CCSD3ZF0000100000001NJPL3IF0PDS200000001 = SFDU_LABEL RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 80 SPACECRAFT_NAME = MAGELLAN TARGET_NAME = VENUS OBJECT = TEXT PUBLICATION_DATE = 1991-02-27 NOTE = "COMMENTS ON MAGELLAN MIDR CD-ROMS" END_OBJECT END COMMENTS ON MAGELLAN MIDR CD-ROMS 1. Purpose of This Document This document is a cumulative summary of comments concerning the Magellan MIDR data set published on CD-ROMs. It contains comments on all the MIDR CD-ROMs published so far. Comments include information of interest to users of the data and notes about any known errors on the CD-ROMs. 2. Comments on volume MG_0001, version 2 2.1 Difference between Standard and Engineering F-MIDRs The ten F-MIDRs on this CD-ROM are not standard products; rather, they are engineering test products generated before delivery of operational software. There are three primary differences between Engineering F-MIDRs and standard F-MIDRs. First, Engineering F-MIDRs consist of 20 orbits whereas standard F-MIDRs contain 24 orbits. Second, Engineering F-MIDRs are not necessarily complete on the trailing edge and may contain missing orbits within the body of the mosaic that have since been processed. The gap at the trailing edge of some of the F-MIDRs is a start of mapping gap and is present because processing began with orbit 376. Gaps in the bodies of the F-MIDRs are present because the Image Data Processing Team (IDPT) was instructed not to wait until all orbits had completed the processing queue. Therefore, orbits that the SAR Data Processing Team (SDPT) had trouble processing appear as gaps in the Engineering F-MIDRs even though the orbit has now been processed. 2.2 Error in VICAR2 Keyword PROJSAMP This CD-ROM has an error in the VICAR2 labels for the mosaic framelets. Specifically, the value for PROJSAMP does not vary from framelet to framelet. 2.3 Error in Pixel Values The eight-bit digital number (DN) for each MIDR pixel on this CD-ROM was computed by taking the estimated backscatter cross section, dividing that value by the value for the Muhleman scattering law at the same incidence angle, converting to decibels, and scaling to a byte. The Muhleman Law has the following expression: sigma naught = A cos(i) / (sin(i) + B cos(i)) ** 3 Values for A and B were meant to be 0.0188 and 0.111, respectively. Through an error, the A value used for the Muhleman Law estimate was 0.0118. As a result of this mistake, the pixel values are systematically about 10 DN (2 dB) greater than they should be according to the planned algorithm. 2.4 VICAR2 Labels of Browse Images The browse versions of MIDRs were created by reducing the original 7168 by 8192 MIDR by a factor of 8. The VICAR2 labels for the browse images do not accurately reflect the size reduction, in that parameters PROJSAMP, PIXSIZ, and SPECLINE still pertain to the original MIDR size. 2.5 VICAR2 Labels of MIDR Framelets Some of the MIDR framelet files have a VICAR2 label embedded at the end of the file, in addition to the normal VICAR2 label at the beginning of the file. This extra label is not documented in the PDS detached label. 2.6 Product ID in VICAR2 Labels In two of the data subdirectories, the product ID given in the VICAR2 embedded labels is not correct. Specifically, the VICAR2 label has an N for north latitude instead of an S for south latitude. The correct IDs are as follows: directory product ID in VICAR2 labels correct product ID --------- --------------------------- ------------------ [F05S335] F-MIDR.05N335 F-MIDR.05S335 [F40S342] F-MIDR.40N342 F-MIDR.40S342 2.7 Error in BROWSE.LBL Files The PDS label files for the browse images on this CD-ROM (all named BROWSE.LBL) contain incorrect entries in the IMAGE object. The keyword LINES, which shows a value of 1024, should have the value 896. The keyword LINE_SAMPLES, which shows a value of 896, should have the value 1024.