AGPS - protocol specs?

[Bartlomiej Zdanowski AutoGuard Ltd.]

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.
>
> http://www.mauve.demon.co.uk/gps-average.gif 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.
> 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)
So *military* devices does it. But not civil.


-- 
*Bartlomiej Zdanowski*
Programmer
Product Research Department
AutoGuard & Insurance Ltd.

Omulewska 27 street
04-128 Warsaw
Poland
phone +48 22 611 69 23
www.autoguard.pl <http://www.autoguard.pl>
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