COMMENTS ON MARS PATHFINDER IMP EDR CD-ROMS


This document contains comments and errors concerning the Mars Pathfinder IMP EDR image archive published on CD-ROM.

Table of Contents

  1. Command sequence number 1520
  2. Imaging of overlap area between left and right frames.
  3. ATMOS_FLAG images
  4. Duplicate IMAGE_IDs
  5. DN values in excess of 4095
  6. Coordinate systems

1. Command sequence number 1520

There is a command sequence number 1520, whose corresponding observation name is "SUNSET_EVENING_AEROSOL_MEASUREMENTS". This number is the result of a typographical error and is identical with command sequence number 152.

2. Imaging of overlap area between left and right frames.

A few images appear to have a black strip across the bottom of the image, followed by more image data, apparently discontinuous with the data above the line. (An example of this is the image mars/seq0179/c1248xxx/i811350l.img on volume MPIM_0002.) This artifact is due to the way the image frames are laid out on the CCD array. (See the DATASET.CAT file on this CD for more information.) The subframing of these images was erroneously commanded such that the area of six pixels of overlap between the left and right frames was captured in the image. Normally this area is not imaged.

3. ATMOS_FLAG images

There is a subset of the sun images known as "ATMOS FLAG" images. (The image ids of these images can be identified from the ATMOS_FLAG field in the COMMAND.TAB file.) For these images, there was no predetermined starting line and sample for image subframing; the IMP camera performed a Sun search, then automatically subframed a 31 x 31 image centered on the Sun. The starting line and starting sample of these automatically chosen subframes was not returned in a timely enough manner to be correctly correlated with the images. Therefore, no FIRST_LINE or FIRST_LINE_SAMPLE values are available for these images.

4. Duplicate IMAGE_IDs

The IMAGE_IDs were intended to uniquely describe a set of observation parameters used to command the acquisition of an image or series of images. Unfortunately, in a few cases IMAGE_IDs were re-used with different command parameters. The telemetry processing software was not designed to keep track of these duplicate commands, so the older values were over-written in the command table. We have attempted to re-construct all of the original IMAGE_IDs and have added a COMMAND_VERSION_NUMBER to the COMMAND.TAB file in the INDEX directory to reflect these multiple versions. While we are fairly confident of the accuracy of the information provided, we cannot guarantee that we have successfully recovered every command, or that the command versions are always in the correct order.

In cases where there are multiple IMAGE_IDs, it is not possible to directly correlate an individual image with the command that produced it. The only way this can be done is to manually compare each of the command parameters for those IMAGE_IDs with the values in the image label (or the EDRINDEX.TAB file). Note that because earlier versions of command parameters will disagree with the most recent version of a command as described in the command table, images with previous version IMAGE_IDs will have the TLM_CMD_DISCREPANCY_FLAG set to TRUE.

5. DN values in excess of 4095

The Mars Pathfinder IMP data is 12-bit data, and should nominally have a DN range of 0 to 4095. However, due to the lossly JPEG compression algorithm used on board the spacecraft, it is possible to generate DN values in excess of 4095. The label item ERROR_PIXELS will identify the number of pixels that have a DN value greater than this.

6. Coordinate systems

There has been a great deal of debate about which coordinate system should be used to define camera pointing and solar position values. The system chosen for this dataset, while not a Project defined coordinate system, is believed to reflect the most common usage in the planetary science community, and additionally matches the system used for the Viking Lander data. Further details on the coordinate systems are provided in the DATASET.CAT file, which can be found in the CATALOG directory, and the IMP EDR Software Interface Specification (EDRSIS), which can be found in the DOCUMENT directory.