Hi The original “we cracked GPS” paper back in the 1980’s (that unlimitedly lead to the end of SA) used a medium sized dish ( think of the good old C-band antennas) to pick out a single sat.
Bob > On Aug 30, 2018, at 9:54 PM, Brooke Clarke <bro...@pacific.net> wrote: > > Hi Gregory: > > I wonder if anyone has tried using a small parabolic dish, like used for Free > To Air satellite TV and aimed it at a GPS satellite track or at a WAAS > geostationary satellite using a feed antenna with reverse polarization from a > normal GPS antenna? > http://www.prc68.com/I/FTA.shtml > > -- > Have Fun, > > Brooke Clarke > https://www.PRC68.com > https://www.end2partygovernment.com/2012Issues.html > axioms: > 1. The extent to which you can fix or improve something will be limited by > how well you understand how it works. > 2. Everybody, with no exceptions, holds false beliefs. > > -------- Original Message -------- >> On Thu, Aug 30, 2018 at 9:43 PM Brooke Clarke <bro...@pacific.net> wrote: >>> I would disagree in that ease of jamming/spoofing is strongly related to >>> wavelength. That's because antenna efficiency >>> goes down as the size of the antenna gets smaller than 1/4 wave. >>> So, it's easy to make a GPS jammer (1,100 to 1,600MHz) since a 1/4 >>> wavelength is a few inches, something that you can >>> hold in your hand. >> However, the short wavelengths of GPS make beam forming a reasonable >> countermeasure against jamming. >> >> By having a small array of GPS antennas a receiver can digitally form >> beams that both aim directly at the relevant satellites (so even >> reducing intersatellite interference) while also steering a deep null >> in the direction of the jammer. If the jammer is powerful enough to >> overload the front-end then this won't help, but against a >> non-targeted area denying jammer it should be fairly effective. >> >> There are many papers on GNSS beamforming. ( e.g. >> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5134596/ >> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5134483/ ) >> >> This kind of anti-jamming solution should even be pretty inexpensive >> -- really no more than the cost of N receivers. Except that it is >> specialized technology and thus very expensive. :) >> >> Seeing some open source software implementing beam-forming was one of >> the things I hoped to see result from the open hardware multi-band >> GNSS receivers like the GNSS firehose project ( >> http://pmonta.com/blog/2017/05/05/gnss-firehose-update/ ) since once >> you're going through the trouble of running three coherent receivers >> for three bands, stacking three more of them and locking them to the >> same clock doesn't seem like a big engineering challenge... and the >> rest is just DSP work. >> >> Even absent fancy beam forming, for GNSS timing with a surveyed >> position except at high latitudes it should be possible to use a >> relatively high gain antenna pointed straight up and by doing so blind >> yourself to terrestrial jammers at a cost of fewer SVs being >> available. But I've never tried it. >> >> In an urban area I noticed my own GPSDOs losing signal multiple times >> per week. Monitoring with an SDR showed what appeared to be jammers. >> >> As others have noted intermittent jamming is pretty benign to a GPSDO. >> Spoofing, OTOH, can trivially mess up the timing. It's my view that >> if you need timing for a security critical purpose there isn't really >> any GNSS based solution commercially available to the general public >> right now, the best bet is a local atomic reference with a GPSDO used >> to monitor and initially set it. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.