Download PDF Button

 

Often, GNSS users ask the question “Can your GNSS receiver operate effectively in GNSS-denied conditions?”

Apart from the obvious response, “No”, the deeper answers to this question are highly dependent on the scenario and what’s causing the interference. The many categories of interference range from signal blocking by structures (e.g., tall buildings or a tunnel) to unintentional interference from a nearby leaky antenna, to an intentional jamming attack.

In each case, mitigation can be very different and highly dependent on your objectives—there is no “one size fits all” measure to alleviate interference.

For continued performance during periods of signal outage, one can use a heterogeneous sensor. A common approach is to add an inertial measurement unit (IMU), which can also help in transient jamming scenarios. For mission-critical applications or longer periods of jamming, one might also consider employing an anti-jam antenna. If the anticipated problem is with spoofing, then a multi-frequency/constellation GNSS receiver and IMU integration will certainly be useful and an authorized user can of course use receivers that can operate with encrypted signals (currently GPS P (Y) Code, and GPS M-Code and Galileo PRS to follow).

Bottom line, it takes three steps to effectively mitigate GNSS interference: detection (establish that interference is really present), So In characterization (define the interference type) and localization (find it).

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.

Find the Problem

Once the interference is detected and characterized, the operator can pinpoint or geolocate the problem if multiple sensors are used as a basis for triangulation by using time difference of arrival (TDOA) techniques. As a GNSS receiver, each sensor can self-survey its own position. In order to be able to continue sampling RF data during interference which would otherwise swamp it, it is necessary to have an external timing source. For outages that last for some hours, this can be
done with a chip scale atomic clock (CSAC). NovAtel has demonstrated that mobile interference sources (both trial units and illegal jammers observed “in the wild”) can be successfully located using this technique that uses commercial-off-the-shelf (COTS) components. NovAtel focuses on the sensors, i.e. the GNSS receivers receive output logs to users to be used in their visualizations or other integrations.

Embedded with Possibilities

Defining the presence and characteristics of interference allows users to make informed choices about GNSS interference.

Then, instead of the question “Can your GNSS receiver operate effectively in GNSS-denied conditions?,” informed users might ask “Can your GNSS receiver mitigate my interference condition?”

It may be that the interference is less dramatic than an intentional jamming attack, in which case the potential solution could be as simple as filtering within the receiver. If it’s a mission-critical system that depends on precise position, navigation and timing (PNT) and you are worried about a deliberate jamming attack, then the answer might be to use all available mitigation techniques. However, for the standard tasks (such as mapping), or when the interference is known and predictable (maybe another systems is interfering with GNSS), the answer can be much simpler, less costly and less complex.