[time-nuts] NIST

Brooke Clarke brooke at pacific.net
Fri Aug 31 13:07:40 EDT 2018


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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