A Revised Target Calibration Procedure for Imager for Mars Pathfinder and Surface Stereo Imager for Mars Polar Lander Datasets

Introduction: The Imager for Mars Pathfinder (IMP) returned over 16,000 high-quality images from the surface of Mars in 1997. The Surface Stereo Imager (SSI), a near duplicate of IMP on the Mars Polar Lander, will acquire images near the South Pole of Mars beginning December 3, 1999. Most scientific analyses of these datasets require some level of calibration , either into radiance units, or into units of re-flectance. The instrument calibration algorithm, called CCDCAL during the Mars Pathfinder mission (MPF), removed instrument-dependent sources of noise based on the laboratory calibration, such as flat field, dark current, bad pixels, and electronic shutter. This algorithm was improved to version 2, which includes correction to intensity units based on the absolute respon-sivity, better handling of pixel averaging, and the elimination of known bugs in the original version [1]. The target calibration algorithm, called SPECTCAL during MPF, uses measurements of ra-diometric targets (RT) to calibrate scene images to reflectance units. The RTs have a bull's eye design, with a inner white, middle gray, and outer black ring with a shadow post in the center. The original SPECTCAL was intended to provide a first-order calibration of scene images to reflectance units in order to facilitate spectrophotometric analysis. Since it was designed to be used during operations, several simplifications were required so that real-time analysis could be performed [1]. Now, in order to improve the rigorousness of the Pathfinder calibration, and to prepare for upcoming Mars Polar Lander operations, the target calibration procedure is undergoing significant revision. The revision of the IMP/SSI target calibration procedure falls into three categories: 1) Elimination of software bugs in the original calibration; 2) Revision of correction algorithms used in the original calibration ; 3) Addition of new algorithms to correct for sources of uncertainty that were ignored or unknown in the original calibration. The primary changes to the SPECTCAL v1 procedure are described below. Bugs: Two significant bugs were discovered in the original CCDCAL algorithm. Since all images used for target calibration are first corrected for instrumental sources of noise, errors in CCDCAL affect the results of SPECTCAL. They have been corrected in v2 of the software. Software Offset: The software offset was not removed from the raw image data. A bias of 16 data numbers (DN) was added to each image after readout to prevent underflow during processing. These 16 DN were not subtracted from any image processed with …