[OSM-talk] Cooperative Differential GPS

[Stefan Baebler]

Gora Mohanty wrote:
> (Please note that I am copying this message to the OpenStreetMap
>  folk: http://openstreetmap.org 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 kabelweb.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.
>   
As far as I understand from globallocate's (globallocate.com) 
implementation of AGPS in my iPaq it seems that AGPS means obtaining 
orbital data of all GPS satellites in advance. Once downloaded (looks 
like same set of files regardless of the device's position in the 
world!) such data can be used for up to 52 hours to assist the GPS in 
locating the device. This can only mean that the downloaded files 
contain precisely predicted orbital data of all GPS satellites, which 
can drastically shorten the time to get a GPS fix after a "cold start", 
saving time and battery (and potentially lives in marketing speech :) ).

I can try getting the files' content (or URL), but that is most likely 
encrypted somehow, with some part of encryption/decryption done in their 
hardware (GPS receiver chip) to ensure sales of own chipset, and 
stopping others from piggybacking on their orbital data.

As far as i am familiar with Neo 1973, its GPS chipset is also made by 
globallocate (recently acquired by Broadcom), so the principle might be 
very similar to the one i described above.

DGPS requires a fixed, stationary GPS nearby, broadcasting position 
"drifts" (due to atmospheric effects) either via internet or a radio 
channel.
> 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.
>   
Yes, this would (to my understanding) be DGPS, cooperative meaning that 
errors (drifts of averaged position) between stationary GPS recievers 
are interpolated to obtain local correction (drift) data. The denser the 
grid, the more precisely the error can be interpolated to accommodate 
for local atmospheric disturbance.
> 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.
>   
Hope i got this right :)

Stefan