On Wed, 09 Dec 2020 09:03:23 +0000, Andy Burns wrote:

> Martin Gregorie wrote:
>
>> Theo wrote:
>>
>>> How long with a GPS unit take to get a time fix from cold, out of
>>> interest?
>>
>> That depends on the make, model, how good its view of the sky is and
>> how long since it last had a fix.
>
> I think Theo was only asking about a time fix, not a location fix.

Sure, but I don't think that affects a GPS receiver's startup process: it
still needs the almanac data to determine the exact time: its location
matters to it when correcting for signal travel time from satellite to
receiver and, if any satellite clocks have been reset recently, what
their UTC offset is now. If the receiver hasn't been run for a while it
will need an updated almanac and, as Andy said, it may have to wait up to
12.5 minutes for this to be retransmitted.

If a GPS receiver is designed to be a time source, it will emit NMEA-0183
ZDA messages which only contain UTC date/time information, but equally
there's no reason why the time recipient shouldn't accept NMEA-0183
message types RMC, GGA or GNS and discard everything except the time
field because all these messages contain a UTC time stamp accurate to 10
mS.


--
--
Martin    | martin at
Gregorie  | gregorie dot org

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