thoughts on A-GPS offline

Helge Hafting helge.hafting at
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> 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.

Helge Hafting

More information about the community mailing list