation messages; the stage of interest is the raw in-phase and quadrature (I&Q) data.

This technique allows one to establish the presence of interference and therefore to decide what to do about it. It works by taking samples in the GNSS frequency bands and uses fast Fourier transforms (FFT) to compute Power Spectral Density (PSD) for interference detection and estimation; together with power detection this provides improved out-of-band interference performance as well as in-band interference mitigation.

Characterizing the Cause

By comparing the received data with the expected shape of the known GNSS signals, the nature of the interference may be deduced and from that appropriate mitigation techniques can be selected. For broadband jamming such as that shown in Figure 1, mitigation by the use of an anti-jam antenna and possibly an additional sensor may be best.

The interference may not, however, be a blunt attack such as broadband jamming. For example unintentional interference from another part of a vehicle system could be causing an issue within the GNSS frequencies.

Manufacturers such as NovAtel have developed techniques to recognize that interference and therefore deal with it.

For example filters may be manually or automatically applied. Generally two different types of digital filters are employed: bandpass filters (BPF) and notch filters (NF). Bandpass filters allow signal to pass within the frequency window the user specifies, and rejects/suppresses all other frequencies. Notch filters reject/suppress frequencies within a specified frequency window, and pass the rest. Users can chain multiple filters together on a single signal path, but note that over use
of filters will eventually degrade overall performance of the receiver.