[FSO] Speed gps time to firxt fix up?

Daniel Willmann daniel at openmoko.org
Tue Jan 13 16:20:15 CET 2009


On Sat, 03 Jan 2009 13:50:18 +0100
Fox Mulder <Quakeman1 at gmx.net> wrote:

> Since a few weeks i'm using fso-gpsd without any big problems so far.
> The only problem is that the TTFF (time to first fix) is always >1
> minute and sometimes it needs quite a lot of time. The only solution
> for me to be sure to get a fix within <30 sec is to use the agps tool
> which downloads the almanac and ephemeris from ublox for my region.

yes, I tried integrating hot-start into ogpsd, but somehow I had
problems with supplying ephemeris. Just some days ago I noticed
something that made me think the culprit is still the time calculation.
The SVs that zhone claimed were visible suddenly made a jolt and were
positioned differently. It was more than one, so it wasn't just one
almanac that got updated. I'm not completely sure, though, didn't look
too closely.

> The problem is, that the gps chip doesn't have any internal memory why
> it needs to download all relevant gps data every time it is powered
> up. So i thought if it is possible to save the relevant gps data
> before shutting down the gps which can be done by fso-gpsd or kernel
> automatically. Than after starting the gps fso-gpsd/kernel feeds back
> the data to speed up the TTFF. This would be the same way most gps
> receivers work.

It actually does have memory, but lacks a backup battery which is
supposed to power the memory of the GPS while main power is off.
ogpsd already stores and restores almanac, GPS/UTC timeoffset and
approximate last position. It's not restoring ephemeris because I had
weird problems with that (TTFF increased to several minutes).

> Maybe this is already implemented somewhere but i don't think so
> because the TTFF without agps is always quite slow.

Some things are there, some are missing. At the moment I'm busy getting
SMS stable so I wont have too much time for GPS in the near future.

Daniel Willmann
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