Satellites over Texas by Jim Naismith

Courtesy of Javad Positioning Systems www.javad.com

 

Several months ago I was in north of Terlingua, Texas, around midday (115 Fahrenheit) surveying with GPS equipment. The gear was tracking several satellites. At the next check, the equipment was not tracking any satellites. It wasn’t long before I noticed a B-2 bomber was making a low pass over the area. After a brief time, all the satellites once again appeared on the screen. But, I did not make any connection between the two. 

Last week I was surveying again. This time in South Texas, using JPS equipment. The equipment was tracking 3 satellites, then 4, then 3 again. A quick call to JPS confirmed that they were tracking eight satellites in California at the same time I was tracking only three in Texas. I checked with other surveyors in the area. They had seen the same problem with other brands of GPS receivers. 

A thought came to my mind. I turned on the in-band interference suppression option on one of the receivers. Immediately, it started tracking 8 satellites. My other receiver and those of all other surveyors that I checked were tracking only three. This scenario lasted for about one hour. 

A day later the same thing happened. This time I happened to be looking at the sky and I saw a Navy airplane. Suddenly, the image of my previous encounter, three months ago, rushed to my mind. I can't blame it on Navy airplanes. So, I thought, there must be ghosts in Texas. But thanks to my in-band interference option, I was able to get my job done. My friends call it the Ghost Bustin' option. 

Jim Naismith

In-band interference is when the interfering signal falls within the GPS band. It is the hardest type of interference to defend against. In some publications terms such as "interference rejection" or "interference suppression" are used, which omit the "in-band" qualifier. They usually refer to "out of band" interference rejections, a common capability that most GPS receivers have.

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