External GPS antenna question

Al Johnson openmoko at mazikeen.demon.co.uk
Sat Jul 4 20:56:36 CEST 2009

On Saturday 04 July 2009, Paul Fertser wrote:
> Sebastian Krzyszkowiak <seba.dos1 at gmail.com> writes:
> > On 7/4/09, Hammer Armin <ahammer at datazug.ch> wrote:
> >> With the internal gps antenna, the first fix last quite long -
> >> with the external - within 5min to 10min I get a fix.
> >
> > WTF? I'm getting fix easly with *internal* GPS antenna in 30 seconds -
> > 2 minutes...
> Because you already have almanac (and sometimes ephemeris) data.

With a good sky view and suitable constellation TTFF from cold is ~40s with 
the internal antenna, rootfs on SD and an early A05 without the capacitor on 
SD. For it to take >2min I need heavy urban canyoning, being indoors or in a 
vehicle, or an unusually poor constellation.

Having almanac and ephemeris _should_ make it faster, as should initialisation 
with correct time and approximate location. Feed it incorrect data at startup 
and you can get a very long TTFF, or no fix at all, and ogpsd has done this at 
some stages of its development.

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