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| Firstly, convergence time should not be confused with the amount of time that it takes for the receiver to start tracking satellites. This step is known as "acquisition", and it is usually quoted as "Time To First Fix", or "Cold/Warm Start time". This is a necessary step before any kind of positioning (GPS, WAAS, DGPS or RTK) can happen.
In the early days of RTK, the receivers (particularly single frequency systems) needed to do a lot of averaging of the measurements in order to reach the required level of accuracy and, more importantly, reliability. In order for any RTK receiver to go from a "normal" DGPS position to a more accurate RTK quality position, they need to perform a step in the computation known as "fixing integers", "ambiguity resolution". The amount of time it takes to perform this step is typically known as "convergence time".
Current RTK systems typically quote their time to start up as "Initialisation Time" instead of convergence, but it is basically the same thing. Here are some examples of what is on the web at the moment:
Leica quotes "Initialization typically 8 seconds."
http://www.leica-geosystems.com/common/shared/downloads/inc/downloa...
Trimble "With RTK, it takes one minute to initialize. "
http://www.trimble.com/ag_gps.shtml
Novatel states "Rapid Time to Narrow Lane (TTNL) at variable baseline lengths", and I guess "rapid" means faster than the 60 seconds quoted for the single frequency "RT-2 L1TE" on the same page.
http://www.novatel.ca/products/advanceRTK.htm
Javad have an interesting article that gives a great explanation of some of the issues with RTK. About three quarters down the page I found this: "Everything else being equal (i.e. for a given set of numbers for all other factors), it may require 10 seconds to resolve ambiguities with 99% confidence but 100 seconds to get to 99.9% confidence."
http://www.javad.com/jns/index.html?/jns/gpstutorial/Chapter4.html
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Convergence time for best repeatibility using CORS
