CCSD3ZF0000100000001NJPL3IF0PDS200000001 = SFDU_LABEL RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 80 SPACECRAFT_NAME = MAGELLAN TARGET_NAME = VENUS OBJECT = TEXT PUBLICATION_DATE = 1992-03-03 BYTES = 80 NOTE = "Magellan GxDR Cumulative Error Notes" END_OBJECT = TEXT END ======================================================================== RELEASE NOTES FOR THE CURRENT GxDR DISK +-------------+------------------+---------------+---------------------+ | VOLUME_ID | PRODUCT_IDs | ORBIT RANGE | PRODUCTION DATE | +-------------+------------------+---------------+---------------------+ | | GEDR.1;2 | 376-2586 | Feb 11, 1992 | | MG_3001 | GREDR.1;1 | 376-2586 | Mar 2, 1992 | | | GSDR.1;2 | 376-2586 | Feb 13, 1992 | | | GTDR.1;2 | 376-2586 | Feb 11, 1992 | +-------------+------------------+---------------+---------------------+ The format of the individual image framelets corresponds to the MIT Software Interface Specification document MIT-MGN-GxDR v2.3 (Magellan Project document MIT001). The following notes and restrictions apply: [G*DR.SINUS]Fnn.LBL and [GTDR.ERROR]Fnn.LBL: In the PDS detached label files that describe the 4 corner frame- lets of the SINUS and ERROR images, numbered 01, 08, 25, and 32, the keywords MINIMUM_LATITUDE, MAXIMUM_LATITUDE, MINIMUM_LONGITUDE, and MAXIMUM_LONGITUDE have the value 'N/A' to indicate that these framelets contains no data whatever, since they lie "outside" the sinusoidal map projection. [G*DR.SINUS]FRAME.TAB: The corner framelets, numbered 01, 08, 25, and 32, contain no physical pixels, so the corresponding latitude and longitude limits are all set to 999. If the "edge" of the projection (longitude 180) bisects a given framelet, the corresponding longitude limit of that framelet is set to 180 degrees. [G*DR.*]HIST.TAB: The histograms of GTDR and GEDR images (Topography and Emissivity) are not smooth functions of DN value due to arithmetic round-off when creating these 16-bit images. [GREDR.*]*.IMG: A systematic error of (currently) unknown origin appears to have corrupted the reflectivity values derived from altimeter data. This error leads to an apparent increase in radar reflectivity with latitude, so that the average values in polar regions are more than double those in equatorial regions. The increase is not believed to be real since it has not been observed in prior experiments, nor in the Magellan radiometry experiment, where it would have led to a decrease in emissivity at high latitudes, instead of the modest increase that was observed. The global image in the [GREDR.SINUS] directory and its accompany- ing BROWSE.IMG have not been compensated in any way, but the images in the remaining three directories, [GREDR.MERC], [GREDR.NORTH], and [GREDR.SOUTH], have been corrected with a "flat fielding" algorithm to attempt to remove the latitudinal bias. The radar reflectivity rho' is computed from the sum of two factors, rho derived from the strength of altimetry echoes, and rhocor, a correction due to diffuse scattering that is derived from the SAR backscatter cross-section. These parameters were taken from the ar_rho and ar_rhocor fields of altimetry data files in the ARCDR product. To compensate for the latitudinal error, all rho values were multiplied by a latitude-dependent polynomial: rho' = rho*(1+x*(p1+x*(p2+x*(p3+x*p4)))) + rhocor where x = (lat-lat0)/90, lat0 = periapsis latitude (~10N), and p1 = 0.0904652, p2 = 0.0124626, p3 = 0.154636, p4 = -0.0495502. The polynomial coefficients p1 through p4 have been adjusted to retain the same average rho at all latitudes, ignoring areas known to possess anomalous reflectivity (Maxwell Montes, Beta Regio, and Aphrodite Terra). [SOFTWARE.MAC]IMAGE.HQX: This compressed archive contains a Macintosh image display program that requires an 8- or 24-bit frame buffer and a math co-processor or emulator. When using a 24-bit frame buffer, the Macintosh must either be running System 7 or a Color Quickdraw INIT file must be installed in the System Folder. pgf 03/03/92 ========================================================================