Mike,

On Thu, 8 Dec 2005 13:09:20 -0000, Michael Rigby-Jones wrote:

> I thought that differential GPS required two GPS receivers, one fixed at a known location and the other 
mobile receiver.

That's the whole system (with a link between the two) but the practical arrangement is that you use someone 
else's fixed receiver.  That calculates the error and it's sent by radio of one sort or another, and your 
mobile GPS has a receiver for the Differential data which is then passed to the GPS itself, which then uses 
the error data to provide the corrected fix.

In some places the Differential data is provided free (around the coasts of the USA, I believe) and in other 
places you have to pay for it, either getting it from a broadcast radio signal that carries the data as part 
of RDS (encrypted so only paid-up subscribers can use it) or on some other radio system, possibly set up just 
for the purpose.  I believe in the UK if you want 10cm accuracy DGPS data, you have to pay many thousands of 
pounds a year for it.

Every GPS unit I've seen in recent years will receive DGPS data on its serial port (I think there's a NEMA 
sentence specifically for it), and do the corrections automatically.  The tricky bit is getting the data in 
the first place!

Cheers,


Howard Winter
St.Albans, England


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