GPS sensitivity

[Joel Newkirk]

On Tue, 28 Oct 2008 01:36:36 +0530, Gora Mohanty <gora at sarai.net> wrote:
> On Mon, 27 Oct 2008 20:56:54 +0100
> David Garabana Barro <david at garabana.com> wrote:
> [...]
>> I allways though Freerunner had AGPS. That's what wiki says:
>>
>> http://wiki.openmoko.org/wiki/Neo_FreeRunner_Hardware#AGPS
>>
>> Isn't it true?
> [...]
> 
> Yes, I believe that it does have AGPS capabilities. But AGPS
> needs someone sending out the AGPS data. I don't know that
> that is being done with OpenMoko.
> 
> Regards,
> Gora

http://wiki.openmoko.org/wiki/AGPS

http://svn.openmoko.org/developers/matt_hsu/agps-online/

aGPS works nicely but isn't fully utilized ATM in most cases. You can
register for a free account at u-blox.com (chipset manufacturer for the GPS
in the FR - send empty email to agps-account at u-blox.com and get password
back) and use a tool called 'agps-onlinec' (at the SVN above) in the
instance I tested, and feed it some relevant data:

agps-onlinec -c full -u agps at newkirk.us -k Fvhcv -la 35.22 -lo -79.46 -al
250 -p 100.00 &

the -u is email address used to register above, '-k' is the password
provided in the reply email, '-la' and '-lo' and '-al' are current lat and
long and altitude, and the '-p' is precision in kilometers. If you have a
very accurate position already (eg you're at home and know precisely what
the GPS /should/ read) you can get a fix in a very short time with "-p 1",
provided the location data prefed to agps-onlinec and the current time
(system clock) are accurate enough. (clock doesn't need to be
second-precise, but if it's off several hours or more then you've got
problems)  On warm restart of the GPS it retains this data within the GPS,
on cold restart it is lost and must be resupplied from outside or it takes
significantly longer again to get a fix.  

Frameworkd (FSO) apparently stores almanac data automatically and feeds it
back into the GPS on startup, so if you're within a couple hours of
previous GPS usage (I presume) you get a faster fix.  Ideal in my mind
would be to check cell tower ID and/or wifi AP MAC against
previously-cached records and feed the location data from those records
(with appropriate precision data, perhaps 10km for cell-tower?) into
agps-onlinec and let it retrieve data via GPRS from the assistance server. 
(even better would be a local service ON the FR that retrieves the full
2-week dataset and serves up the appropriate data on demand, but I'm not
even sure if that can be done without careful dissection of both protocols
- the long-term data isn't supported by the GPS inside the FR)

j