DATA_SET_DESCRIPTION |
Data Set Overview : Mars Pathfinder bounced down and rolled to a stop on the surface of Mars on July 4, 1997. It landed in an ancient floodplain in the Ares Vallis region of Chryse Planitia at 19.17 degrees North latitude, and 33.21 degrees West longitude. (See below for details.) 16,661 images were collected by the Imager for Mars Pathfinder (IMP) camera. The IMP is a stereo camera with twelve filter positions. This permitted the imaging of the landscape surrounding the lander at a variety of wavelengths, lighting conditions, and resolutions. Most of the landscape was also imaged in stereo, allowing for the generation of a topographic map of the area around the lander. A large portion of the image data was collected for a series of panoramas of the landscape. These panoramas differed from each other in terms of the coverage and compression of the data, the number of filters used, and whether they were stereo or monoscopic. Additional imaging was done to support various other investigations. Numerous images were taken of the Sun, Phobos and Deimos, a few stars, and the sky. Images were also taken of the Rover and the sites it investigated. Additional imaging targets included APXS investigation sites, the wind socks, and magnetic and radiometric calibration targets on the lander. Landing Site : Several prominent features are visible around the landing site, which are also visible in Viking Orbiter and Mars Global Surveyor images. These include 'North Knob' 1.7 km to the north of the lander, 'Southeast Knob' 21.2 km to the southeast, 'Big Crater' 2.2 km to the southeast, 'Far Knob' 30.6 km to the south, and the 'Twin Peaks' 0.86 km (North Twin) and 1.0 km (South Twin) to the west. Using these features, the landing site has been pinpointed at 19.17 degrees north and 33.21 degrees west relative to the U.S. Geological Survey cartographic network. However, two-way ranging and Doppler tracking of the lander place the site at 19.30 degrees north and 33.52 degrees west in inertial space, suggesting that the USGS grid may be displaced about 19 km to the north and 7 km to the west. In addition to the above features, other small craters are visible in the orbiter and lander views. 'Little Crater' and 'Rimshot Crater' (later identified as four small craters, A through D), all lie on the northwest outer flank of the rim of Big Crater. The azimuthal directions of these features from the landing site are shown below. Azimuthal Direction Feature Name (in degrees E of N) --------------------------------------------- North Knob 1 - 8 Little Crater (crest) 102 - 112 Southeast Knob 133 Big Crater (rimcrest only) 138 - 159 Rimshot Crater A 156 Rimshot Crater B 153 Rimshot Crater C 158 Rimshot Crater D 150 Far Knob 176 - 180 South Twin 242 North Twin 260 The lander is on the southeast-facing flank of a low ridge, giving the overall landing site (out to about 50-100 meters) a slight slope of two or three degrees, dipping to the southeast. Very distant features to the south and east are in view, whereas relatively nearby features to the north are partially or completely obscured. Only the tip of North Knob, which appears larger in the Viking orbiter images than the Twin Peaks, projects above the local horizon. A 300 m crater, 1.2 km to the northeast, is completely obscured. The radius of Mars at the landing site is 3,389.714 km. Processing : Prior to retrieval of the data on the ground, a number of processing steps were executed onboard the spacecraft, primarily with the intent of cleaning up the data in preparation for compression. They include 'shutter effect' correction, dark current subtraction, flat fielding, bad pixel correction, and sub-framing as necessary. (Although the flat fielding capability existed, it was never actually used.) Compression is also a part of onboard processing, but it will be discussed in the section titled 'Data Compression'. The onboard data processing stream begins by dropping the anti-blooming gates to clear charge in the imaging area of the 512 square CCD, which comprises only the upper 256 rows. Designed for stereo imaging, the first 256 columns in the imaging area correspond to the left image and the second 256 columns are reserved for the right image. Exposure begins when the anti-blooming gates are raised and electrons begin to accumulate in the pixel charge wells. At the end of the set exposure time, the image is shifted row by row to the lower 256 rows in the CCD that make up the storage area, taking less than 0.5 milliseconds. During this time additional electrons build up in the active area, causing the last row transferred to have a longer exposure than the first. The DN's are then read out to the frame buffer board, which takes about 1.25 seconds, and digitized to 12 bits. Optionally, a zero-exposed frame can be subtracted at this point. One complete set of left and right IMP images can remain in the frame buffer board long enough to be read by the onboard computer into main memory. This frame transfer capability of the CCD avoids the need for a mechanical shutter by effectively working as an electronic shutter. The complete transfer time is referred to as the 'shutter effect'. Note that the flight software is capable of reading out each 256x256 storage area, but border effects were minimized by ignoring the top two lines and bottom six lines, yielding a nominal image area of 248 lines by 256 samples. Additional CCD features include 4 'null' pixels in the readout register, an 8-pixel wide covered dark strip, and a 2-pixel wide border on each edge of the CCD. They provide for the generation of 4 calibration images that can be used for the calibration of each IMP scene. These are the zero-exposure image, dark current, zero-exposure dark current, and flat field. Only the actual scene exposed image is required for scene correction, since the other images can be generated based on previous exposures. The dark current is the sum of currents produced in the CCD imaging section, the storage section, and the serial readout register, added to the hardware offset. It is an exponential function of temperature and a linear function of integration time. A small light leak into the 8-pixel wide dark strip required modification to the onboard algorithm so that it could be used to scale dark current corrections in later images. A flat field image attempts to model the responsivity variation across the CCD field-of-view, and is a function of wavelength. Because of onboard memory constraints, only one flat fielding correction table was permitted, and was comprised of the average of the tables for spectral filters 5 through 11 (excluding the diopter filter in the right eye). This limits onboard flat field corrections to only filters from which the average was computed. Optimum correction is also limited by the scene noise and the accuracy with which the responsivity variation is known. Bad pixels fall into three categories: hot strip (have higher than average dark current but fail the variability or linearity test), high variability (highly fluctuating dark currents, or fail linearity test), and vignetted pixels (severe vignetting causes order of magnitude higher variability). Their locations are identified in a bad pixel map for corrective application. Filter 0 turned out to be the most critical for bad pixel detection. Key onboard processes were flagged, and they include the following: AUTO_EXPOSURE_DATA_CUT .......... A value indicating a base for pixel values using AUTO_EXPOSURE_PIXEL_FRACTION as the percentage of pixels wanted above that value. AUTO_EXPOSURE_PIXEL_FRACTION .... Fraction of pixels wanted to exceed value of AUTO_EXPOSURE_DATA_CUT, expressed as a percentage. Field is valid only if value of EXPOSURE_TYPE is 'AUTO' or 'INCR'. BAD_PIXEL_REPLACEMENT_FLAG ...... Indicates whether or not bad pixel replacement processing was completed. If set TRUE, certain pixels of the image will be replaced based on the bad pixel table. DOWNLOAD_TYPE ................... Specifies which image data to download; any or all of the Left or Right image data (IM), dark current strip (DS), and null pixel data (NS). EXPOSURE_COUNT .................. Maximum number of exposures to take. Valid values are dependent on EXPOSURE_TYPE. FLAT_FIELD_CORRECTION_FLAG ...... Indicates whether or not flat field correction processing was completed. If set TRUE, the image has been adjusted by a flat field correction image. SHUTTER_EFFECT_CORRECTION_FLAG .. A command flag set in the IMP Flight Software Command to remove the 'shutter', or fixed-pattern, from the image. SQRT_COMPRESSION_FLAG ........... Flag for on-board square root compression of a 12-bit pixel down to an 8-bit pixel. 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 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. Finally, each image was run through the VICAR program MPFPDSLBL to convert the labels to PDS format. The IMP Experiment Data Records were created at the Multimission Image Processing Laboratory at the Jet Propulsion Laboratory. Data Compression : Because the IMP can generate data at a much higher rate than can be downlinked, various compression techniques were used, depending on the nature of the image data. Lossless compression used the JPL-developed Rice algorithm. For lossy compression during non-science or low data rate scenarios, a modified JPEG compressor developed at the Technical University of Braunschweig was used. Nominally, JPEG compression involves a discrete cosine transform (DCT), but can be enhanced by a local cosine transform (LCT). Note that the instrument compression block size for JPEG compression was 8x8, and was variable for RICE compression. For the JPEG compression, there are two types of entropy encoding: Huffman and arithmetic. In the Mars Pathfinder scenario, the possibility of transmission errors using 'adaptive' Huffman coding (in which the coding table is adapted to data statistics and transmitted to the ground) required instead the use of a 'fixed' Huffman table. On the other hand, the arithmetic encoder scheme did not require a table, as it adapted to the input data on-the-fly while using a state machine for probability estimation on both the encoder and decoder side. The JPEG compression was operated via one of two user specified criteria: Quality and Ratio. Specifying a Quality value targets a desired level of image quality which the compression must satisfy. The lower the selected Quality, the higher the gain in compression factor. Use of the second type of criteria, a Ratio, was more straightforward in that it defined the compression factor a priori. On the first Sol, lossy JPEG compression enabled the downlink and assembly of a full 360 degree, 670 nm filter panorama halfway through Sol 1 operations. The compression rates fell steadily after the first week of operation as downlink rates increased above the expected rates. The valid Compression Types are listed below: User Specified Compression Type Encoding Criteria --------------------------------------- ---------- -------------- #1 JPEG Discrete Cosine Transform (DCT) HUFFMAN QUALITY #2 JPEG Discrete Cosine Transform (DCT) HUFFMAN RATIO #3 JPEG Discrete Cosine Transform (DCT) ARITHMETIC QUALITY #4 JPEG Discrete Cosine Transform (DCT) ARITHMETIC RATIO #5 JPEG Discrete Cosine Transform (DCT) HUFFMAN QUALITY (LCT) #6 JPEG Discrete Cosine Transform (DCT) HUFFMAN RATIO (LCT) #7 JPEG Discrete Cosine Transform (DCT) ARITHMETIC QUALITY (LCT) #8 JPEG Discrete Cosine Transform (DCT) ARITHMETIC RATIO (LCT) #9 RICE Adaptive Variable-length Coding - - The number of completed IMP images (ie., images for which 100% of image packets were received) are listed below by Compression Type: Average Average Compression Number of Compression Compression Type Completed Images Ratio Per Image Rate Per Image ------------ ---------------- ----------------- -------------- none 2277 - - #1 6 25.02 0.4871 #2 2842 62.74 0.3200 #3 358 289.99 0.4632 #4 546 39.27 0.3506 #5 (not used) - - #6 (not used) - - #7 46 5873.07 0.0022 #8 6990 9.40 2.8591 #9 3126 1.38 9.2347 TOTAL 16191 46.28 3.6023 NOTE: The ratio multiplied by the rate is not exactly 12 (bits) because the numbers in the table are averaged over the number of images and not the total number of bits. The instrument JPEG compression quantization table, listed in 'zigzag' order, is shown below: 16, 11, 12, 14, 12, 10, 16, 14, 13, 14, 18, 17, 16, 19, 24, 40, 26, 24, 22, 22, 24, 49, 35, 37, 29, 40, 58, 51, 61, 60, 57, 51, 56, 55, 64, 72, 92, 78, 64, 68, 87, 69, 55, 56, 80, 109, 81, 87, 95, 98, 103, 104, 103, 62, 77, 113, Other methods of conserving downlink resources included sub-framing the image, row and column sums, and pixel averaging (in addition to JPEG), all of which were useful for atmospheric science images. For highest compression, these methods were used in combination with each other. Sub-framing was used in returning 31x31 pixel blocks of the Sun. (A subcategory of these images were the so-called 'ATMOS FLAG' images. They are distinct from the regular Sun images in that the location of the Sun was determined automatically on-board the spacecraft. The position of these ATMOS FLAG sub-frames within the CCD array was not returned to the ground.) Row and column sums were used for sky images, as they gave the gradient and the edges of cloud features, but not the high resolution of an image. Pixel averaging was sometimes done prior to data compression. This involved replacing each 'block' of pixels in an image by the average of the pixels in that block. The size of the block or area over which the averaging was done was defined in the horizontal or sample direction by the pixel_averaging_width, and over the vertical or line direction by the pixel_averaging_height. Rectangular blocks were permitted. Although the pixel averaging technique reduces the spatial resolution of an image, the color information was largely retained and was overlaid onto higher resolution monochrome images for 'first look' color characterization. Data : A variety of types of data were returned, and as mentioned in the 'Compression' section, were submitted to lower and lower compression rates as downlink rates increased above the expected rates. Eventually, large sets of image data were returned lossless, contributing to a 'hi-res' panorama called the Superpan. This table from the 'End of Mission Report' provides a detailed correlation between data type and compression rates. The items in the Description column are shown in the format exxx(comp,pa):n, where e:(R)ight or (L)eft eye, xxx : wavelength (nm), comp : compression ratio, pa : pixel averaging size, and n : number of images. 'q' : quality factor and 'los' : lossless (Rice) compression. Only regular images are included in the table; dark strips, null strips, and dark current images are excluded. Observation Description Number of Observation Time Images -------------------------------------------------------------------- R440(12:1,2x2):32 R670(24:1,1x1):19 RL670(12:1,1x1):26 First Look R670(80:1,1x1):15 (Mission Success) R530(12:1,2x2):32 145 Sol 1 Morning Panorama R440(q:09,2x2):07 R670(q:15,1x1):07 R530(q:09,2x2):07 R670(12:1,1x1):30 Predeploy Red/IR R670(q:15,1x1):20 139 Sol 1 Morning Panorama R965(6:1,1x1):89 RL440(los,1x1):168 Insurance R530(los,1x1):84 420 Sol 2 Afternoon Panorama R600(los,1x1):84 R750(los,1x1):84 R440(6:1,2x2):50 R530(6:1,2x2):50 Monster RL670(6:1,1x1):195 395 Sol 3 Various Panorama R750(6:1,2x2):50 R965(6:1,2x2):50 R440(q:65,1x1):33 Presidential R440(6:1,1x1):132 (Gallery) R530(q:65,1x1):33 495* Sol 9, 10, 11 Panorama R530(6:1,1x1):132 Morning R670(q:65,1x1):33 R670(6:1,1x1):132 RL440(los,1x1):238** R480(2:1,1x1):119 R530(2:1,1x1):119 R600(2:1,1x1):119 RL670(los,1x1):238 Super Panorama R750(2:1,1x1):119 1785*** Various L800(2:1,1x1):119 L860(2:1,1x1):119 L900(2:1,1x1):119 L930(2:1,1x1):119 RL965(2:1,1x1):238 L1000(2:1,1x1):119 * excludes 22 duplicate images from two aborted sequence 164 runs ** compression in RL440 began as 2:1 JPEG and was later changed to lossless during acquisition *** includes only complete cubes The returned data types are listed as follows: Pixel Image Size Size Image Type (lines x samples) (bits) Description ------------ ----------------- ------ ------------------------------- Image Data 248 x 256 12 Image data is unsigned and archived top side up. Right images have been rotated counter-clockwise; left have been rotated clockwise. Dark Strip 248 x 8 12 Calibration data retrieved from covered area of the CCD. Null Strip 248 x 4 12 Calibration data retrieved from the readout register of the CCD. Flat Field 248 x 256 12 Flat Field data is unsigned and archived top side up. Right images have been rotated counter-clockwise, left images have been rotated clockwise. Dark Frame 248 x 256 12 Dark Frame data is unsigned and archived top side up. Right images have been rotated counter-clockwise, left images have been rotated clockwise. NOTE: Two additional data types, 'Histogram' images and 'Summed' images, were originally included as possible data types. However, since no images with these data types were returned, they are not described here. Coordinate Systems and Image Geometry : Of the many coordinate systems defined by the Mars Pathfinder Project, four are of particular interest in understanding the IMP data. These are the Mars Pathfinder Lander Coordinate Frame (L Frame), the IMP Coordinate Frame (IMP Frame), the Martian Local Level Coordinate Frame (M Frame), and the Mars Surface Fixed Coordinate Frame (MFX Frame). The Mars Pathfinder Lander (L) Coordinate Frame ----------------------------------------------- The Mars Pathfinder Lander is a tetrahedral structure. One of its faces, the one upon which it sits, is called the base petal and houses most of the lander equipment. The other three faces, or petals, open after surface impact to expose these systems. The rover is mounted on one of these petals. The Mars Pathfinder Lander Coordinate Frame, or 'L' Frame, has the lander base petal as its reference plane and its center coincident with the geometric center of the base petal. The YL-axis of this coordinate system passes through the geometric center of the rover petal, and defines the reference direction. The ZL-axis is normal to the reference plane and coincident with the nominal spacecraft spin vector. When the lander is upright on the surface, the ZL-axis is directed positively downward into the ground. The IMP Coordinate Frame ------------------------ The IMP Coordinate Frame is co-linear with the Mars Pathfinder Lander Coordinate Frame. However, its origin is offset, and is located at (-198.4, -9.0, -436.7) mm with respect to the origin of the L Frame. The XIMP-YIMP plane is defined as the interface plane between the lander structure and the IMP camera base. The Martian Local Level (M) Coordinate Frame -------------------------------------------- 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 sytem is defined relative to the Mars areocentric coordinate system. The Mars Surface Fixed (MFX) Coordinate Frame --------------------------------------------- This coordinate frame is defined as being initially equivalent to the Martian Local Level Coordinate Frame. While the origin of the Local Level frame shifts with the origin of the Lander Coordinate Frame as the lander itself settles and shifts, the origin of the Mars Surface Fixed Coordinate Frame becomes fixed once the initial orientation and position of the lander has been determined. However, in practice, the position of the lander changed so little after the determination of the origin of the MFX frame, that it essentially remained equivalent to the M Frame throughout the mission. Other coordinate systems were under discussion at the time this archive was assembled and may be used for future data releases. The Lander-Surface quaternion defining the conversion between the Lander Coordinate Frame and the Mars Surface Fixed frame was determined four times during the mission. The greatest change occurred between the first and second determinations, during which time the rover petal and airbags were repositioned. The following table shows the four values of the quaternion listed in the NAIF defined order of 'cosine, x, y, z'. (MIPL determinations of camera pointing for images in this archive were done using only the first two quaternions.) Also shown in this table is the direction of North, the tilt of the lander, and the azimuth of the lander tilt, all measured with respect to the Lander Coordinate Frame. North Tilt TiltAz # SCLK cosine x y z (deg) (deg) (deg) --------------------------------------------------------------------- 1 1246747789 0.2100 -0.0169 -0.0169 0.9774 155.75 2.74 147.12 2 1246761006 0.2102 -0.0146 -0.0293 0.9771 155.75 3.75 165.65 3 1247191866 0.2102 -0.0148 -0.0317 0.9770 155.71 4.01 167.09 4 1248364781 0.2102 -0.0149 -0.0333 0.9770 155.71 4.18 168.09 (The first SCLK value listed was chosen as ten seconds before the first image was acquired.) Azimuth and Elevation Determination ----------------------------------- Contrary to the nadir-pointing Z axes of the above coordinate systems, measurements of elevation for camera pointing and solar position in this IMP EDR archive were measured positively upwards from their horizontal reference planes, in order to be consistent with common usage in planetary science and astronomy. This change has been emphasized with the use of the POSITIVE_ELEVATION_DIRECTION : UP keyword in the PDS labels. At the time this CD archive was created, three well-established methods were available for generating instrument azimuth and elevation values (ie., those measured with respect to the lander coordinate frame) from the azimuth and elevation motor clicks. (In the following discussion, azimuth and elevation refer to boresight or camera head pointing, as opposed to the pointing of the individual camera eyes.) These three methods were called the TELEMETRY, MPFNAV-MIPS, and BACKLASH-UOFA methods. The method that was used for the images in this archive was that employed during the realtime processing, the TELEMETRY method, which used the following simple algorithms: IMP stowed: instrument_azimuth : (436 - azimuth_motor_clicks) * 0.5535 instrument_elevation : (elevation_motor_clicks - 173.86) * 0.5535 IMP deployed: instrument_azimuth : (380 - azimuth_motor_clicks) * 0.5535 instrument_elevation : (elevation_motor_clicks - 173.86) * 0.5535 The MPFNAV-MIPS method uses the VICAR MPFNAV program which adjusts the commanded pointing values in a set of image labels until the images mosaic properly. MPFNAV performs the pointing adjustment by minimizing the discrepancy between a set of tiepoints measured across adjacent image borders and their predicted locations based upon a camera model. While this method provides more accurate pointing information by removing random pointing inaccuracies, it is more labor intensive than the TELEMETRY method, and had not been completed for most of the images at the time this archive was made. The third method for determination of instrument pointing is the BACKLASH-UOFA method. This method is based on an attempt to characterize the backlash effect of the camera on image pointing. The University of Arizona has implemented an algorithm to attempt to compensate for this. For the most part, this produces accurate results. The only drawback is that all previous camera movements must be accurately known, which they aren't. Both the lander surface quaternion and the instrument pointing are likely to undergo changes as improved values are determined in the future. The most up-to-date spacecraft and instrument kernels available can be obtained from the anonymous FTP site of the Navigation and Ancillary Information Facility at JPL, ftp://naif.jpl.nasa.gov/. For further assistance, contact Boris Semenov of the PDS NAIF node: Address: Boris Semenov M/S 301-150 Jet Propulsion Laboratory Pasadena, CA 91109-8099 Phone: (818) 354-8136 Email: Boris.V.Semenov@jpl.nasa.gov WWW URL: http://pds.nasa.gov/naif.html FTP Site: ftp://naif.jpl.nasa.gov/ Incidence, phase, and emission angles were not included for the images in this archive. This was primarily due to the difficulty of determining these values in a lander setting and the ambiguities that would be associated with any values provided. Due to the large field of view of the camera and the irregular feature surfaces and landscape variations, the emission angle (and therefore the phase angle) can vary significantly across an image scene. Instead, the position of the Sun at the time each image was acquired has been included in the solar azimuth and solar elevation keywords. From these, users can determine the local incidence, phase, and emission values that will be of particular interest to them. For more information on Mars Pathfinder coordinate systems, see the [MELLSTRAUM&LAU1996], [WELLMAN1996B], and [VAUGHAN1995] references. The [WELLMAN1996B] document in particular provides good coverage of the coordinate systems related to the IMP camera. Please note that where he used elevation ranges of 0 to 180 degrees, the MPF Project used -90 to +90 degrees. Calibration : In addition to the IMP camera calibration discussed in the IMP instrument catalog object, there are a couple of calibration issues relating to targets the camera imaged. Specifically, these are the investigation of wind speed and direction through imaging of the wind socks on the ASI/MET mast, and the magnetic properties investigation performed by imaging the magnets mounted on the lander. The calibration files and algorithms for these investigations are not yet completed. Calibration information for the IMP camera is available in [REIDETAL1998]; information about the magnetic properties experiment is available in [GUNNLAUGSSONETAL1998]. Software : The IMP EDR images can be displayed on UNIX, Macintosh, and PC platforms using the PDS developed program, NASAView. This software is freely available from the PDS Central Node and may be obtained from their web site at http://pds.nasa.gov/. For more information or help in obtaining the software, contact the PDS operator at the following address: Address: Planetary Data System, PDS Operator Jet Propulsion Laboratory 4800 Oak Grove Drive Pasadena, CA 91109 Phone: (818) 354-4321 Email: pds_operator@jpl.nasa.gov WWW URL: http://pds.nasa.gov/ The real time processing of the VICAR-formatted IMP data was done using the VICAR suite of image processing programs. For information on obtaining VICAR, please contact: Address: Danika Jensen M/S 168-514 Jet Propulsion Laboratory 4800 Oak Grove Drive Pasadena, CA 91109 Phone: (818) 354-6269 Email: Danika.Jensen@jpl.nasa.gov WWW URL: http://www-mipl.jpl.nasa.gov/vicar.html Some IMP calibration and processing programs have been written in IDL (Interactive Data Language). IDL is available from Research Systems Incorporated at http://www.rsinc.com/. Media / Format : The IMP EDR data will be stored on compact disc-read only memory (CD-ROM) media. The CDs will be formatted according to ISO-9660 and PDS standards. The data files will not include extended attribute records (XARs), and will therefore not be readable on some older VMS operating systems.
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