Dale DePriest
Guest
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 Posted:
Wed Oct 26, 2005 9:23 pm Post subject:
Re: GPS Satellite Searching in Connecticut |
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Jack Yeazel wrote:
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Bob Greschke wrote:
"Jack Yeazel" <jack@finalapproach.net> wrote in message
news:435D6033.204DCA8E@finalapproach.net...
Incidentally, I've been e-mailing Garmin asking if they will tell me
what constitutes a cold start and a warm start, but nothing yet... I'll
let the group know when they 'divulge' it...
A "cold start" on the internal GPS on one of our seismic recorders (don't
know who makes the GPS engine) is defined as dumping the almanac and
ephemeris data and any sense of what time the GPS thinks it is and searching
from scratch.
Well, here's Garmin's reply:
"Thank You for contacting Garmin International. I will be happy to help
you.
Anywhere between 30 minutes to an hour is considered a "warm start."
The
longer the unit remains powered off the more the satellites will change
overhead. This will take the unit longer to re-acquire the signal thus
indicating a "cold start.""
-As I remember it, they used to have a hard and fast time limit for
warm starts, but maybe they now they 'examin the situation' before
doing a cold start...??? The satellites themselves will indicate when
their last valid ephemeris was obtained and if different from that
stored in the unit will cause a cold start (at least for that
satellite)...
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As I said in my earlier post, Garmin defines things differently.
Ephemeris data is guaranteed valid for 2 hours and can often be used for
another 2 hours so as long as the currect visible satellites remain in
the sky the emphemeris data that was collected once is usually ok. There
should be no timeout done by the receiver based on the age of the data.
The satellite sends a update every 2 hours saying if it is ok for the
next period or updates it. So long as 3 satellites still exist in that
transit that the unit has ephemeris data for it can compute a 2D fix
without regathering data. Most people except Garmin call this condition
a hot fix. Since this condition does not require downloading new data it
can be done in a few seconds, typically 10 or less.
There is no cold start for a particular satellite. The term is not
defined since a fix is not based on any particular satellite. A new fix
can use entirely different satellites than were used for the original
fix so long as the data is available. If sufficient satellites are
available with ephemeris data these will be used for the fix. Other
satellite data will be gathered in the background. If not then the it
will be a warm (everyone except Garmin /cold (Garmin) start.
Dale
--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs |
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