The radio occultation (RO) sounding technique that uses signals transmitted by the Global Positioning System (GPS) has evolved as an important global observing technology. In this paper, we compare RO refractivity profiles from the CHAMP (CHAllenging Minisatellite Payload) satellite mission with those calculated from radiosonde soundings over five geographical areas, each of which uses a different type of radiosonde. CHAMP RO soundings that occur within 2 hours and 300 km of radiosonde soundings are used during the period from June 2001 through March 2004. The comparison was made between the altitudes of 5--25 km, where RO soundings are most accurate. These results indicate that the RO soundings are of sufficiently high accuracy to differentiate performance of various types of radiosonde. The differences in performance among various types of radiosonde present a challenge for climate analysis. In this regard, RO is a considerably more robust measurement technique for climate monitoring.