AGPS - protocol specs?

[Al Johnson]

On Friday 09 March 2007 08:58, 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).
> To get *any* GPS reading device must see 3 satellites but you cannot be
> sure the position. It can vary even up to hundreds of meters!
> Of course you can get very accurate reading but device must see many
> satellites and stand up still for many hours! Then the mean value is
> computed and can be assumed that GPS position reading is accurate to
> centimeters...

This is true for a single civilian GPS receiver without extra sources of 
information. However this is not what was being described in the earlier 
posts. These were describing Differential GPS systems where data from a 
nearby stationary GPS receiver is used to compensate for the location error. 
This works because for small separations of receivers the location error 
sources are the same, and the error increases as separation increases. This 
is howGPS-based surveying systems achieve high accuracy with civilian GPS, 
and is the basis of GPS enhancement systems like WAAS.

There are other ways of improving things too. IIRC RaceLogic use the doppler 
shift of the GPS signal for speed and acceleration calculations. These can 
also be used to improve the accuracy of the location. I think they claim it's 
good enough to compare the line taken by the driver on successive laps of the 
race circuit, and the speed and acceleration measurements are as accurate as 
those from 5th wheel systems.

> You cannot depend on GPS reading to count quite accurate velocity,
> acceleration and accurate position. Sorry.

Conventionally this is true, but there are other ways. IIRC RaceLogic use the 
doppler shift of the GPS signal for speed and acceleration calculations. 
These can also be used to improve the accuracy of the location. I think they 
claim it's good enough to compare the line taken by the driver on successive 
laps of the race circuit.

> Regards,