Current generation GPS systems take a little over 30s to cold start.
They do this by acquiring a set of satellites during the first few
seconds, then downloading ephemeris and almanac over the next 15-30s.

This isn't usually necessary though - in nearly every case a warm
start can be done instead. The almanac is already available in
battery-backed RAM so the unit guesses which satellites are in view
based on the time and its last known location. This means that unless
it's moved a long distance it can usually start up in about around 25s
rather than 35s.

Some GPSs can also hot start, meaning that if they have a sufficiently
accurate time mark and recent ephemeris information available they can
use the ephemeris information straight away,  allowing them to fire up
in 5-15s. If the ephemeris is out of date or the time mark isn't
accurate enough the unit has to fall back to warm or cold start.

Cheers,
Zik

On 12/6/06, Robert Rolf <Robert.Rolf@ualberta.ca> wrote:
> Brooke Clarke wrote:
> > out of a garage or tunnel), not cold start.  That data rate coming from
> > the satellites means that it will take about 12 minutes to download an
> > ephemeris and that can not be speeded up by receiver horsepower.
>
> Is the ephemeris data transmitted by all satellites 'in phase' or is it
> "store and forward" with staggered phases?
>
> Seems that if one can find one satellite in 'search' mode, one could find
> more than one, and so download different pieces of the almanac and ephemera in parallel
> to shorten the cold start time.
>
> And with software correlaters, having more horsepower would mean more correlaters
> so more satellites found sooner (only if the data transmissions are NOT synchronized
> so parallel loading of the data was feasible).
>
> And if this ISN'T the case, wouldn't changing the system to staggered data
> transmission allow for this to happen? Patent anyone?
>
> Expert comment anyone?
>
> Robert
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist