Data Set Information
DATA_SET_NAME MAGELLAN SURFACE CHARACTERISTICS VECTOR DATA RECORD
DATA_SET_ID MGN-V-RDRS-5-SCVDR-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : The Surface Characteristics Vector Data Record (SCVDR) is an orbit-by-orbit reduction of Magellan scattering and emission measurements carried out at Stanford University. The SCVDR includes near-nadir scattering functions obtained by numerical inversion from altimetry (ALT) echoes, results (e.g., rms surface slopes and Fresnel reflectivity) from fitting analytic functions to those inversions, scattering function segments at oblique incidence angles derived from synthetic aperture radar (SAR) echoes, and estimates of surface emissivity derived from thermal microwave radiometry (RAD) measurements. The SCVDR is one of several inputs to the Global Vector Data Record (GVDR), a gridded summary of scattering results, also produced at Stanford. The SCVDR parallels the Altimetry and Radiometry Composite Data Record (ARCDR) produced at the Massachusetts Institute of Technology in its orbit-by-orbit organization. But the SCVDR differs from the ARCDR in that no a priori assumptions are made regarding the form of the scattering function. The SCVDR was originally recorded on a set of 8 mm Unix 'tar' tapes. Data were blocked typically in groups of 100 orbits, but the coverage in a few cases was as large as several hundred orbits or as few as a dozen depending on mapping activity or mission scheduling. For transfer to CD-WO (the SCVDRCD) blocking was adjusted to match the capacity of the CD media. Parameters : The Magellan raw data set comprises three basic data types: echoes from the nadir-viewing altimeter (ALT), echoes from the oblique backscatter synthetic aperture radar (SAR) imaging system, and passive radiothermal emission measurements (RAD) made using the SAR high-gain antenna (HGA). The objective in compiling the SCVDR is to obtain an accurate estimate of the surface backscattering function (sometimes called the specific backscatter function or 'sigma-zero') for Venus from these three data types and to show its variation with incidence (polar) angle, azimuthal angle, and surface location. The Magellan data have been 'inverted' using techniques described by [TYLER1992]. If a surface is statistically homogeneous and scatters isotropically, the distribution of echo signal can be predicted using the radar equation on small surface elements, then sorting and accumulating the incremental power contributions by time-delay and Doppler frequency. Given a measured radar echo versus range and Doppler, the process may be inverted to yield the specific radar cross section as a function of incidence angle. In the SCVDR the Magellan empirical backscattering function has been obtained from ALT data at nadir to as much as 10 degrees from normal incidence in steps of 0.5 degrees. The empirical backscattering function at oblique angles (15 to 50 degrees from normal incidence) has been obtained over narrow ranges (1-4 degrees) from the SAR data. The emissivity, which in some models is the complement of the Fresnel reflectivity, has been obtained from the RAD measurements. Processing : ALT and SAR data have been processed at Stanford University using 'inversion' methods, whereby the radar equation is converted to a matrix-vector relationship and that is solved (using least-squares techniques) to obtain an 'empirical' scattering function for each radar echo [TYLER1992]. In the case of ALT data, stability of the solution requires considerable attention; in the case of SAR data, it is the variability of the surface scattering itself that leads to the most uncertainty. Stanford has also independently confirmed the results of the MIT processing of emissivity data but without developing separate algorithms. At Stanford ALT-EDR tapes were the input for calculation of near-nadir empirical backscattering functions. For oblique backscatter, C-BIDR tapes from the Magellan Project and F-BIDR files obtained via Internet from Washington University were the input products. Emissivity results were calculated from radar burst headers, which are included among both the ALT-EDR and C/F-BIDR files. Output was collected on an orbit-by-orbit basis into the SCVDR. Data : Each SCVDR includes data from several orbits. For each orbit, there are six files: (1) Orbit Header File (2) Altimetry Inversion File (ANF) (3) Inversion Fit File (NNF) (4) Sinusoidal Image File (SIF) (5) Oblique Image File (OIF) (6) Emissivity Data File (EDF) The Orbit Header File contains basic information about the Magellan orbit and summary data from the other five files. OHF information includes average Keplerian orbit elements, the number of entries in each of the other files, and starting and ending times for data in the other files. The Altimetry Inversion file contains the results of the altimetry echo inversion (sigma-zero versus incidence angle at spacings typically of 0.5 degree to a maximum of about 10 degrees from nadir). The ANF also includes uncertainties in sigma-zero, geometry for the ALT observations, parameters of the radar system used in computing the absolute backscatter, and echo timing estimates which could be converted to Venus radius values. The Inversion Fit File contains results from fitting up to five scattering laws to the experimentally derived near-nadir sigma-zero function (the ANF). The five laws include: Hagfors, exponential, gaussian, Rayleigh, and Muhleman. Free parameters (for example, the roughness parameter C and the Fresnel reflectivity R for the Hagfors law) are also reported, as is an estimate of the rms surface roughness corresponding to each fit. Residuals between the analytic function and the data indicate which function best matches the experimental results. The Sinusoidal Image File contains the results of fitting a quadratic function to the average SAR pixel values across a sinusoidal equal area image strip. The image strips are from either C-BIDR or F-BIDR files; the best fit quadratic represents the sigma-zero function over a few degrees of incidence angle for a surface area about 20 km across track and about 2 km along track. Geometrical data, uncertainties, and other information is also included in the SIF. The Oblique Image File contains the results of fitting a quadratic approximation to pixel values stored in the Magellan oblique sinusoidal equal area projection. The OIF and SIF are identical except for the projection of the input pixel data. The OIF contains data for north and south polar regions. The Emissivity Data File contains results of analysis of Magellan RAD data. Surface emissivities, uncertainties, geometrical data, and system parameter values are included. Ancillary Data : Inversion of the altimetry data requires use of matrices incorporating information about the geometry of the observations and the relationships of geometry to radar range, Doppler frequency offset, and incidence angle on the planet's surface. Matrices were computed at approximately one degree increments in spacecraft latitude for orbits on which viewing conditions were distinctly different. These matrices are included with the SCVDR data in the GEOMETRY directory on the CD-WO. Ancillary data for most processing at Stanford was obtained from the data tapes and files received from the Magellan Project. These included trajectory and pointing information for the spacecraft, clock conversion tables, spacecraft engineering data, and SAR processing parameters. For calibration of the radar instrument itself, Magellan Project reports (including some received from Hughes Aircraft Co. [BARRY1987; CUEVAS1989; SE011]) were used. Documentation on handling of data at the Jet Propulsion Laboratory was also used [BRILL&MEISL1990; SCIEDR; SDPS101]. Coordinate System : Most information about spacecraft positions and velocities is stored using J2000 inertial coordinates. Most information about surface locations is stored using Venus Body Fixed 1985 coordinates [LYONS1988]. Software : A special library and several example programs are provided in source code form for reading the SCVDR data files. One main program is provided for each file type (including the geometry files). These are provided to illustrate access to the data using the reader subroutine(s); the main programs themselves do nothing beyond returning diagnostic messages. Source code and include files are in the C language; a Unix makefile (for creating executables) and a Perl script (to list contents of SCVDR files) are also provided. Media/Format : The SCVDR will be delivered using compact disc write once (CD-WO) media. Formats will be based on standards for such products established by the Planetary Data System (PDS) [PDSSR1992].
DATA_SET_RELEASE_DATE 1995-01-01T00:00:00.000Z
START_TIME 1990-09-15T04:22:14.000Z
STOP_TIME 1992-09-14T02:28:41.000Z
MISSION_NAME MAGELLAN
MISSION_START_DATE 1989-05-04T12:00:00.000Z
MISSION_STOP_DATE 1994-10-12T12:00:00.000Z
TARGET_NAME VENUS
TARGET_TYPE PLANET
INSTRUMENT_HOST_ID MGN
INSTRUMENT_NAME RADAR SYSTEM
INSTRUMENT_ID RDRS
INSTRUMENT_TYPE RADAR
NODE_NAME Geosciences
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Overview : The SCVDR is intended to be a systematic and comprehensive reduction of radar and radio data acquired by the Magellan Mission. It emphasizes extraction of electromagnetic properties of the Venus surface with minimal assumptions regarding the form of the backscatter function. Limitations implicit in the analysis approach and in the resulting data are summarized below. Review : The SCVDR has been reviewed internally by the Magellan Project prior to release to the planetary community. The review included compilation of the Global Vector Data Record (GVDR) at Stanford and independent confirmation of formats and values at M.I.T. Data Coverage and Quality : Because the orbit of Magellan was elliptical during most of its mapping operations, the character of the signal varied considerably over an orbit as well as from place to place on the surface. Although the SCVDR quantifies much of the spatial variation, systematic limitations of both the instrument and the mission mean that some variability was missed or only partially captured. General Limitations ------------------- It is important to remember that, since the SAR and ALT antennas were aimed at different parts of the planet during each orbit, building up a collection of composite scattering data for any single surface region requires that results from several orbits be integrated. In the case of data from polar regions, where only the SAR was able to probe, there will be no ALT data. When scheduling or other factors interrupted the systematic collection of data, there may be ALT data for some regions but no comparable SAR or radiometry data (or vice versa). Outages played an important role in determining coverage for all Cycles. For example, although a goal of Cycle 3 radar mapping was radar stereo, early orbits were used to collect data at nominal incidence angles that had been missed during Cycle 1 because of thermal problems with the spacecraft. A transmitter failure during Cycle 3 caused a further loss of data. It is not within the scope of this description to provide detailed information on data coverage. Leading Edge: Identification of the echo leading edge is critical for both topographic analysis and extraction of the near-nadir scattering function from ALT data. For topography, a timing error translates directly into an error in planetary radius. For calculation of the scattering function, both the inferred echo dispersion and the inferred echo amplitude will be in error if the initial rise of the echo is improperly identified. This limitation applies most commonly when the spacecraft ALT antenna was aimed away from nadir and the surface was rough (e.g., during Cycle 1 over Aphrodite Terra). Inversion Stability: Although the ALT inversions require no assumptions about the form of the backscatter function, inversion processes are unstable in the presence of noise. Second differences were constrained during ALT inversions to improve stability. The quantitative constraint was varied from periapsis to the poles based on whether the 'noise' was radar echo speckle or speckle plus thermal system noise in the receiver. Instabilities affect primarily the inversion results at the largest incidence angles. Noise Baseline: During ALT inversion, an estimate of 'thermal' noise was derived from the data. At low spacecraft altitudes noise was primarily radar clutter (for example, aliased signal from frequencies outside the nominal Doppler window). At high spacecraft altitudes, both range aliasing and real system thermal noise contributed. The erroneous inclusion of high-angle scattered signal in the 'noise' estimate leads to the following consequences: (1) the derived scattering function amplitude will be underestimated (inferred reflectivity will be too small); (2) the echo decay rate with incidence angle will be overestimated (inferred rms surface slope will be too small); (3) uncertainty in the derived scattering function will be too small (especially at the largest angles). Orbit Eccentricity: Periapsis throughout the radar mapping mission was near 10 degrees N latitude at altitudes of approximately 300 km. The altitude near the poles, on the other hand, was on the order of 3000 km. For all data types this means lower confidence in results obtained at the poles than near the equator. Dynamic Antenna Pointing: The spacecraft attitude varied during each orbit to compensate partially for the changing SAR range to the surface and to provide scattering at higher incidence angles when the echo signal was expected to be strongest. The SAR antenna was pointed at about 45 degrees from nadir near periapsis; this was reduced to about 15 degrees at the poles. The ALT antenna, at a constant 25 degree offset from the SAR antenna, followed in tandem but at angles which were not optimized for obtaining the best altimetry echo. Footprints: A nominal nadir footprint can be assigned to altimetry results, but this footprint is biased near periapsis because the ALT antenna is rotated about 20 degrees from nadir (during Cycle 1). Over polar regions in Cycle 1, the ALT antenna is rotated about 10 degrees to the opposite side of nadir. A more important consideration in polar regions is that the area illuminated by the ALT antenna is approximately 100 times as large as near periapsis because of the higher spacecraft altitude. The region contributing to echoes in polar regions -- and therefore the region over which estimates of Fresnel reflectivity and rms surface tilts apply -- is much larger than at periapsis. Cycle 1 Mapping --------------- During Mapping Cycle 1 almost half the orbits provided SAR images of the north pole; because of the orbit inclination, ALT data never extended beyond about 85N latitude in the north and 85S in the south. No SAR images of the south pole were acquired during Mapping Cycle 1 because the SAR antenna was always pointed to the left of the ground track; Cycle 1 SAR image strips near the south pole were at latitudes equatorward of 85S. Beginning at approximately orbit 600 and extending until nearly orbit 1100, onboard recording of radar data slowly degraded. Most of these data were subsequently reprocessed, with some improvement in data quality. Where available, the reprocessed data have been included in the SCVDR. Cycle 2 Mapping --------------- During much of Mapping Cycle 2, the spacecraft was flown 'backwards' so as to provide SAR images of the same terrain but with 'opposite side' illumination. This adjustment also meant that the SAR could image near the Venus south pole (but not near the north pole). The ALT data continued to be limited to latitudes equatorward of 85N and 85S. SCVDR processing of image data within about two degrees of the south pole suffers from errors. This apparently resulted from reprojection of the oblique sinusoidal pixels but the precise cause of the problem was never isolated and corrected. This problem potentially affects all south pole OIF data, including any obtained in Cycle 3 as well as the larger volume from Cycle 2. Cycle 3 Mapping --------------- During Mapping Cycle 3 the emphasis was on obtaining SAR data at the same viewing azimuth as in Cycle 1 but at different incidence angles (for radar stereo). In fact, most data were acquired at an incidence angle of about 25 degrees, which meant that the ALT antenna was usually aimed directly at nadir instead of drifting from side to side, as had been the case in Cycles 1 and 2. These Cycle 3 data, therefore, may be among the best from the altimeter. Dynamic range in SAR data was larger than in Cycles 1 and 2 because the incidence angle was fixed rather than varying to compensate for the changing spacecraft altitude. Other Comments : Although information on radar echo timing is included in the ANF, a thorough analysis for topography was not attempted in the SCVDR. Comparison of the preliminary SCVDR results with results in the ARCDR showed consistency in the altitude estimates. Systematic errors were more carefully addressed in the ARCDR. ARCDR topography was carried forward to the GVDR. Likewise, the EDF results in the SCVDR are consistent with emissivity data in the ARCDR. The ARCDR emissivity results were carried forward to the GVDR.
CITATION_DESCRIPTION Tyler, G. L., MAGELLAN SURFACE CHARACTERISTICS VECTOR DATA RECORD, MGN-V-RDRS-5-SCVDR-V1.0, NASA Planetary Data System, 1995
ABSTRACT_TEXT This data set contains the Magellan Surface Characteristics Vector Data Record (SCVDR) which is an orbit-by-orbit reduction of Magellan scattering and emission measurements carried out at Stanford University. The SCVDR includes near-nadir scattering functions obtained by numerical inversion from altimetry (ALT) echoes, results (e.g., rms surface slopes and Fresnel reflectivity) from fitting analytic functions to those inversions, scattering function segments at oblique incidence angles derived from synthetic aperture radar (SAR) echoes, and estimates of surface emissivity derived from thermal microwave radiometry (RAD) measurements. The SCVDR is one of several inputs to the Global Vector Data Record (GVDR), a gridded summary of scattering results, also produced at Stanford.
PRODUCER_FULL_NAME DR. G. LEONARD TYLER
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