Many scanning sensors produce raw outputs in which their response to the signal is superimposed on a large background. The signal changes rapidly, due to the scanned input, while the background varies more slowly, due to thermal drifts and 1/f noise. Whenever such a sensor is used as a radiometer, it is necessary to perform a differential measurement: to measure a known reference and subtract it from the raw signals, cancelling the common-mode background. Calibration is also a differential measurement: the difference between each channel's response to two known inputs is divided by the difference between these two inputs to determine the linear gain of the channel. The GOES-I Imager obtains its background subtraction references by viewing space, with radiance virtually equal to zero, during the turn- around intervals at the ends of scan lines. It used a temperature-monitored blackbody as a second reference to measure the gain. We have verified our analytical predictions by computer simulations. Gaussian and 1/f noise were generated and combined, filtered, and processed using the differential measurement algorithms. Excellent agreement was demonstrated between these simulations and the analytical model.