thoughts on A-GPS offline

[Helge Hafting]

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!
>> http://git.freesmartphone.org/?p=framework.git;a=commitdiff;h=ad2ec48cdb5ab9bdddc15adfc05cbd2e5b8a2cee
> 
> 7s TTFF is pretty good. I got that once, but usually I have about 16s
> TTFF.
> 
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.

>> 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.

Helge Hafting