[time-nuts] NIST

Gregory Maxwell gmaxwell at gmail.com
Thu Aug 30 19:05:48 EDT 2018


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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