On 2011-08-30, [email protected] <[email protected]> wrote: > Chris Albertson <[email protected]> wrote: >> On Mon, Aug 29, 2011 at 1:46 PM, <[email protected]> wrote: >> >>> Harlan Stenn <[email protected]> wrote: >>> > GPS can be done very affordably and can offer great time. There are >>> > several *potential* pitfalls: >>> > >>> > - It is *possible* for the US Gov't to detune the GPS system (locally or >>> > in-general). Since GPS is now increasingly used for "human safety" >>> > things, the costs/risks for doing this have gone up significantly so >>> > this risk may now be more of a threat than a reality >>> >>> GPS aviation navigation is now so deeply entrenched that if you live near >>> a major airport the US Gov't "detunes" GPS, you will have much bigger >>> things to worry about than the current time and should start filling >>> things with water... >>> >>> >> In the context of an NTP server any "detuning" of GPS will not matter. >> It's called "Selective Availability" and the way it works (in short) is >> they encrypt the lower few bits. Or in English, they scramble least >> significant decimal points so that GPS users who don't have the "secret key" >> see much reduced precision. But how does this effect TIME? Remember that >> light travels at about 1000 feet per microsecond. So if you GPS is >> "detuned" such that it is seeing a random 1,000 foot error then the time is >> also "detuned" by about one microsecond. NTP will hardly notice. >> >> The reason this may seem counter intuitive is that it is hard to picture in >> your mind the incredible time precision we get using GPS. In normal use the >> error is about one part in a hundred million. If they "detune" it so as to >> make the error 1000 times larger we still get less error than NTP can track. > > But then there goes the 10 ns accuracy. > > How could anyone survive with only microsecond accuracy? > >> The thing to worry about with GPS is if the system were totally shut down or >> (more likely) if it was jammed by some local radio transmitter. The common >> way you address this (and all cell towers have to address this. I think the >> requirement they have is to handle a 24 hour loss of GPS) is to have a good >> stable local oscillator. They use either an ovenized crystal or >> rubidium oscillator. Sounds exotic but the price of good surplus is about >> $100 to $150. In simple terms the GPS keeps the oscillator running at >> correct speed and then if GPS goes away the oscillator is stable enough that >> it will serve as a clock until GPS comes back. Cell towers need to track >> time at the uS level and they all work like this and can work for a long >> time with no GPS signal. (for more info google the term "gps holdover". >> That is the term for keeping time when GPS is down) >> >> Chris Albertson >> Redondo Beach, California > > It would be more realistic to worry about a swarm of locusts and a plague > than GPS going away for 24 hours absent WWIII, in which case you will have > much bigger things to worry about than the current time at any accuracy. > > As far as being jammed goes, the FCC is rather sensitive and very aggressive > about the subject as LightSquared has discovered.
Have they discovered that? I thought it was still up in the air. And we keep hearing about the UK jamming GPS for hours at a time in regions of the UK. > > > _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
