<i>Most RTK systems can guide for a couple of minutes with out signal from the base.</i>

The EZ Guide 500 and the 262 can go for 40 seconds. The 252 for 20 seconds.

Assume that you lose 1 millimeter per second this way.  40 seconds then equals about 3.5 inches offline.  The JD claim of being able to run for 15 minutes is a downfall more than an advantage (no matter how they spin it) since at the end of fifteen minutes you can be up to 3 feet off.  Wouldn't advocate counting on it.