ASTER User's Guide - Aster - NASA

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ASTER Users Handbook Subsystem Band Spectral Range (µm) Spatial No. Resolution, m 1 0.52-0.60 VNIR 2 0.63-0.69 15 3N 0.78-0.86 3B 0.78-0.86 4 1.60-1.70 5 2.145-2.185 SWIR 6 2.185-2.225 30 7 2.235-2.285 8 2.295-2.365 9 2.360-2.430 10 8.125-8.475 11 8.475-8.825 TIR 12 8.925-9.275 90 13 10.25-10.95 14 10.95-11.65 Table 1: Characteristics of the 3 ASTER Sensor Systems. Quantization Levels 8 bits 8 bits 12 bits Figure 2: Comparison of Spectral Bands between ASTER and Landsat-7 Thematic Mapper. (Note: % Ref is reflectance percent). The Terra spacecraft is flying in a circular, near-polar orbit at an altitude of 705 km. The orbit is sun-synchronous with equatorial crossing at local time of 10:30 a.m., returning to the same orbit 10
ASTER Users Handbook every 16 days. The orbit parameters are the same as those of Landsat 7, except for the local equatorial crossing time. 2.1 The VNIR Instrument The VNIR subsystem consists of two independent telescope assemblies to minimize image distortion in the backward and nadir looking telescopes (Figure 3). The detectors for each of the bands consist of 5000 element silicon charge-coupled detectors (CCD's). Only 4000 of these detectors are used at any one time. A time lag occurs between the acquisition of the backward image and the nadir image. During this time earth rotation displaces the image center. The VNIR subsystem automatically extracts the correct 4000 pixels based on orbit position information supplied by the EOS platform. The VNIR optical system is a reflecting-refracting improved Schmidt design. The backward looking telescope focal plane contains only a single detector array and uses an interference filter for wavelength discrimination. The focal plane of the nadir telescope contains 3 line arrays and uses a dichroic prism and interference filters for spectral separation allowing all three bands to view the same area simultaneously. The telescope and detectors are maintained at 296 ± 3K using thermal control and cooling from a platform-provided cold plate. On-board calibration of the two VNIR telescopes is accomplished with either of two independent calibration devices for each telescope. The radiation source is a halogen lamp. A diverging beam from the lamp filament is input to the first optical element (Schmidt corrector) of the telescope subsystem filling part of the aperture. The detector elements are uniformly irradiated by this beam. In each calibration device, two silicon photo-diodes are used to monitor the radiance of the lamp. One photo-diode monitors the filament directly and the second monitors the calibration beam just in front of the first optical element of the telescope. The temperatures of the lamp base and the photo-diodes are also monitored. Provision for electrical calibration of the electronic components is also provided. The system signal-to-noise is controlled by specifying the NE delta rho (ρ) to be < 0.5% referenced to a diffuse target with a 70% albedo at the equator during equinox. The absolute radiometric accuracy is ± 4% or better. The VNIR subsystem produces by far the highest data rate of the three ASTER imaging subsystems. With all four bands operating (3 nadir and 1 backward) the data rate including image data, supplemental information and subsystem engineering data is 62 Mbps. 11
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ASTER User's Guide - Aster - NASA

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