In Band Interference Rejection

This letter appeared in the June 2008 JAVAD GNSS ad in The American Surveyor:

In Band Interference Rejection

Consider your favorite FM radio station. It is assigned a frequency band and it cannot broadcast outside that band (to disturb others) and no one else can broadcast within its band (to disturb it). Assigning and protecting frequency bands is a highly contested and challenging task. Radio amateurs are often caught "trespassing" with their home made transmitters onto the properties of others. By that I mean transmitting within bands which belong to others.

At the global level, countries and organizations compete to obtain global frequency allocations for their national projects. GPS, GLONASS, and Galileo had to engage in international campaigns to receive their frequency allocations. A few years back the GPS frequency band was in danger of being "walked on" by international wide-band civil communications. This could have damaged GPS receiver performance. The efforts led by the US GPS Industry Council and Charlie Trimble saved the GPS band from that threat.

The GNSS frequency bands are in constant danger of being attacked by the nearby radio and TV stations, radars and other signal transmitters. Yes, no one is allowed to transmit outside of its allocated frequency band, but in reality all transmitters transmit not only their allocated frequency but they also transmit what are called "harmonics" of their frequency. Harmonics are the integer multiples of an allocated frequency. Such harmonics are much weaker than the main signal but they can interfere with the GNSS signal when a GNSS receiver gets close enough to a transmitter and when one of its harmonics falls within a GNSS band. Remember that the GNSS signal strength is comparable to a 100 watt light bulb 20,000 kilometers away.

When you tune your FM radio to a particular station you actually move a filter, which is like an electronic window. The filter allows the desired frequency band to enter the radio receiver and blocks all others. In GNSS receiver design, we filter out all signals outside of GNSS and make an open window for the GNSS signal band. This task is easy. The problem is when an undesired signal falls within the desired band. An example of an undesried signal would be a harmonic of a nearby radio station that falls within the GPS band. Defending against this phenomenon, which we call "In Band Interference," is not easy.

Marc Cheves, the editor of The American Surveyor, told me about an event that happened while he was driving near Memphis: both of his GPS receivers (from different manufacturers) stopped working for about an hour. Texas surveyor Jim Naismith reported to me that for two hours every day near Dallas-Fort Worth all of his GPS receivers stopped working. Now, it could be that an illegal or inadvertent signal was responsible, but I believe it’s more likely that the GPS signal was being walked on by harmonics.

I’m pleased to announce that we have paid particular attention to this problem and have come up with what we consider to be a unique method to deal with harmonic interference: we call it In Band Interference Rejection (IBIR), and we believe no other civilian GNSS is equipped to protect against In Band Interference. We scan the GNSS bands, detect the interfering signal, determine its characteristics, and then generate an equal signal to cancel it.

Our IBIR can defend against all signals typical of civilian environments, including narrow-band continuous wave CW signals such as the harmonics that come from radio and TV stations and civilian communications. We cannot defend against sophisticated warfare jamming which occurs near battlefields. I say this to make it clear that we don’t have "military anti-jamming capabilities," and while we are helping civilian users worldwide we will not cross certain boundaries.

Now you know a possible reason for why your GPS receiver sometimes stops working without any obvious reason. Our IBIR will easily pay for itself within a month by increasing your productivity.

Until the next issue