Running Interference in GPS Receivers

It goes without saying that any RF device as sensitive as a GPS receiver will be inherently vulnerable to interference. Clearly, care needs to be taken at both the design and integration stages to minimise interference effects. But what interference sources need to be considered? And how do you know if your receiver can deal with them?

Most potential sources of interference are obvious and predictable: The effects of fixed-frequency transmitters for TV, radio and the like can easily be modelled and accounted for. Indeed, one advantage of working with multi-GNSS receivers is that some are multiple-frequency devices, and therefore inherently more resistant to interference on any specific frequency.

However, there are two sources of interference that you ignore at your peril: the first of these is internally generated interference, and this is particularly relevant in devices such as location-enabled mobile handsets. The typical handset has a transmitter capable of transmitting more than 1W in very close proximity to the GNSS receiver (and two may even share some signal-path components). Careful design partitioning is essential to prevent interference.

The second is more subtle, but will increase over time, and concerns the increasing number of constellations and signals coming on stream. So say, for example, you have designed your multi-GNSS receiver to work with both GPS and GLONASS, and then the Galileo system comes on-line, then the receiver will view the uncorrelated signals from Galileo satellites as interference. At a simpler level, a GPS-only receiver will view both GLONASS and Galileo as interference. In a similar way, all civilian receivers need to work in the presence of classified signals such as GPS Y and M codes and Galileo PRS.

Fortunately, even if you are still working with single-GNSS receivers then you can test your designs for interference from other systems using a Spirent Constellation simulator equipped with the interference option. And this can even be used to simulate unwanted GNSS signals.

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