[time-nuts] NIST

Bob kb8tq kb8tq at n1k.org
Thu Aug 30 20:24:38 EDT 2018


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. 


> On Aug 30, 2018, at 7:05 PM, Gregory Maxwell <gmaxwell at gmail.com> wrote:
> On Thu, Aug 30, 2018 at 9:43 PM Brooke Clarke <brooke at 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.
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