First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: On-Orbit Radiometer Characterization

The WMAP satellite has completed one year of measurements of the Cosmic Microwave Background (CMB) radiation using 20 differential high-electron-mobility-transistor (HEMT) based radiometers. All the radiometers are functioning nominally, and characterizations of the on-orbit radiometer performance are presented, with an emphasis on properties that are required for the production of sky maps from the time ordered data. A radiometer gain model, used to smooth and interpolate the CMB dipole gain measurements is also presented. No degradation in the sensitivity of any of the radiometers has been observed during the first year of observations.