On Mon, Aug 29, 2011 at 1:46 PM, <j...@specsol.spam.sux.com> wrote: > Harlan Stenn <st...@ntp.org> 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. 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 _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
