Spaceborne Moderate
Resolution Spectrometer Zhimin
Zhang The space-bone Moderate Resolution Imaging
Spectrometer, which is presently under development, is a remote-sensor,
for acquiring dynamic spectral and image data from the surface of land,
water and from lower atmosphere. Shanghai Institute of Technical Physics, Academy of Sciences 500 Yu Tian Road, shanghai 200083, China Technical Specification Table 1 summarizes the specifications of the sensor. Thirty contiguous spectral channels cover the visible (VIS) and near infrared (NIR) bands. Design features The design is derived from the 91-band Moderate Resolution Imaging Sepectrometer that was developed by Shanghai Institute of Technical Physics and was successfully test flown in November 1993. The detectors of the 91-band Moderate Resolution Imaging Spectrometer were constructed from the detector arrays. The FPA is adopted with a 22 line parallel scanning scheme for the Space-borne Moderate Resolution Imaging Spectrometer.One axis of the FPA is for the spectral dimension and the other for the spatial dimension. A continuously rotating, double-sides scan mirror is used to parallel scan across the flight track. A double optical paths scheme, as shown in Fig. 1, is used: once for the infrared (IR) channels and once for VIS and NIR channels. Since the whole target scene needs to be focused on the FPA, the off-axis imaging quality of the system is very important. To obtain high optical efficiency throughout the wide detection spectrum of the sensor, reflective and flat field device optical components are used. The signal of IR channels are focused by the optical system and are imaged onto four detector arrays each consisted of 22 elements, in the infrared FPA. Three thermal IR arrays use photoconductive Mercury Cadmium Telluride (PC MCT) derives, while the short wave infrared (SWIR) used photovoltaic mercury Cadmium telluride (PV MCT) detectors with a self-scanning readout circuit. Individual thin film interference filters with different spectral response functions are inlaid on the array. The infrared FPA is in a box cooled to 80 K by balanced Sterling cycle coolers. The VIS and NIR channels are focused by other optical path and imaged on plane with two slots. Beams, one for VIS and the other for NIR from the slots, are reflected by a prism and then passed to individual concave grating. The VIS signal (0.4-8um) is imaged onto 22x20-element silicon FPA by a grating. The 22 elements in one dimension of the FPA detect the 22 spatial pixels of the target by means of parallel scanning, while the 20 elements in the other dimension acquire the spectral information. The NIR signal (0.84-1.04um) is imaged on the silicon FPA with 22x10 elements by another concave grating. Similarly, the 2 elements detect the spatial pixels, and the 10 elements of the other dimension acquire the NIR spectral information. An electronic timing circuit drives the signal output from the FPA. The signal is amplified by a low noise amplifier and then passed to an A/D converter. The spectral images data are buffered in a Ping-Pong RAM and then output in special format with a data rate of 7.2 Mbps. The main features of the instrument design are:
Using an integrating sphere and radiation black body, some measurements have been made to validate the design. The results show signal-to-noise ratios of 700 that are better than requirements. From the screen of a ground detecting equipment designed specially for checking the sensor, clear target images were seen. To ensure the quality of raw spectral images of the sensor, the inherent non-uniformity of the FPA must be corrected in real time. Because the double-side reflective scanning mirror is used, the non-uniformity of reflectance of double sides should also be corrected. The method of correction for non-uniformity in real time has been developed. A ROM device is used to save the correcting table. The input of the ROM address lines consists of the output of the 12 bit A/D, the flag bit of a side of the double-sides scanning mirror, and the timing logic address of each detector readout circuit of the FPA. The output of the ROM is the 12 bit data that has been corrected. The results of measurements indicate that the non-uniformity of raw spectral image output by the Moderate Resolution Imaging Spectrometer is about 1% in laboratory using the integrating sphere radiation source. Airborne test flights had been carried out in summer this year to check the Moderate Resolution Imaging Spectrometer design and applications. The 45 G byte data, obtained from 4 flights totaling 12 hours and recorded by an EXABYTE 8900 tape in the rate of 7.2 Mbps, have shown that the performance of the Moderate Resolution Imaging Spectrometer meet the requirements and that high image quality has been achieved. The preliminary applications show that these data are useful.
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