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Proceedings Paper

On-Board Radiometric Preprocessing For Multispectral Linear Arrays (MLA)
Author(s): Leslie L. Thompson; Richard A. Tracy; David G. Frankel
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Paper Abstract

Currently, NASA is planning to develop a new generation earth resources survey sensor which uses solid-state line arrays of detectors in a pushbroom scan mode. Precision radiometric calibration of data from detector arrays with thousands of elements presents one of the more difficult systems challenges. In the general case, each element exhibits its own characteristic signal at dark, and its own transfer function (volts out vs. light in). For monolithic arrays of detectors, this pattern of response variations must be accepted as a fact of life; i.e., individual detectors are not accessible to be individually "tweaked" to match responses. A program was undertaken to design, fabricate, and test a real-time hardwired data preprocessor which applies a calibration normalization to each detector in a 576-element linear photodiode array. The raw video data was quantized to 8 bits. The particular algorithm selected was driven by the large element-to-element variations for this particular array. The most significant bit in dynamic range was lost for a majority of the elements leaving an effective dynamic range of 127 counts. A limited number of elements had a dynamic range as low as 43 counts. After normalization and scaling back to 255 counts, various calibration problems were uncovered: (1) due to system noise in recording the calibration tables; (2) due to thermal drift; and (3) due to the original quantization process. In this experiment, noise and thermal drift led to fixed errors in the normalization of responses on the order of ±10 counts out of 255 counts for many of the detectors. This level of coherent error is of course readily observable as a stripe in a pushbroom scan image. Quantization thresholding was a second order error, and was not separable in the test images. These results lead to recommending: focal plane cooling to minimize dark leakage currents and improve dynamic range; the use of some form of multiline averaging to reduce the noise in the calibration tables; and in the case of systems with very large elemental offsets, to use analog offset corrections ahead of digitization to maximize dynamic range for the high offset elements. Application of this experiment to potential spaceflight hardware indicates a significant increase in electronic hardware complexity, as well as some power penalty.

Paper Details

Date Published: 20 August 1979
PDF: 8 pages
Proc. SPIE 0178, Smart Sensors, (20 August 1979); doi: 10.1117/12.957268
Show Author Affiliations
Leslie L. Thompson, NASA Goddard Space Flight Center (United States)
Richard A. Tracy, Westinghouse Electric Corporation (United States)
David G. Frankel, Westinghouse Electric Corporation (United States)

Published in SPIE Proceedings Vol. 0178:
Smart Sensors
David F. Barbe, Editor(s)

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