In order to measure the absolute brightness temperature of a scene a passive microwave radiometer must be accurately calibrated if the drifts in the instrument receiver gain are not to be misinterpreted as changes in scene brightness temperature. This requires periodic calibration and a method commonly adopted is to view an onboard calibration source and/or cold space. Passive microwave instruments often, however, have large antennas and important considerations are the way these sources are viewed and the errors that arise. These issues are explored in this paper and the overall error that occurs in microwave instruments is quantified. The errors that occur in calibration have a wide range of timescales ranging from an uncertainty in measurement due to system noise to the long timescale deterioration of the onboard calibration source. This can be a change in temperature or emissivity of the source. Another important issue is the location of objects, such as the spacecraft or other instrumentation, in the nearfield of the antenna. These can be particularly damaging for cold space calibration because of the relative temperatures of these objects.