Chopping, scanning, and image motion compensation

The basic radiometer configuration shown in Fig. 5.2.1 functions adequately in the visible and near infrared where radiation from the object under study is likely to dominate radiation emitted from the instrument. However, this must not be tme 

An alternative way to separate radiation from the object under study from that of the instrument is to hold the instrument temperature constant and occasionally intersperse measurements of deep space. For most purposes deep space is a nonemitting sink. The weak emission of the cosmic background ( 2.7 K), of stars, and of galaxies is negligible compared with that of objects in our Solar System. To cancel instrument emission one subtracts deep space readings from those of the object of interest. Deep space observations must occur often enough so that the effect of a residual drift in the instrument temperature, and in atmospheric conditions in case of ground-based observations, can be kept small in comparison with the planetary radiance to be measured. Deep space observations must not occur too often, however, so that data collection can proceed undisturbed in between space observations. The optimum duration of planetary and space measurements depends on the planetary intensity, the instrument temperature, and on the thermal time constants of the components involved. 

While scanning across a planetary disk, deep space observations come naturally just before and after the field of view crosses the limb. If the field of view is permitted to complete a full 360° rotation at uniform angular speed, sufficient time exists to observe a blackbody of known temperature mounted inside the instrument. Assuming the radiometer response to be linear, the planetary measurements can be scaled to the readings from space and the blackbody, permitting absolute calibration (see Section 5.13). 

To demonstrate the principles involved we discuss the conceptual design of a scanning radiometer for a spacecraft in a low, circular orbit. The field of view and 

To an instrument at an altitude, h, above the surface, the planetary disk appears at a half angle, 9, 

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