On 10/8/13 4:17 AM, Bob Camp wrote:
Hi
But there's obviously something wrong with the 400 KM number.
1) If +10 dbm is good enough to burry a useful signal at that distance, it
should be good enough to communicate at that distance. That's pretty impressive
QRP without high gain / directional antennas involved.
2) The radios (at least the modern ones) do have CW signal immunity. Weather
that's 60 db or something else probably varies with the make / model of the
GPS. How well that works with a VCO jammer - again, a that depends sort of
thing.
3) There's a (maybe) 40 db variation in GPS signals. To deny service you need
to take out the strong ones, not just the weak ones.
4) Even without specific anti-jam in the GPS, the code it's self does have some
immunity to a jammer.
Of course you don't have to look very far into the archives to find wonderful
examples of slipped decimal points in my posts….
You're right, I forgot the process gain of the despreading.
Assuming you're going from 1 Mchip/sec for the C/A code to 50 bps for
the nav message, that's 43 dB
That alone gets you to 2km jamming range from 400km, but that also
assumes that the jamming signal doesn't inhibit acquiring the code and
despreading.
As Dixon's book on Spread Spectrum says, "acquisition is the hard part";
because you don't have the process gain yet. I suppose with a parallel
acquisition strategy, you're basically trying all codes, and that might
be able to work.
But even so, those sorts of process gain arguments don't necessarily
work if the receiver has a hard limiter or quantizer in it.
I don't know that modern consumer GPSes have CW immunity. If they're
using a 1 or 2 bit quantizer, a strong CW signal pretty much captures
the front end.
Easy to try. Let me just fire up my kilowatt 1.5 GHz transmitter here<grin>
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