Spaceborne Moderate Resolution Spectrometer
Zhimin Zhang
Shanghai Institute of Technical Physics,
Academy of Sciences
500 Yu Tian Road, shanghai 200083, China
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.
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:
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Use of FPA devices with readout circuitry and micro interference filters for the VIS, NIR and IR channels.
- Use of balanced Sterling cycle devices to cool the infrared FPA effectively.
- Use of a double-sided scanning mirror for parallel scanning, and a stepper motor with subdivided frequency drives scan mirror.
- Use of high-order concave and flat field devices for reflective surfaces of the optical systems and the double optical system for infrared and VIS/NIR channels individually.
- Use of the type III concave grating with flat field imaging for the spectrometer, leading to a compact optical structure.
Modular structural design.
- Use of programmable logic devices for electronic data processing.
