Detecting GPS Trackers With an AM Radio

Neat tool that could be all sorts of useful for defending yourself from illegal tracking. It turns out a (poorly-filtered) GPS receiver may emit noise at 1.023 MHz, which you can pick up with an AM radio.

The link also mentions a separate document by James Atkinson of TSCM-L
fame, “Detecting_Covert_GPS_Tracking_Systems.txt,” but it returns a 404. If anyone can find a copy, I’d love to see it. I recently built the “coathanger, diode, and $10 [12€ in this case] multimeter” ‘bug detector’ from the TSCM-L archive — in about five minutes, using an SMA connector and whip instead of a coathanger — and it does indeed do a good job of finding signals.

(Next up when I feel like it — building in a mixer, signal generator, and bandpass filter a la the Russian Sinitsa [http://www.cryptomuseum.com/df/sinitsa/] elegantly simple intercept receiver. Even though I still have yet to fully comprehend the utility of the injected oscillator signal: I feel like it can’t possibly change the receiver’s ability to detect a signal, though it might help decode the audio from one so the operator could listen to it. Anyone want to explain what I’m missing?)

Note: The link author is comically right-wing, anti-Muslim, and anti-2600 Magazine. But someone’s political beliefs don’t change the merit of their technical ideas.

http://72.52.208.92/~gbpprorg/mil/gps1/index.html

“It is possible to detect some GPS receivers by the electromagnetic radiation they emit. GPS L1 receivers all generate a pseudo-random digital “noise” source at a chiprate of 1.023 MHz. These pseudo-random digital signals are then used to correlate against the data received from a GPS satellite’s transmitted Coarse Acquisition (CA) data stream at 1575.42 MHz. By searching for any unintentional radiation of the receiver’s 1.023 MHz digital noise signal (or its harmonics) you can pin-point the general location of an operating GPS receiver. By verifing that you “hear” a data stream at 1.023 MHz, 2.046 MHz, 3.069 MHz, etc. you can be sure that you’ve found a GPS receiver.

All you need to do this is a cheap AM radio receiver. One with the fancy PLL tuning will be useful, though. Tune the AM radio to approximately 1.023 MHz and pass it over an operating GPS receiver. You should hear a distinct, high-pitched digital “whine.” That is, if you are not receiving any nearby AM radio broadcasts, which can be a problem. This receiving method will work best indoors or inside a shielded metal or concrete enclosure, which reduces the chance of external RF interference.

If you wish to get fancy, you can replace the Intermediate Frequency (IF) filter on the AM radio with one which has a much narrower bandwidth. This will help to eliminate any interference and reduce false signals. The ARRL Handbook for Radio Amateurs will cover the designing and building of suitable crystal IF filters. You could also try using a high-end shortwave receiver fed with a tuned ferrite stick or loop antenna.[…]

Signal Strength Meter Option

A visual signal strength meter can be a quite handy addition. It is made by directly tapping the speaker output from the AM radio and rectifying it with a common 1N4001 diode. The resulting DC voltage “pulses” are then used to drive a small analog meter movement, which can be salvaged from an old stereo receiver or CB radio system.”

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