CCSD3ZF0000100000001NJPL3IF0PDSX00000001 PDS_VERSION_ID = PDS3 RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 80 SPACECRAFT_NAME = MAGELLAN TARGET_NAME = VENUS OBJECT = DATA_SET DATA_SET_ID = "MGN-V-RDRS-5-DIM-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "MGN V RDRS DERIVED DIGITAL IMAGE MAP DATA RECORD V1.0" DATA_SET_COLLECTION_MEMBER_FLG = "N" START_TIME = 1990-09-15 STOP_TIME = 1992-09-14 DATA_SET_RELEASE_DATE = 1994-06-01 PRODUCER_FULL_NAME = "HAIG F. MORGAN" DETAILED_CATALOG_FLAG = "N" DATA_SET_DESC = " Data Set Overview ================= The FMAP is a full-resolution (75 meters/pixel) global mosaic produced by the U.S. Geological Survey from Magellan F-BIDR data. The complete dataset consists of 340 quadrangles in Sinusoidal Equal-Area projection. Quadrangles extend approximately 12 degrees in latitude, except for those between 84 and 90 degrees North and South. Quadrangles near the equator extend 12 degrees in longitude; at higher latitudes, the longitudinal extent is increased to maintain a roughly constant number of samples. The data between 84 and 90 degrees North are also available in Polar Stereographic projection on a separate volume. Parameters ========== N/A Processing ========== The FMAP was produced by using the Planetary Imaging Cartography System (PICS). Processing consisted of the following steps: 1) Verification of the validity of the F-BIDR tapes used as input. Each tape was checked for readability, correct number of files and records per file, and agreement between label information in the files and written labels on the tape cover. 2) Latitude-dependent trim of pixels at the edges of each line of data. The number of pixels trimmed, NTRIM, was calculated as follows: NTRIM = NINT( ( NSAMPS - WIDTH ) / 2 ) where NSAMPS is the number of valid pixels on the line and WIDTH = ( 0.204 / SCALE ) * ( 1 + OVER / 100 ). Here, 0.204 is the nominal width of the BIDR swath in degrees, so that (0.204/SCALE) is the nominal width in pixels. OVER is the percent overlap. OVER=30 was used for latitudes between -60 and 60 degrees, and OVER=10 was used outside this range. 'Standard' and 'substandard' pixels flagged in the BIDRs were treated identically in this process. 3) Spatial filtration to suppress radiometric artifacts caused by by errors in the correction for the antenna sensitivity pattern (see CONFIDENCE_LEVEL_NOTE). Filtration consisted of the following steps. (a) For each pixel, the average data value in a box 701 lines (701 lines was chosen based upon visual testing to produce a cosmetically appealing product) by 1 sample, centered on that pixel, was computed. (b) The average of these averages for all pixels on a line was computed. (c) The box average for each pixel was subtracted from the pixel data value, and the average-of-averages was added. 4) Reprojection of the trimmed and filtered data to the appropriate center longitude of the mosaic being made. Reprojection was done by using nearest-neighbor interpolation. 5) Mosaicking of the data. The feathering algorithm developed by Mike Girard of JPL and utilized in Magellan Project mosaics was used [MGNFRD1992REVC]. This algorithm, applied independently to each pair of overlapping BIDRs and to each line, may be summarized as follows: (a) The average data value for each image in the region of overlap, Ai, was computed, as was the average of these averages, A. (b) A correction was added to the data values in each BIDR. This correction was chosen to vary linearly from zero at the midline of each BIDR to (A-Ai) at the midline of the overlap area. Data values farther from the overlap region than the midline of the BIDR were left unchanged. (c) The corrected data values were written to the output. Values from the left image were used to the left of the midline of the overlap, and values from the right image to the right of the midline. Ancillary Data ============== N/A Coordinate System ================= N/A Software ======== Refer to the SOFTINFO.TXT file in the SOFTWARE directory on this CD-ROM volume for information on the software provided with this data set. Media/Format ============ The FMAP dataset will be delivered to the Magellan Project (or its successor) using CD-ROM media. Formats will be based on standards for such products established by the Planetary Data System (PDS) [PDSSR1992]. " CONFIDENCE_LEVEL_NOTE = " Overview ======== These image mosaics should be considered a preliminary product. They were assembled by the U.S. Geological Survey from Magellan Full-resolution Basic Image Data Records (F-BIDRs) [PETTENGILLETAL1992] [SAUNDERSETAL1992] processed and provided by the Jet Propulsion Laboratory. There are several known problems with the Magellan image data that either have led to reprocessing of a subset of the FBIDRs or that may be of sufficient importance to warrant future processing. The mosaics on this disk were created on a 'best effort' basis, using the most current versions of the constituent FBIDRs available to the USGS at the time of processing (1993-1994). Completed mosaics were visually inspected for quality; where known problems were identified, and alternative versions of the component F-BIDRs were available, the mosaics were reprocessed with these versions. The present FMAP may be superseded by later versions utilizing reprocessed and improved data. The major known problems with the Magellan image data are the following: 1) The synthetic-aperture radar (SAR) image data were reprojected from the radar coordinates (delay-doppler) onto the surface of Venus by using information about the spacecraft location based on a priori predictions and earth-based tracking; self-consistency of the image data from multiple orbits was not used to refine the ephemeris accuracy as is commonly done with optical imaging data. The FBIDR data contain known positional errors of up to several kilometers, and, in particular, discontinuities of this magnitude between adjacent BIDRs whose ephemerides were processed separately. Reprocessing of the Magellan SAR data with ephemerides refined by matching features in multiple BIDRs is technically feasible and might be carried out at some future date. For the orientation of the Magellan spacecraft, only a priori information was available. The positional accuracy of SAR imaging is not dependent upon knowledge of the spacecraft orientation and hence was not affected. Orientation information is needed, however, to correct for the varying sensitivity of the instrument across the width of the radar beam. There are radiometric (tonal) mismatches between almost all neighboring F-BIDRS because of errors in the correction for beam sensitivity. Additional, more severe tonal variations resulted from errors in the topographic model used to project the data into map coordinates. See DATA_SET_DESC for a description of the spatial filtration and feathering techniques used to suppress these tonal variations. The combination of filters used does not lead to a systematic change in radiometric calibration (i.e., the average data value in bland regions is left unchanged) but it can lead to localized artifacts. Bright streaks appear to the North and South of dark surface features, and vice versa. 2) A topographic model of the surface of Venus is needed in order to project the SAR data into map coordinates. Most BIDRs were processed with a topographic model based on pre-Magellan (Pioneer Venus and Venera 15/16) data; the limited resolution of this dataset leads to planimetric inaccuracies in the BIDRs and FMAP mosaics. During the Magellan mission, an attempt was made to process the BIDRs with a higher resolution topographic model (version 5.0) based on Magellan data. This topographic model proved to have a higher resolution than could be handled by the Magellan SAR processor, leading to severe interpolation errors and distortion of the data. The affected BIDRs (orbits in the range 2696-3282, crossing the equator at longitudes ~76-194 degrees; not all orbits in this range were affected) were reprocessed with the earlier topographic model, and the reprocessed versions were used in this mosaic when available, but the FMAP may contain some data with this defect. 3) An error in the Magellan SAR processor resulted in the production of BIDRs with limited dynamic range (specifically, all data values less than about 75 DN were set to this value). This problem affected orbits 2366-4515. Three blocks of data in this range were obtained with illumination from the west ('left looking SAR') and were used in the FMAP. These were orbits 2465-2586 (longitudes 30-54 degrees at the equator), 3669-3962 (272-331 degrees) and 4031-4515 (345-82 degrees). Again, reprocessed versions of these BIDRs were incorporated in the mosaic as available, but some erroneously processed data may be present. It was not feasible to track the version of each F-BIDR incorporated in the FMAP. Review ====== The FMAP will be reviewed internally by the Magellan Project prior to release to the planetary community. The FMAP will also be reviewed by the PDS. Data Coverage and Quality ========================= The FMAP incorporates all Magellan images of Venus obtained with the nominal incidence angle profile for left-looking (illuminated from the west) observations. Data obtained during the first 243-day cycle of operations cover approximately 84% of the surface; some additional left-look data filling gaps in this coverage were obtained in the second cycle [SAUNDERSETAL1992] and third cycle, resulting in a total coverage of ~92% of Venus. The division of the mosaicked data into quadrangles covering 12 degrees of latitude and longitude at the equator, and the division of the quadrangles into tiles 2 by 2 degrees at the equator is described under the keyword DATA_SET_DESC and in more detail by [KIRKETAL1993A]. For a variety of reasons, data may be unavailable for portions of the FMAP ranging from a few kilometers in north-south extent to entire F-BIDRs. Regions of missing data are flagged by a data value of 0; valid data are in the range from 1 to 250. The F-BIDRs contain information flagging certain pixels as 'substandard' because they contain information from less than four statistically independent 'looks' at the surface [PETTENGILLETAL1991]. Both standard and substandard pixels were used identically in producing the FMAP and are not flagged in it. Limitations =========== N/A " END_OBJECT = DATA_SET_INFORMATION OBJECT = DATA_SET_TARGET TARGET_NAME = VENUS END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_HOST INSTRUMENT_HOST_ID = MGN INSTRUMENT_ID = RDRS END_OBJECT = DATA_SET_HOST OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "KIRKETAL1993A" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MGNFRD1992REVC" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "PDSSR1992" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "PETTENGILLETAL1991" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "PETTENGILLETAL1992" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "SAUNDERSETAL1992" END_OBJECT = DATA_SET_REFERENCE_INFORMATION END_OBJECT = DATA_SET END