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Two Active Cold Loads (ACLs) for microwave radiometer calibration, operating at X-band, are evaluated with respect to important stability parameters. Using a stable radiometer system as test bed, absolute levels of 77 K and 55 K are found. This paper identifies and summarizes potential challenges and uncertainties in future application of ACLs in air and space borne missions, and important parameters, such as temperature sensitivity and long term stability are addressed. For the devices under test, temperature sensitivities are found to be in the range from 0.2 K/°C to 0.4 K/°C, and typical long term drift up to 1.5 K/year is estimated.