On Tue, 08 Dec 2020 12:40:33 +0000, 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. The older trekking units, e.g. Garmin GPS
II, generally took 1-2 minutes to get a fix outdoors and didn't work
indoors. The slightly more recent GPS-35, a puck with no controls or
screen, was quite a lot faster and IIRC would get a fix if left on a
south-facing window sill in the UK.

How long since last the last time a GPS had a fix matters because each
receiver has a copy of the ephemeris, which contains orbital parameters
and time adjustments for all the satellites in the GPS constellation. If
this is out of date, the receiver can't get a fix until it has downloaded
a fresh ephemeris. GPS satellites broadcast their ID and time every
second and a new ephemeris rather less frequently. To get a fix the
receiver needs to know the ID and time at each satellite. It looks up
orbital details for the satellites it can hear in the ephemeris and from
this and the broadcast time at each satellite it knows where it was when
that satellite sent its last time message. This lets it work out times of
flight for the signals and, from that, its distance from the satellite.
Then it combines information about all the satellites to work out exactly
where it is and what the exact time is at its location.

> The OP's situation sounds like one where a RTC would be quicker than a
> cold GPS, although maybe there's a way to get a time out of GPS before
> it has seen enough satellites for a position fix.
>
I'm certain thats true, particularly with an old GPS receiver.

Another caveat: I have a pair of old Garmin GPS II+ units, used
originally for model aircraft tracking and later for full-size glider
navigation, bought in 2001 and 2004. They still work because they have
always had good AA cells in them. That's important for early GPS units,
which used battery-backed RAM to store waypoints and, most importantly,
the GPS epoch counter. The old Garmins used a soldered-in battery (2032
or similar) to keep the RAM active while you changed the AA cells, but if
left for any time with AA cells removed, the coin cell will die and the
RAM will be wiped and the epoch counter will be zeroed. However, none of
these units have any ability to configure the epoch, so once that
happens, the GPS receiver is bricked. My pair still work because having
good batteries in them has kept the coin cell from being used except for
a minute every 2 years or so when the AA cells get changed.

More recent receivers use non-volatile memory: these should still be
usable but I don't know which specific makes or models these would be.

I also have a pair of PNAs (a Binatone and a Medion S3747) which run on
Li-ion batteries and store vital data in non-volatile memory, both are
used to run LK8000, a glider navigation and flight logging app. The Medion
normally comes up from cold in about a minute. The Binatone was bought in
around 2008 and the Medion in 2014.

However, when I suggested using a GPS as time source I was thinking that
in at least some cases it may be better to treat the GPS receiver as its
prime time source, particularly if the RPi isn't more or less constantly
online to an internet time source.

> (and can you do A-GPS by configuring the lat/long of your stationary
> Pi?)
>
Probably not - seeing that the GPS still needs to have current ephemeris
data, which it picks up when a satellite it can see broadcasts it.

I'm only suggesting that using an old trekking or puck GPS is a good idea
for somebody who already has one lurking, unused in a drawer.

Otherwise the obvious way to do it is to buy one of the RPi GPS expansion
cards. Most of them seem to use uBlox GPS receivers, which appear to be
well-regarded devices.


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

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