What is contained in "GPS Corrections"?

Nate The Surveyor, post: 333915, member: 291 wrote: Is it a complete copy of the GPS signal(s) received at the base? That is, is it a direct copy? Or is it a refined version of it? Just what is contained in "GPS Corrections"? This is from DGPS, or Differential GPS.
It is something we use, but what is it?

Thanks!

Nate

A GPS vector is tight, but the endpoints are uncertain.
If you constrain one endpoint, the other endpoint becomes "tight".

Collect a bunch of these vectors, put them in a Least Squares network and voila, you have good points.

As I understand it DGPS, or differential GPS, only uses the L1 C/A code to position the rover with a precision of (depending on the equipment) 30cm or more, typically "sub meter". The message that is sent from the base to the rover is the range correction to each satellite based on comparing the position derived from the pseudoranges to the position that the base is constrained to.

RTK, on the other hand, transmits the actual observation data from the base to the rover, as well as the base position (the position isn't transmitted every epoch, which is why, if you use Trimble, you frequently hear "waiting for information from the base" on startup - it's waiting for the position message). I don't know if there are any refinements made to the base observations before they're transmitted, or if things like Doppler are accounted for. The rover calculates the vector by fixing the carrier phase ambiguities and constrains the positions to the base position.

From what I've read when L5 is fully implemented both DGPS and RTK will be greatly enhanced. I haven't tried using L5 for RTK yet and there aren't enough satellites transmitting it to have dedicated L5 DGPS receivers.

As someone posted here a couple weeks ago, there's a new edition of Jan Van Sickle's "GPS For Land Surveyors" out - if you really want to learn about GPS I'd highly recommend it. It explains GPS in practical terms and you don't have to be a math major to understand it.