thoughts on A-GPS offline
helge.hafting at hist.no
Tue Mar 10 15:18:23 CET 2009
Daniel Willmann wrote:
> On Wed, 04 Mar 2009 13:59:07 +0100
> Helge Hafting <helge.hafting at hist.no> wrote:
>> Thanks a lot!
>> I needed this one too, and now get 7s warm starts!
> 7s TTFF is pretty good. I got that once, but usually I have about 16s
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
>> I also raised pacc from 3km to 9km, as I often enough travel a bit
>> more than 3km with the gps unit off.
> I'm interested to hear how that affects TTFF. The way I understand it
> initial position is only used to calculate which SVs are in view 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.
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