[time-nuts] NIST

Bob kb8tq kb8tq at n1k.org
Thu Aug 30 21:00:50 EDT 2018


Hi

….. ok, so you are dealing with city wide jammers that take out all of New York City on a daily basis? 
Again, that was the original example tossed out. “A cigarette pack sized jammer that takes out an entire
city”.  A jammer with that sort of range is an easy jammer to spot. 

Somehow I find that a bit difficult to believe. What I’ve seen and gone after are *far* shorter range than 
that magic device.  A short rang mobile jammer aimed at an ankle bracelet takes out an infrastructure  device 
for minutes. That’s why those devices have holdover capabilities. 

Indeed at the point they *do* start interfering with major systems over a wide range…. bigger gear gets brought in. 
Jamming that actually takes utility systems down is very rare. No cell phone service anywhere in New York is 
something that gets noticed pretty fast. It also fires up meetings that last quite literally for years …..

Bob

> On Aug 30, 2018, at 8:43 PM, Scott McGrath <scmcgrath at gmail.com> wrote:
> 
> Just ask the NY Port authority how ‘easy’ knocking these jammers offline is.   Usually done by vehicle to vehicle inspection with a SA.
> 
> And yes the day job all too frequently searching for and identifying interference sources.
> 
> One of the more interesting ones was a halogen leak detector wiping out WiFi at a manufacturing plant.   So my opinions on interference location are informed by leading teams of people doing just that.
> 
> Content by Scott
> Typos by Siri
> 
> On Aug 30, 2018, at 8:24 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> Hi
> 
> 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. 
> 
> Bob
> 
>> 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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