John Green wrote:
jamming anyone's GPS. A while back, I was looking at one of those It doesn't look capable of putting out more than 50 milliwatts or so into a 2 inch antenna
The GPS antenna is perhaps 35 feet away
with a cinder block wall, a brick wall, and a metal roof in between. I also put a 15 Db attenuator between it and the antenna with almost the same result.
down. Has anyone here had any actual experience testing GPS receivers for susceptibility?
OK... typical received signal at a GPS receiver (L1) is on the order of -130dBm. Thermal noise floor (assuming noiseless receiver and no losses) is -114 dBm in 1 MHz BW.
Remember, the typical GPS is a single bit quantizer, which works just fine considering the signal is 20dB below the noise floor.
So, let's do a little link calculation: 32+20log10(1500)+20log10(.010) between isotropic antennas (which is not a bad starting point for your jammer and the GPS).. 32+64-40 -> a link loss of 56 dB.. you're radiating +17dBm, so let's call it -40dBm into the GPS.. Yep, jamming is almost assured..
But at that kind of power, you'd jam almost ANY receiver that's trying to receive a signal at -130dBm. 90dB instantaneous dynamic range is pretty good when you can't use a filter to remove the interfering signal (e.g. a HF receiver has a narrow band filter in the IF to solve this problem).
realistically, you need about, say, 10 dB J/S so you'd need -120 dBm into the receiver from the jammer. A microwatt 10 meters away would do it nicely. Inverse square helps a bit.. if you were 1 km away, your interfering 50mW signal would be down another 40 dB.. -80dBm.
10km away, your jammer is down into the area where it probably won't jam all the time.
Obviously, *real* radios that need reliable GPS reception do things to make life easier. Aside from using 1.5-2 bit detection, or signal excisers, etc. There are also techniques that rely on looking at the post correlation signal (where the interferer is suppressed to a certain extent): with modern signal processing, you can correlate against all possible phases of the code in one shot, for instance.
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