A pointing mirror internal to the instrument allows different scanning modes. In the nominal mode, the entire planet is covered by contiguous footprints about 22 km across in about 12 hours. Other modes are used to obtain multiple zenith angle coverage, for dust and condensate cloud studies, and radiometric calibration. Figure 4.7 shows a schematic drawing of the instrument and the table below its principal characteristics. A steerable mirror achieves the scanning modes described above and allows frequent views of a black body calibration target.
Cold space is viewed during the rotation of the spacecraft to obtain a zero radiance reference.
The instrument design has no moving parts except a scan mirror and an optical chopper in the infrared module. The system will use commercially-available amplifiers and planar filters in a hybrid configuration to minimise the size and mass. Part of the signal power is split off to two 20 MHz bandwidth filter channels at 80 and 150 MHz from the line centre, to measure the baseline. The infrared channels are similar in concept to the infrared sounder instrument on the Pioneer Venus Orbiter. The infrared radiation transmitted by the dichroic beam splitter passes through a field stop into a simple Littrow grating spectrometer before being focussed onto the focal plane.