Friday, August 05, 2011

How to Jam GPS

This is about as fun as breaking RFIDs.  There are  about 500 million GPS-dependent devices is the United States.Being tracked by GPS is creepy, but being rescued because of GPS is not creepy, and Apple using it to gauge traffic congestion is pretty useful. You can decided where your own personal comfort level is. 

So here's the good news, GPS satellites only broadcasting at a power of about 10 watts.  Being 12,000 miles they're quite easily overpowered, even by devices broadcasting just a couple watts.There have been several large-scale incidents of accidental GPS jamming already. In 2007, the Navy was conducting an exercise in communications failure. They jammed their own radio communications in the exercise and "accidentally" jammed the radio signals from GPS satellites over a large portion of San Diego. I'll quote New Scientist about the scope of the error:
"In the tower at the airport, air-traffic controllers peered at their monitors only to find that their system for tracking incoming planes was malfunctioning. At the Naval Medical Center, emergency pagers used for summoning doctors stopped working. Chaos threatened in the busy harbour, too, after the traffic-management system used for guiding boats failed. On the streets, people reaching for their cellphones found they had no signal and bank customers trying to withdraw cash from local ATMs were refused. Problems persisted for another 2 hours."
In all likelihood they did intend to jam GPS, they just didn't intend to do so over such a large area. they probably miscalculated how much power it would require... or how many civilian devices would be effected. It's so simple it can happen by accident. LightSquared, a wireless company operating in the US has had several problems with GPS interference that have been hampering their roll-out. [More here] All you have to do is broadcast noise on the same frequency as the GPS satellites. One jammer with a simple GPS jamming device could take out GPS over a few square miles if his signal were unobstructed.  FYI: This is a type of failure the LORAN system wasn't sensitive to.

There are dozens of different commerciall y available devices the smallest and cheapest of which  (pictured above) is made by the Hong Kong-based BrandoWorkshop. It's a basic GPS jammer that plugs into a car's cigarette lighter.  There were over 40 of them on ebay when I checked.

For the record the FCC has been quite clear in their legal opinion. Michele Ellison, Chief of the FCC's Enforcement Bureau stated the following back in February:
"While people who use jammers may think they are only silencing disruptive conversations or disabling unwanted GPS capabilities, they could also be preventing a scared teenager from calling 9-1-1, an elderly person from placing an urgent call to a doctor, or a rescue team from homing in on the location of a severely injured person. The price for one person's moment of peace or privacy, could be the safety and well-being of others,"

The NAVSYS Corporation (a military contractor) developed a GPS Jammer Detection and Location system called JLOC. It's run by  National Geospatial Intelligence Agency. Much like the way the FCC catches pirates with triangulation, JLOC uses a network of GPS receivers capable of detecting regions of unusually high signal levels and low signal-to-noise ratios. How precise it can be while GPS isn't working... they don't exactly say. More here.