RESURS/SCARAB 2 Appendix C.4

ScaRaB is a 4-channel cross-track scanning radiometer with angular resolution of 48 milliradians. Scanning is obtained by rotation of a cylinder (the rotor) carrying the optics, filters, detector, choppers and analog-detection electronics about an axis parallel to the direction of rnotion of the spacecraft, within a cylinder (stator) rnounted on the spacecraft. The four channels (Table C.4-1) include two broad spectral bands from which the reflected SW and emitted LW radiances are derived, and two narrower bands similar to operational weather satellite imager channels, one corresponding to the infrared atmospheric window, the other to the visible (green to red) portion of the solar spectrum. Note that the upper 50-m limit of the TW channel is nominal; there is no abrupt cut-off in spectral response and no practical way to measure the response in this range. The lower 0.2-m limit of SW and TW channel response corresponds to the cut-off in reflectance of the aluminum mirror optics.

Daytime radiation in the LW band (nominally 4 to 50 m) is determined by appropriately weighted subtraction of the SW signal from the TW signal. Note that although the ERBE scanner had a LW channel, the unfiltered LW radiances were in practice determined from such TW-SW subtraction. The CERES scanner has an IR window channel in place of the ERBE LW channel, and LW radiances will also be determined by spectral subtraction.

The four channels of identical structure are mounted in parallel inside the rotor cylinder, perpendicular to its axis. Incoming radiation is focused by a spherical aluminum mirror directly onto the detector field stop, with no secondary reflection. Each detector is mounted at the prime focus of a telescope with a single mirror, and each optical system is centered so as to be insensitive to polarization. The scanning is produced by rotating the ensemble of the four telescopes around an axis parallel to the satellite's orbital motion vector. The field of view is defined by a square field stop, whose diagonal is parallel to the scan direction. Half-diagonal overlapping of pixels reduces aliasing. For ScaRaB FM1, which flew on Meteor-3/7 at 1200-km altitude, the projection on the ground, at nadir, of the instantaneous field of view (pixel) is a 60-km square, with pixel spacing on a square grid of 42.5 km.

The detectors are windowless pyroelectrics coated with black paint. The radiation arriving on each detector is chopped at the pixel sampling frequency. Each of the two choppers is a rotating hemispherical mirror with two openings, each detector alternately receives radiation transmitted directly from outside and radiation reflected from a small internal reference black body. This is not a calibration source; its temperature is not actively controlled. The temperature of the reference blackbody for channel 3 is measured by a platinum resistance thermometric sonde and included in the scientific telemetry. These data and thermal modeling of the instrument confirm that these temperatures are stable on time scales as long as several scan cycles.

The pixel signal is in fact the difference between the signal integrated (for 20 ms) when the chopper is fully open to the external scene and the signal obtained when it is fully closed and reflecting the internal black body. Note that because the filters used for channels 1, 2 and 4 are located between the chopper and the detector, their thermal emission is eliminated. Radiometer bias is canceled using a space look at the beginning or end of each scan line (depending on the direction of the Sun, which is avoided). It may be noted that the CERES instrument is similar to the ERBE scanner, but with the imperfect broad LW band replaced by an infrared window channel (8-13 m) analogous to, but somewhat broader than, ScaRaB IRW channel 4 (10.5-12.5 m).