Measurements on Active Cold Loads for Radiometer Calibration

Two semiconductor active cold loads (ACLs) to be used as cold references in spaceborne microwave radiometers have been developed. An X-band frequency was chosen, and the target noise temperature value was in the 50-100-K range. The ACLs are characterized in the operating temperature range of 0degC-50degC, and long-term stability is assessed. To this end, a test bed has been developed. This test bed is actually a stable radiometer, and its design and performance are discussed. The test setup is described, and test campaign results indicate output temperatures of 77 and 56 K for the two ACLs. The temperature sensitivity is slightly below 0.4 K/degC for the units, and long-term stability within 2 K/year is observed.