627 images were collected by the three cameras mounted on the Microrover Flight Experiment (also known as the Mars Pathfinder Rover or Sojourner). The rover has two monochromatic cameras mounted in front which provide stereo viewing, and a separate 'color' camera mounted in the rear. The rear camera obtains color pictures through the use of red, green, and blue pixels arranged in a 'field-staggered 3G' mosaic filter pattern on the CCD.
During the rover's 83 Sol mission on Mars, it nearly circumnavigated the lander, staying within a 12 meter radius of it.
The Rover Camera EDRs are contained in the RVR_EDR/ directory on the CD. The front left images are stored in the RVR_EDR/RVR_LEFT/ subdirectory, the front right images are in the RVR_EDR/RVRRIGHT/ directory, and the single-banded rear images are in the RVR_EDR/RVR_REAR/ directory. The triple-banded rear images are kept in the RVR_EDR/RVR_CLR/ directory.
Individual filenames consist of the instrument identifier 'R', followed by the seven least significant digits of the spacecraft clock start count, followed by the file extension '.IMG'. The filenames are only unique within their respective directories, not within the entire data set.
Each image file is stored with a fixed length record format. There is an attached PDS label at the beginning of each image file that describes the content and format of the image. If the PDS label size is not an exact multiple of the file record length, padding is added after the end of the PDS label. Thus, the image object always starts on a record boundary.
PDS labels are object-oriented. The object to which the label refers (e.g., IMAGE, TABLE, etc.) is denoted by a statement of the form:
^object = locationin which the carat character ('^', also called a pointer in this context) indicates that the object starts at the given location. For an object in the same file as the label (as is the case for the Rover EDRs), the location is an integer representing the starting record number of the object (the first record in the file is record 1). For example:
^IMAGE = 22indicates that the IMAGE object begins at record 22 of the file.
A more detailed specification of the rover image file format is given in the ROVERSIS file in the DOCUMENT directory. A complete description of the PDS file format is available from the Planetary Data System. (Complete contact information for the PDS is available in the AAREADME file in the root directory of this CD.)
Image capture was executed by applying low-level functions to manage the acquisition, readout, and optionally, compression of the data. Valid image data were in rows 6 through 489 and columns 0 through 767; row 490 contained dark reference pixels. The data was packetized for telemetry, with enclosed identifiers describing the exposure (in milliseconds) and image region.
Auto exposure determination was indicated by an exposure time of zero. Given an image region, an overexposed image was taken and the brightest (saturated) pixel found. Then a geometric search of exposure times was performed, until an image was found with an average pixel value between 40% and 50% of saturation. Finally, exposure time was reduced by 25% as many times as necessary until no more than 1% of the pixels were within 5% of saturation. An exposure time of 1 indicated that the last computed auto exposure time was re-used. If no auto exposure had been performed since the last wakeup, a new auto exposure time was determined.
As part of the auto exposure, a shift factor was employed to manage the A/D conversion from 12 bits to 8 bits. Proper auto exposure for the front B&W cameras generally required a shift factor of 2, while the aft color camera used a shift factor of 1.
Used optionally on front camera B&W images only, the block truncation coding (BTC) algorithm gave a fixed 4.9:1 compression ratio, helping to reduce the communication time without unduly sacrificing image quality. In pre-flight tests, a S/N ratio of 145:1 was achieved. Compression was performed on 16 column by 4 row blocks of image data. When the BTC compression was erroneously performed on aft camera images (indicated by a command sequence number of zero), a loss of color information resulted.
The images were extracted from the telemetry via 'real time processing', specifically using the VICAR (Video Image Communication and Retrieval) program MPFTELEMPROC. This program obtained the Standard Formatted Data Unit (SFDU) records from the Telemetry Delivery Subsystem (TDS), constructed the image files from the telemetry data, and decompressed them. The rear camera images were rotated 90 degrees within MPFTELEMPROC so that the output image data had its vertical dimension along the image's line dimension, similar to the front camera data. The images were compared to any previously downloaded versions to determine which contained the most data. The versions with the greatest amount of data were retained. The program resulted in VICAR formatted image files with a subset of descriptive label items. The label information was then supplemented with data derived from the mission catalog and SPICE kernels.
The rear camera images are archived in both single-band and triple-band format. Each pixel in the rear camera CCD is designated a specific color: red, green, or blue. The VICAR program MPFRVRCLR uses a color map to split these into three separate bands. The missing pixels are filled in using bilinear spatial interpolation.
Finally, each image was run through the VICAR program MPFPDSLBL to convert the labels to PDS format.
The Rover Camera Experiment Data Records were created at the Multimission Image Processing Laboratory at the Jet Propulsion Laboratory.
The Martian Local Level Coordinate Frame is a right handed, orthogonal, frame whose origin is co-incident with the origin of the Lander Coordinate Frame. The XM axis points north, the YM axis points east, and the ZM axis points down. This system is defined relative to the Mars areocentric coordinate system.
For more information on Mars Pathfinder coordinate systems, see the [MELLSTROM&LAU1996], [WELLMAN1996B], and [VAUGHAN1995] references.
|Address:||Planetary Data System, PDS Operator
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
The real time processing of the VICAR-formatted Rover data was done using the VICAR suite of image processing programs. The latest version of the VICAR program XVD may also be used for displaying PDS formatted images. For information on obtaining VICAR, please contact:
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
As mentioned above, there were a four instances when the rear camera acquired images in an emergency mode and erroneously compressed them. As a result, the color information was lost for these images, and they were therefore not converted to three-banded color images. The rear camera images which exist as single-banded images but which were not converted to three-banded images are the following:
Early in the mission, there were a few rear camera images in which one scan line of the images was skipped; however, this was due to a commanding error, not an error in the onboard software.
During the mission, some data packets were lost (indicated in the image labels by the RECEIVED_PACKETS value being lower than the EXPECTED_PACKETS value); this was due to communication problems between the lander and rover, or the lander and Earth.
As the mission progressed, dust accumulated on the lenses, so artifacts did become visible during the mission. This was expected.
A table of the rover images received and relevant comments about their quality is available in the 'Flight Rover Image Status Report', contained in the DOCUMENT/ROVERDOC/DOWNLINK/TABLIMAG.HTM file on this CD.
Complete details of the rover clock problem are provided in the data set description for the Rover Engineering data (RVRENGDS on this CD).