[time-nuts] NIST

Bob kb8tq kb8tq at n1k.org
Fri Aug 31 13:17:33 EDT 2018


Hi

It’s back in the FCS archives. I don’t think it’s one of the ones you can hit without going through a
paywall. It was a fun paper to attend. The chatter in the room was “interesting” to say the least.

Bob

> On Aug 31, 2018, at 1:07 PM, Brooke Clarke <brooke at pacific.net> wrote:
> 
> Hi Bob:
> 
> Do you have and info on that article that would allow me to read it?
> 
> -- 
> 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 --------
>> 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 <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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>>> 
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> 
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