Last time in this series we posted some recent examples of GPS jamming.  I wanted to really address the issue but my experience is limited to the practical aspects … I’m not an engineer, let alone a true scientist.  Also, some of my readers don’t care too much for the in-depth articles.

Better, then, that I crib off a man who has taught me tons about GPS, Richard langley … a real scientist, and one with a gift for explaining even deep technical concepts in the way that everyday users can gain valuable insights.  If you aren’t already a reader of GPS World, and Richard’s excellent "Innovations" columns I highly recommend them:

INNOVATION INSIGHTS with Richard Langley

Richard Langley

AS WE ALL KNOW, GPS SIGNALS ARE WEAK. At a receiver’s antenna, in the open air, their strength is about –160 dBW or 1 × 10^-16 watts. Compare this to a cell-phone signal, which might be –60 dBW or 1 × 10^-6 watts — 10 billion times stronger! While code correlation in the receiver lifts the GPS signals above the background noise floor, the signals are still relatively fragile, and building walls and other obstructions can significantly attenuate the received signal power so that they cannot be tracked by a conventional receiver.

It is the ratio of the signal power to the noise power per unit bandwidth that determines the trackability of the signal. Accordingly, if the receiver’s noise floor should increase sufficiently, even in an outdoor environment, the signals may also become untrackable. This can happen when the receiver is subjected to intentional or unintentional radio-frequency interference (RFI) by a transmitter operating on or near GPS frequencies. If the interference is strong enough, it can jam the receiver. Although intentional jamming is typically of concern only to military GPS users, unintentional jamming can occur anywhere and anytime and can affect large numbers of users within the range of the jamming transmitter. The jamming incident in San Diego harbor in January 2007, for example, affected all GPS users within a range of about 15 kilometers including a medical services paging network.

Such jamming renders a GPS receiver inoperable. But how do users know that their receivers are being jammed and not suffering some other type of malfunction? Clearly it would be advantageous for users to receive a heads-up when jamming signals are present and, if possible, for the receiver to take corrective action automatically.

In this month’s column, we look at some simple techniques, which can be easily incorporated into the design of a GNSS receiver, to detect, characterize, and actually mitigate RFI. Such receiver enhancements will benefit civilian and military users alike…. read the rest of Richard’s very useful column here.