Very precise position reference, but only 1 hz output.

http://www.scribd.com/doc/91055018/19/Fast-Update-Synchronized-RTK-5-or-10-Hz

Synchronized RTK is the most widely used technique to achieve centimeter-level position estimates between a fixed base station and a roving receiver. Typically, the update rate for Synchronized RTK is once per second (1Hz). With Synchronized RTK, the rover receiver must wait until the base station measurements are received before computing a baseline vector. The latency of the synchronized position fixes is dominated by the data link delay (see Figure2.1). Given a 4800 baud data link, the latency of the Synchronized RTK fixes will approach 0.5 seconds. The solution latency could be reduced by using a 9600 baud or higher bandwidth data link.

The Synchronized RTK solution yields the highest precision possible and suits low dynamic applications such as human-mounted guidance. Airborne applications such as photogrammetry, or aircraft landing system calibration, demand update rates in excess of 1 Hz, to sample the platform trajectory.