Performance measurements on active cold loads for radiometer calibration

Two semi-conductor Active Cold Loads (ACLs) to be used as cold references in spaceborne microwave radiometers have been developed. An X-band frequency has been chosen, and the target noise temperature value is in the 50 to 100 K range. The ACLs are to be characterized in the operating temperature range 0-50degC, and long term stability must be assessed. To this end a Test Bed has been developed. This Test Bed is actually a stable radiometer, and its design and development is discussed. The test setup is described, and preliminary test campaign results indicate output temperatures of 73 K and 51 K for the two ACLs. The temperature sensitivity is slightly below 0.4 K/degC for the units, and excellent stability seems to prevail.