External GPS antenna question

[William Kenworthy]

On Sat, 2009-07-04 at 19:56 +0100, Al Johnson wrote:
> 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.

and if you are carrying the FR in a pocket, yes it gets a lock most of
the time, but the track wobbles all over the place - up to a hundred
meters.  And if driving through an "urban canyon", similar track errors
can occur.  Ive wanted a small external powered antenna for some time to
improve this.

BillK