[ntp:questions] Questions about joining pool.ntp.org
albertson.chris at gmail.com
Mon Aug 29 23:19:02 UTC 2011
On Mon, Aug 29, 2011 at 1:46 PM, <jimp at specsol.spam.sux.com> wrote:
> Harlan Stenn <stenn at 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)
Redondo Beach, California
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