CCSD3ZF0000100000001NJPL3IF0PDS200000001 = SFDU_LABEL RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 80 SPACECRAFT_NAME = MAGELLAN TARGET_NAME = VENUS OBJECT = TEXT PUBLICATION_DATE = 1992-02-01 BYTES = 80 NOTE = "MAGELLAN ARCDR CUMULATIVE ERROR NOTES" END_OBJECT = TEXT END ======================================================================== RELEASE NOTES FOR THE CURRENT ARCDR DISK (see below for information about previous ARCDR products) +-------------+-----------------------+--------------------------------+ | VOLUME_ID | PRODUCT_IDs | EPHEMERIS CORRECTION | +-------------+-----------------------+--------------------------------+ | | ARCDR.00541-00600;4 | MGMORB FIT12 | | MG_2002 | ARCDR.00601-00660;4 | July 8, 1991 | | | ARCDR.00661-00720;4 | using orbits 376-676 | +-------------+-----------------------+--------------------------------+ +-------------+--------------+--------------+--------------------------+ | Orbit Range | Altimetry | Radiometry | Comments | +-------------+--------------+--------------+--------------------------+ | 0590-0661 | partial loss | partial loss | DTR-A failure | | 0677-0720 | no data | no data | Superior conjunction | +-------------+--------------+--------------+--------------------------+ Altimeter Data Files (ADF): (a) Data recorder A (DTR-A) degraded progressively from orbit 590 through orbit 1097, after which it was no longer used. During this period, much altimetric and radiometric data was lost. This results in missing footprints and degraded signal strength for those that remaining. Reflectivity measurements (ar_rho) are worse affected than planetary radius (ar_radius) and RMS meter-scale slope (ar_slope). The worst region spans footprints -200 through 735 of orbits 627 through 661. (b) Bands of anomalous RMS slope (ar_slope) and reflectivity (ar_rho) at latitudes of 9S and 23S are caused by an altimeter mode change confused by missing data (due to the data recorder failure). (c) The west face of Maxwell Montes rises almost 10 kilometers within a few altimeter footprints. The altimeter itself has an inherent range ambiguity of 10.02 kilometers, which makes it impossible to locate the west face from the altimeter echoes alone. The radius values in two regions (0.5E, 64.8N and 2.0E, 63.1N in orbits 578 through 583) have therefore been adjusted with the help of side- looking images of Maxwell from Magellan and Arecibo, adding or subtracting 10.02 km from each ar_radius value, but the assignment is tentative. The ambiguity will remain unless and until Magellan views that part of Maxwell at a different incidence angle. Radiometer Data Files (RDF): (d) The progressive failure of DTR-A from orbit 590 through 1097 results in many missing radiometer footprints. The notes for MG_2001 also apply to this disk. ======================================================================== CUMULATIVE ERROR NOTES FOR PREVIOUS ARCDR DISKS +-------------+-----------------------+--------------------------------+ | VOLUME_ID | PRODUCT_IDs | EPHEMERIS CORRECTION | +-------------+-----------------------+--------------------------------+ | | ARCDR.00361-00420;4 | MGMORB FIT12 | | MG_2001 | ARCDR.00421-00480;4 | July 8, 1991 | | | ARCDR.00481-00540;4 | using orbits 376-676 | +-------------+-----------------------+--------------------------------+ +-------------+--------------+--------------+--------------------------+ | Orbit Range | Altimetry | Radiometry | Comments | +-------------+--------------+--------------+--------------------------+ | 0361-0375 | no data | no data | Prior to mapping | | 0452-0455 | partial loss | no data | Radar test number 5 | | 0490-0491 | partial loss | partial loss | Madrid DSN outage | +-------------+--------------+--------------+--------------------------+ The data format corresponds to the MIT Software Interface Specification document MIT-MGN-ARCDR v2.3 (Magellan Project document SCI002). The following notes and restrictions apply: Ephemeris Files (EPF): (a) These files are copied directly from the ALT-EDR data tapes, and may therefore inherit errors from them. Their format may NOT correspond precisely either to the NAIF Ephemeris SIS (NAV-135) or to the examples given in MIT-MGN-ARCDR v2.3. In particular, some lines are terminated by too many, or too few, carriage returns and/or line feeds. Altimeter Data Files (ADF): (b) Bad data records have NOT been eliminated from these files. The user should ALWAYS inspect the ar_flag fields and ignore records for which flags such as AR_BAD have been set. (c) All radii in ADF files have been corrected by the mgmorb processing phase, in which radii from crossing and near-crossing orbits are compared and in-plane ephemeris corrections computed. (d) Power reflection coefficient values (ar_rho) show a gradual increase at high latitudes. Since the surface emissivity (rr_emiss in RDF files) shows no corresponding drop in polar regions, the increase in the average value of ar_rho is probably an artifact of the data reduction process. (e) The signal quality index field (ar_sqi) is recorded in IEEE-754 single-precision floating-point format, not in the VAX format used for the other 4-byte floating-point fields. Radiometer Data Files (RDF): (f) The side-looking backscatter cross-section measurements reported in the rr_sar[0] and rr_sar[1] fields are derived from the block- adaptive-quantizer (BAQ) signal levels reported in radar burst headers, not from averaged C-BIDR image swaths as described in the MIT software design document, MIT-MGN-SDD v2.1. (g) There is a systematic bias to all radiometer data that depends on the time elapsed since start of mapping and on the behavior of the altimeter and, possibly, other on-board systems. The magnitude is of the order of 5K in system temperature, or a factor of 0.01 in emissivity. The bias is seen to change with command upload number and all RDF files have been re-processed through a data filter in an attempt to correct the problem. The filter has itself introduced subtle artifacts in the radiometer data (sudden jumps in data value at specific latitudes of the same magnitude as the original bias), but the overall systematic error is greatly reduced. ======================================================================== A data processing error has been discovered that affects all ARCDR products generated before July 1991. Frequency components from separate altimeter bursts were incorrectly summed together, leading to a blurring of altimetry data in the along-track direction. +-------------+-----------------------+--------------------------------+ | VOLUME_ID | PRODUCT_IDs | EPHEMERIS CORRECTION | +-------------+-----------------------+--------------------------------+ | | ARCDR.00361-00420;1 | MGMORB FIT4 | | MG_TEST3 | ARCDR.00421-00480;1 | November 18, 1990 | | | ARCDR.00481-00540;1 | using orbits 376-676 | +-------------+-----------------------+--------------------------------+ The data format corresponds to the MIT Software Interface Specification document MIT-MGN-ARCDR v2.1 (Magellan Project document SCI002). The following notes and restrictions apply: Orbit Header Files (OHF): (a) The SFDU keyword section refers to the processing of altimetry data files, not to radiometry data. (b) There are no RDF files for the 4 orbits comprising Radar Test 5 (452-455). The oh_nrad fields are therefore zero for these orbits. Ephemeris Files (EPF): (c) These files are copied directly from the ALT-EDR data tapes, and may therefore inherit errors from them. Their format may NOT correspond precisely either to the NAIF Ephemeris SIS (NAV-135) or to the examples given in MIT-MGN-ARCDR v2.1. In particular, some lines are terminated by too many, or too few, carriage returns and/or line feeds. (d) All radii in ADF files have been corrected by the MGMORB processing phase, in which radii from crossing and near-crossing orbits are compared and in-plane ephemeris corrections computed. The data in these products were processed by FIT4 of MGMORB on 1990-11-18T13:06:29. Altimeter Data Files (ADF): (e) Bad data records have NOT been eliminated from these files. The user should ALWAYS inspect the ar_flag fields and ignore records for which flags such as AR_BAD have been set. (f) Because of the lack of C-BIDR data (see below), the power reflection coefficients in the ADF files have NOT been corrected for the diffuse scattering component. Therefore no AR_RHOC flag is set in any ar_flag fields, and the corresponding ar_rhocor fields contain zeroes. (g) The template fitting algorithm used to compute the surface power reflection coefficient, ar_rho, doesn't correctly account for echo profiles with multiple peaks. Values of ar_rho over rugged terrain should not be relied upon. A correction for this phenomenon will be introduced in subsequent versions of the ARCDR. (h) A systematic wave-like fluctuation in altimeter range values has been observed in the ADF data covering a wide area of the planet between 325' and 357' longitude, and 45' and 52' North latitude. The amplitude is approximately 50 meters and the wavelength 100 km. Its cause is currently unknown, but it is likely to be an instrumental effect or data processing error. Radiometer Data Files (RDF): (i) No radiometry data is available for the 4 orbits comprising Radar Test 5 (orbits 452-455). The corresponding RDF files are null, i.e. the ARCDR tape contains an ANSI header (HDR) file, followed by two consecutive file marks, followed by an ANSI trailer (EOF) file, followed by a file mark. Software used to copy this tape (ARCDR.00421-00480;1) must treat this double file mark as a null file, NOT as the physical end of tape. (j) Only 26 C-BIDR orbits (376-399 and 403-405) were available when these ARCDR tapes were produced. RDF files generated from C-BIDRs are given version number 1. The remaining RDF files came from ALT-EDR products and bear the version number T1. These latter are all marked with the RR_NOS1 and RR_NOS2 flags in their rr_flag fields and they contain zeroes in their rr_sar and rr_sfoot arrays. (k) The average planetary radius at the footprint location, rr_radius, is derived from the Magellan topography model 4.0, NOT from Magellan altimeter data. Therefore, the RR_RADC flag is NOT set in the rr_flag field, and the surface emission temperature, rr_surftemp, and the emissivity, rr_emiss, have NOT been corrected accordingly. (l) There is a systematic bias to all radiometer data that depends on the time elapsed since start of mapping and on the behavior of the altimeter and, possibly, other on-board systems. The magnitude is of the order of 5K in system temperature, or a factor of 0.01 in emissivity. The bias is seen to change with upload number and all RDF files have been re-processed through a data filter in an attempt to correct the problem. The filter has itself introduced subtle artifacts in the radiometer data, sudden jumps in data value at specific latitudes, of the same magnitude as the original bias, but the overall systematic error is greatly reduced. ========================================================================