Cooperative Differential GPS

Gora Mohanty gora at
Thu Mar 27 18:38:29 CET 2008

(Please note that I am copying this message to the OpenStreetMap
 folk: as we will hopefully find more
 GPS/mapping experts there. Please edit the list of recipients
 if you need to.)

On Thu, 27 Mar 2008 14:24:56 +0100
beren at wrote:

> Hi all!
> After hours of searching the web for information about the new Antaris GPS 
> chip I am still not sure if it will be possible to use the raw data output of 
> the ATR0635 (message type RXM-RAW) on the Neo FreeRunner. This message 
> should, according to the protocol specification [1], contain information 
> about the per-satellite errors necessary to build something like 
> a "cooperative differential GPS" [2]. Such an application could - in theory - 
> greatly improve the accuracy of the Neo-GPS and therefore open it up to a 
> whole new range of use-cases.

Thanks for bringing this up, as this is is a topic that I am very much
interested in. We are just starting to explore this area, but the AGPS
capabilities of the Neo 1973 was one reason that we went ahead and bought
this. However, I do not know what AGPS support means in real terms.

My naive understanding of this is that a cooperative differential GPS needs
no external support. One just sits at a base station, and averages GPS
readings until one knows the position of the base station to an arbitrary
level of accuracy. After that, the reception of realtime GPS signals at
the base station provides information about random errors, which I understand
are largely due to atmospheric fluctuations. Corrections for these are
derived from the difference between the  well-known actual position of the
base station, and the received realtime position. These can be broadcast
through various means, e.g., an Internet DGPS server, and should be good for
a few hundred km around the base station. The cooperative aspect of this
comes in because of the benefits of multiple base stations.

I would love to be corrected on the above by someone who knows better,
as this is little more than a surmise on how DGPS should work.


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