[time-nuts] GPS jamming susceptibility
jimlux
jimlux at earthlink.net
Tue Nov 23 14:36:37 UTC 2010
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.
More information about the time-nuts
mailing list