Hi Since timing receivers are actually going to prefer high angle sats, an antenna that rejects close to the horizon is a pretty common thing. Enhancing that sort of rejection doesn’t take a lot of effort.
Bob > On Aug 30, 2018, at 7:05 PM, Gregory Maxwell <gmaxw...@gmail.com> wrote: > > 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.