thoughts on A-GPS offline

Daniel Willmann daniel at
Wed Mar 11 11:55:26 CET 2009

On Tue, 10 Mar 2009 15:18:23 +0100
Helge Hafting <helge.hafting at> wrote:

> I do not get 7s every time. This was an ideal test, I was standing in
> a place with good visibility. the gps used 9 or so satellites. Then I 
> stopped tangogps and restarted it, and timed it from the moment the 
> tangogps screen showed. (So as to not include the startup time for 
> tangogps itself). The TTFF is usually a little longer inside a bus or 
> building.

Okay, this still sounds really promising and matches my experience.
Even when riding my bike I now usually get a fix within the first
minute, mostly within 30 seconds. Before uploading ephemeris this often
took several minutes (sometimes it wouldn't get a fix until I stood
still for 20 seconds even after riding around for ~10 minutes).

> > so the chip knows which SVs to search for. So a pacc of 3km or 9km
> > shouldn't have any noticable effect. This is just guesswork though,
> > so any results on your part would be greatly appreciated.
> It was just to be safe. The documentation states that you might not
> get a fix _at all_ if either position or time is outside the claimed 
> accuracy. Now, maybe it works with 3km anyway after the fixes that 
> prevents the chip-crashing exception. I happen to live about 6km away 
> from where I work, so 9km was a nice safe value. The default is
> 300km, and "100km allows a more optimistic startup." Perhaps such
> rough estimates is all that is needed, if it is only used to figure
> which satellites that can be seen.

If you don't mind testing please try changing pacc to 100km and see if
it affects TTFF adversely in your case. If not we could just use that
as a default.

Daniel Willmann
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