[time-nuts] NIST

Bob kb8tq kb8tq at n1k.org
Thu Aug 30 22:22:45 EDT 2018


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.


> On Aug 30, 2018, at 9:54 PM, Brooke Clarke <brooke at 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 <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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