[ntp:questions] Slow convergence of NTP with GPS/PPS
david at ex.djwhome.demon.co.uk.invalid
Fri Oct 24 21:26:20 UTC 2008
Nicola Berndt wrote:
> time of 12.5 mins to transmit the full almanach. I don't know, if really
> the entire almanac is needed for precise time, but I'd actually suspect
I don't think the almanac is strictly necessary. What you need is the
detailed ephemeris for each satellite you are using. That takes 30
seconds to transmit.
My old GPS receiver takes a long time to acquire from cold because it
can only read data from one satellite (one spreading code) at a time.
It starts at satellite 1 and tries each one for some time (I suspect it
is trying different frequency offsets and different phases) until it
finds one. It might speed up a bit then, as it will have an approximate
frequency, but it still continues stepping one at a time until it has
enough for a 2D fix. At that point, I think it does use the almanac,
although it may use an old one, to decide which other satellites should
be in view.
A modern, high performance, device, may be able to decode data for the
whole constellation at once, and therefore cold start much faster.
If you know your position, you can get a time lock within 30 seconds of
finding the first satellite.
According to the June 1995 GPS SPS Signal Specification document, the
almanac repeats every 24 "pages". So it will take 12 minutes to
transmit. I presume the extra half minute is to find the start of a
page. 15 of the 45 seconds quoted for cold starts one one receiver may
well be to find a safe starting point for decoding.
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