The National Weather Service (NWS) is developing the Radiosonde Replacement System (RRS) to replace its antiquated Micro-ART system, which has been in operation since the late 1980s. Currently there are two radiosonde vendors who will be producing 1680-MHz GPS radiosondes for the NWS. Before the new RRS is fielded beginning in 2003, NWS is conducting a series of tests to understand the characteristics of the measurements used by operational and research staff. Some of the techniques to be discussed will be functional repeatability, functional precision, and functional comparison with reference radiosondes produced by NASA. Examples of the types of data acquired during each of these tests will be presented.