[ntp:questions] Slow convergence of NTP with GPS/PPS

David Woolley 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 
> that.

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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