AGPS - protocol specs?

Ian Stirling openmoko at
Fri Mar 9 13:21:15 CET 2007

Bartlomiej Zdanowski AutoGuard Ltd. wrote:
> Ian Stirling napisał(a):
>> Bartlomiej Zdanowski AutoGuard Ltd. wrote:
>>> Hello.
>>> Nils Faerber napisał(a):
>>>>> There is all sorts of wacky stuff - for example, peer-peer DGPS 
>>>>> that can
>>>>> be done, where all stationary neos on charge with a GPS signal and a
>>>>> free internet connection contribute to a global ionospheric model.
>>>>> Then any Neo can connect to this model, download 200 bytes or so, and
>>>>> get +-0.3m (or better) position for a short while.
>>>> Exactly.
>>>> Also relative positioning can be made much more precise using the raw
>>>> data (AFAIK in the range of cm not m).
>>> I asked my colleagues who are GPS devices specialists and they said 
>>> that this is all wrong. Military devices can do magic but civil not. 
>>> They said that with civil GPS receiver you can get accuracy up to 
>>> about 5 - 10 meters. For such precision it is required to be seen at 
>>> least 8 satellites, a clear sky and good magnetic and ionosphere  
>>> conditions (also solar magnetic field's change is important).
>> It's a little more complex than that, and not quite as bad.
>> is some data I took a few 
>> years back with a garmin GPS12.
>> The circles show radiuses inside which the stated number of points 
>> fall, in a 10 second average.
>> For example, 99.99% of points fall within 13.8m.
>> 10s averages are red dots,
>> green 100 second, magenta circles 1 hour, cyan squares 6 hours, and
>> black 24h.
> Try it while walking on streets between buildings :(

> Another problem can be when we put Neo out of pocket. I don't know if 
> it's GPC received would need some extra time to compute pos.

Nope - the chipset gets a 5m position in one second according to the 
brief datasheet.

>> If a number of factors in the hardware all come together (that details 
>> are not public on) position under 10cm may be possible after 
>> downloading the few hundred bytes of correction information. (It may 
>> be possible to optimise this down a lot)

The DGPS corrections - per satellite error - helps you a little in urban 
canyons, as if you can see several satellites, and their errors check 
with what you expect, you can throw out reflections that don't correlate 
with expected errors. (well, not throw out, but assume to be a reflection)

> So *military* devices does it. But not civil.

No, I'm _not_ talking about the more precise 'P' code - simply accuracy 
of phase tracking loops, and how 'tunable' the GPS hardware is for the 
C/A code.
DGPS can give results down to centimetre level.

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