[OSM-talk] Cooperative Differential GPS

[simon at mungewell.org]

[Disclaimer, although I work for a company that manufactures GPS equipment
I don't claim to be an expert].

WAAS is broadcast from 2 satellites over the US, and contains a simplistic
model of the atmosphere conditions over the US on a grid based system
(100km I think). This allows a GPS receiver to model the current
conditions and produce a better position fix.


DGPS (as broadcast by US coastguard) is a RTCM correction stream, which
contains similar information about atmospherics but is related to a single
point (the reference station).

DGPS is usually transmited by radio around 110KHz and can be received by a
DGPS beacon (although can be streamed via internet). It is usually fed
into a GPS receiver via serial port.



You can set up your own reference station, but it is not simply a GPS
receiver broadcasting the inaccuracies in it's computed position.

It is more involved than that, you will need a receiver capable of doing
direct carrier/phase measurement (ie. a post processing capable receiver),
at which point the computation might already be in the firmware.

It is possible to kick some consumer grade receivers (garmin etrex for
example) into a debug mode where you get this information, but I am
unaware of any project to compute a RTCM stream from this information.

<subliminal ad ;->
If you want to purchase a pre-built solution see:
http://www.point-inc.com/products/gsr2700rsx.html
</ad>

The further you (the rover) are from the reference station the less
accurate your position fix will be, a base line of less than 20km is
normally required for surveying level accuracy. Corrections are normally
transmitted via UHF/VHF radio modem or GPRS cell modem.


A third option is a virtual reference station which will compute a fake
reference station close to the point at which you are surveying.

Cheers,
Mungewell.