GPS problems, summary

[Marcus Bauer]

On Wed, 2008-07-16 at 21:38 +0200, Alasal wrote:

> What is the problem?
>  - The Openmoko Freerunner have a long TTFF with the SD card in the
> phone. So it takes a long time (10min+) before you get your first GPS
> data.

More specific: the GPS signal level drops by -20dBm, i.e. factor 100.
Signal strength of a GPS satellite above your head is around -127dBm.
Needed strength for a first fix is -145dBm with a minimum of three
satellites. Once the GPS chip has a fix, it can operate at signal levels
of -157dBm, thus making it possible to operate while there is data
transfer from/to the SD card.


> So if we have a first fix, the SD card isn't blocking the GPS anymore?
>  - yes, the SD card isn't blocking the GPS if we have a first fix.
> (Some people even claim it's more stable)

The FR's GPS performance is definitely top notch without the SD card.


> But we can't read of the SD card when the GPS is on?
>  - Wrong, you will be able to read the SD card when the GPS is on. You
> will probably not be able to read the SD card when you're starting the
> GPS (appr. 30 sec), because the GPS will only block the SD card when
> it's searching it's first fix. After that you will be able to read the
> SD card again.

You can always read the SD card. As said above the GPS signal
temporarily drops by -20dBm during i/o.

The kernel patch is a pragmatic and well working solution and should not
effect daily use.

> Summary of the summary:
> You will be able to use the SD card in the same time of the GPS except
> for the first appr 30 seconds. (And that's the worst case, because
> maybe Openmoko can find better hacks/fixes)

With the help of AGPS you can shorten it probably to 15 seconds.