AAtmospheric water vapor is a critical atmospheric constituent for understanding global climate and weather. Variations over a broad range of spatial and temporal scales have proved difficult to characterize. Satellite techniques are needed to complement the radiosonde network to achieve global observations. In this study, we compare water vapor measurements among three methods applied to the lower troposphere (400-1000 mb): GPS radio occultation (RO), AIRS, and radiosonde observation (RAOB). Triple coincidences within a 300-km/3 hour window for observations from the COSMIC/FORMOSAT-3 constellation of six GPS RO satellites, AIRS version 5 retrievals, and radiosonde data from the Integrated Global Radiosonde Archive (IGRA) for the period of 2006-2007 are analyzed. The three measurement techniques differ considerably in horizontal and vertical resolution, with different error characteristics. Our analysis provides valuable information on measurement biases and variances among the three techniques. We will present results showing the difference statistics for GPS RO - AIRS, GPS RO - RAOB, and AIRS - RAOB pairs at several pressure levels and how their differences vary spatially and seasonally. The level of consistency of these results with the expected error characteristics and resolutions will be discussed.