Figure B.1

Depiction of the generic electro-optical sensor.

Since the sizing of scanning mechanisms is driven principally by geometric considerations, technology plays a secondary role in determining size and mass. Technology is certainly a factor in developing lightweight scan mirrors, electromagnetic torquers, and position encoders, but these elements are already well developed and not subject to dramatic improvements. Lightweight beryllium scan mirrors are already in widespread use, for example.

In other cases, specialized performance requirements make it preferable to design sensors with a moving telescope assembly instead of a moving scan mirror. The SeaWiFS (Sea-Viewing Wide Field-of-View Sensor) is designed with a scanning telescope in order to minimize polarization sensitivity and, correspondingly, to maximize the radiometric fidelity of its ocean color measurements.

Still other sensor design approaches dispense with scanning mechanisms altogether by employing wide-field telescope forms. Such designs cannot accommodate more complex telescope optics and detector arrays, but this is often a reasonable trade-off given the current state of technology. Pushbroom designs, utilizing linear detector arrays and optical designs with a moderately wide field of view (~5°) in one dimension, have been in use for some time in sensors such as the French SPOT (Systeme Pour l'Observation de la Terre) satellites; the design remains attractive for high-resolution imaging systems with fields of view to 15°. "Fish-eye" lens designs can be used to provide wider field coverage, but the severe geometric distortion and limited spectral range of such designs usually make them more appropriate for nonimaging applications such as missile launch detection. Pushbroom designs are not suited to moderate-resolution wide-field systems such as MODIS (Moderate-Resolution Imaging Spectrometer) or AVHRR (Advanced Very High Resolution Radiometer), because the extended focal planes of such designs—coupled with relatively short focal lengths—lead to unworkable field angles and geometric distortion problems.

Moving the entire spacecraft with the telescope assembly remaining fixed relative to the body of the spacecraft is yet another scanning alternative, and is particularly suitable for single-sensor small satellites. Here, the

   

rectilinear image. In fact, this technology was developed for wide-angle aerial photography, which has substantial distortion over the field of view of the camera due to its wide field. This example illustrates the increasing potential to use advances in ground processing capability as a means of avoiding development of an otherwise more sophisticated and costly space-borne instrument.



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