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…. Bob On Oct 7, 2013, at 9:11 PM, Jim Lux <[email protected]> wrote: > On 10/7/13 8:31 AM, Chris Albertson wrote: >> OK so let's say you have a receiver and detect a certain about of power at >> the right frequency. How do you determine which of three cases you have >> (1) an actual GPS signal from a satellite. (2) a spoofer (who tries hard to >> look like #1) or (3) a jammer. > > > The jammers put out many milliwatts and have enormous signals that are > obvious on a spectrum analyzer. GPS signals are invisible on a spectrum > analyzer, normally. IN fact, most GPS receivers don't work very well if > there are signals above the noise floor: they depend on the noise to make > them work with their mighty 1 bit quantizers. > > >> >> >> Spoofers are a real problem. > > I doubt anyone is selling spoofers on eBay. > Sure, one can probably find some code to run on a USRP from some grad > student's project. > >> So the easiest thing to detect would be a cheap, GSP jammer that is moving. >> You could use multiple receivers to triangulate the location and then >> determine it is not in orbit and is not a reflection from a metal roof or >> something. The problem is the jammer's very low power. These things are >> inteneded to only cover a tiny area > > They are not designed with coverage area in mind. They are basically > "whatever power the VCO puts out coupled to the antenna" From a jamming > standpoint, they're not very sophisticated. > > As a result they dump out something like +10dBm. > So running a quick Friis formula link budget, and assuming you want to have a > Prec of around -100dBm (10 MHz BW, kTB) > > 110 = 32+ 20*log10(1575) + 20*log10(d) > 110-32 - 25 = 20*log10(d) > d = 400 km... > > This is why they are such a problem > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
