NSSDCA ID: PSPG-00772
Availability: Archived at NSSDC, accessible from elsewhere
This description was generated automatically using input from the Planetary Data System.
This CD contains portions of the MOC Decompressed Standard Data Product (DSDP) Archive, a collection of decompressed images from the Mars Orbiter Camera on the Mars Global Surveyor spacecraft. Images are stored with PDS labels, but are otherwise unprocessed and uncalibrated. This CD contains also ancillary data files and browse images in a JPEG format, HTML documents that support a web browser interface to the CDs, an index file ('imgindx.tab') that tabulates the contents of the CD, and documentation files. For more information on the contents and organization of the CD volume set refer to the 'CD CONTENTS, DIRECTORY, AND FILE NAMING CONVENTIONS ' section of the aareadme.txt file located in the root directory of the data volumes. Using a web browser, open the 'index.htm' file located in the 'root' directory of the CD. The HTML document will direct you to other informational documents and the image browser for rapidly viewing the image collection. Parameters ========== Although this dataset has not been calibrated, and the algorithms for calibration are still being developed, we here describe some of the relevant calibration parameters. The MOC uses programmable gain and offset states, commanded on the ground prior to image acquisition, to condition the CCD output signal prior to its digitization to 8 bits. The very wide potential dynamic range of MOC images has required a large number of gain states (16 for the NA and 20 for the WA) and offset states (256 possible) compared to, for example, the Viking cameras, which had only two gain and two offset states. This leads to the operational complexity of predicting the scene brightness in advance and selecting appropriate parameters. The GAIN_MODE_ID and OFFSET_MODE_ID fields in the image headers describe the gain/offset selection. The GAIN_MODE_ID is a two-digit hexadecimal number which is the value of the MOC hardware register that selects the gain. The allowable flight values are Narrow Angle gain hex gain hex ---- --- ---- --1 F2 7.968 EA 1.465 D2 11.673 CA 2.076 B2 16.542 AA 2.935 92 23.386 8A 4.150 72 33.067 6A 5.866 52 46.740 4A 8.292 32 66.071 2A 11.73 12 93.465 0A Wide Angle gain hex gain hex ---- --- ---- --1.000 9A 16.030 96 1.412 8A 22.634 86 2.002 7A 32.092 76 2.832 6A 45.397 66 4.006 5A 64.216 56 5.666 4A 90.826 46 8.014 3A 128.464 36 11.34 2A 181.780 26 16.03 1A 256.961 16 22.67 0A 363.400 06 where the gain value given is the nomimal multiplicative factor from the lowest gain state. The OFFSET_MODE_ID is the value of the MOC hardware register that selects the offset. Offsets are commanded in units of 5 (five) Data Numbers (DN), so an OFFSET_MODE_ID of '1' would correspond to a DN offset of 5. All offsets are positive. The simplified MOC response equation (without pixel-to-pixel variation terms) is as follows: dn = a*(r*ex+dc*ex+g)+(z-off) where r is the average signal level being generated at the focal plane (in DN/msec at minimum gain), z is the fixed zero offset, off is the commanded variable offset in DN (note that the offset is subtracted), dc is the dark-current term (in DN/msec at minimum gain), g is the gain-dependent offset (in DN at minimum gain), a is the system gain (where minimum gain is 1 and all other gains are >1, as given in the above tables), and ex is the exposure time (given in the image headers as the LINE_EXPOSURE_DURATION.) In-flight values for the fixed parameters in the above equation are still being derived from flight data. The values from ground testing at ambient conditions are system z dc g NA prime 25.5767 -0.0529099 0.381963 NA spare 28.934 -0.0099495 0.371922 WA red 27.5633 0.0013369 0.196468 WA blue 27.9424 0.0008232 0.264303 The significance of the negative dark-current terms for the NA systems is suspected to be due to other system noise sources in ground testing; the NA systems should have negligible dark current, even at room temperature, because of the short exposure times. The calibration algorithm will consist of two independent parts: removal of the pixel-to-pixel variation, which causes the visually apparent 'streaking' in the downtrack direction in MOC images, and conversion to either relative or absolute flux units (for purposes of mosaic construction, photometry, etc.) Work is ongoing to define these algorithms. Future volumes will include more information. Processing ========== Processing included packet decommutation, removal of the MOC communications protocol headers, and decompression. No additional geometric or radiometric processing was done. For most of the pre-mapping phase of the MGS mission, data quality did not allow error-free transmission of the instrument data to Earth. The MOC protocols (in particular, the formats for compressed image data) were designed for the bit error rates expected in mapping. As a result, considerable data losses were incurred in the image data. The majority of processing for pre- mapping data was done to minimize the effects of this data loss. These efforts are ongoing; corrections for significant losses may appear on future volumes. MOC image data are broken up into 'packets' of approximately 1000 bytes. A typical data loss is that of one or two packets, due to uncorrectable bit errors caused by noise in the space-to-Earth communications path, momentary loss of receiver lock caused by a transition between the one-way and two-way tracking modes, or loss in the Earth segment of the Deep Space Network. For uncompressed images, a packet loss leads to loss of 'line sync' in the image. Since the amount of actual image data in a packet is variable and cannot be determined precisely without the packet, such errors must be corrected by hand. This has been done for as many images as practical. The majority of NA images were acquired using the lossless predictive compression mode of the MOC. However, when a packet is lost from this compressed data stream, the decompression algorithm cannot realign itself to the compressed pixel boundaries, and must skip ahead to the next sync marker, which occurs only every 128 lines in the image. The effect of decompressing the data between the site of packet loss and the next sync marker is unpredictable, but usually results in either semi-random variations in pixel brightness (with the general morphology of the original image still visible) or essentially random noise patterns. A second type of loss is that of tens or hundreds of packets caused by bad weather, hardware failure, or operator error at the DSN stations, or miscommanding of the telemetry playback on the spacecraft. For these errors in a compressed data stream, over 128 lines of the image were lost, making it impossible to recover even the original downtrack size of the image. Such images are described as 'PARTIAL' in the NOTE field of each image header. The browse images were subsampled via averaging and then auto-ends stretched to create visually acceptable contrast. No other processing was performed. Subsampling was intended to produce an image of an approximately fixed size, so the subsampling employed varied depending on the original image's dimensions. Media/Format ============ The MOC DSDP archive is delivered to the Planetary Data System using CD media. Formats are based on standards for such products established by the Planetary Data System (PDS) [PDSSR1992]. These data are available on-line from the Planetary Data System (PDS) at: ftp://pdsimage2.wr.usgs.gov/cdroms/mars_global_surveyor/mgsc_0001/
Questions and comments about this data collection can be directed to: Dr. David R. Williams
Name | Role | Original Affiliation | |
---|---|---|---|
Dr. Michael C. Malin | Data Provider | Malin Space Science Systems, Inc | malin@msss.com |
Dr. Michael C. Malin | General Contact | Malin Space Science Systems, Inc | malin@msss.com |