Well everyone just rambled on, so I figured I'd get out the spectrum
analyzer and see what could be sniffed.
I tried to detect a Garmin GPSMap60CSx. The claim is the GPSMap60CSX
uses the SiRFStar III. The local for the chipset is 1571.424Mhz. L1 is
1575.42. So the goal is to look for a low side mixer leaking beneath the
actual GPS signal.
Since this mixer is nearly in the GPS band, the easiest thing to use for
an antenna is a GPS antenna. I was given a passive GPS antenna years
ago, and of course I never used it. Hey, it was passive. Hooking it up
doesn't get the GPS signal (beneath the noise floor of the spectrum
analyzer), but sure gets lots of L-band cellular, plus that annoying
smart meter that pings around 1.2GHz.
The next step was to hook up an active GPS antenna. This was an
interesting experiment in it's own right since I never looked at the
output of a GPS antenna. The active antenna is a HAG-240, good for 37dB.
That brought the GPS signal up to -75dbm. No mixer signal detected.
If you think about it, given the proximity of the mixer frequency to the
actual GPS signal, you would be a fool to leak a lot of mixer. It would
desense the receiver. The Sift III implementations I found on the net
(not necessarily how it is done in the Garmin) use a LNA and SAW filter
cascaded. This should have decent isolation.
As I expected, sniffing the local is simply not the way to detect a GPS.
I would still go with the NLJD.
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