Bluetooth GPS for timekeeping?

[Glenn Dawson]

At 12:19 PM 8/9/2005, David Gilbert wrote:
> >>>>> "Frank" == Frank Mayhar <frank at exit.com> writes:
>
>Frank> On Tue, 2005-08-09 at 14:51 -0400, David Gilbert wrote:
> >> But ... since there are long patches of time where I'm not mobile,
> >> I was wondering if anyone had looked at using a Bluetooth GPS for
> >> timekeeping.  Has anyone also ever had an ntp server sometimes use
> >> a GPS and othertimes use other servers ... depending on the
> >> availability of the GPS?
>
>Frank> The former would depend strongly on the characteristics of the
>Frank> Bluetooth protocols, at least when it comes to accuracy.
>Frank> Keeping time to the half-second or so would be pretty easy, I
>Frank> would guess.
>
>Frank> The latter is the way it already works.  Just configure other
>Frank> peers in your ntp.conf along with your GPS, viz:
>
>How might you determine the accuracy of the GPS ... or the
>"characteristics of the Bluetooth protocols" ?

GPS relies heavily on the accuracy of the clocks in each of the satellites, 
without that accuracy GPS would not be able to give any sort of accurate 
position information.


>The GPS docs "say" that the GPS chipset keep time to within 100ns.
>However (and I assume this is to save power) they also say that the
>position indication is only sent once per second.

The satellites don't send any position information to the GPS receivers 
(well, actually they transmit their own location), pretty much all that 
they do is broadcast their clock information.  The GPS receiver calculates 
the delays from all the satellites that it can hear to determine it's 
relative distance from each satellite.  Using the precise location of each 
satellite the, and the relative distance from each satellite the receiver 
can calculate it's location. (it takes a minimum of 3 satellites to get a 
"fix".  The more satellites the more accurately the receiver can determine 
it's position.)  Because of the way GPS functions, the time that is 
transmitted has to be as accurate as possible.

-Glenn


>In my case, the Bluetooth GPS would be talking to a Bluetooth dongle
>hanging directly out a port of the server in question.
>
>Dave.
>
>--
>============================================================================
>|David Gilbert, Independent Contractor.       | Two things can only be     |
>|Mail:       dave at daveg.ca                    |  equal if and only if they |
>|http://daveg.ca                              |   are precisely opposite.  |
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