urn:nasa:pds:magellan_gvdr:data_gvdr:gvanf_sinu 1.0 1.23.0.0 Product_Observational Magellan Mission urn:nasa:pds:context:investigation:mission.magellan data_to_investigation The Magellan Spacecraft Host urn:nasa:pds:context:instrument_host:spacecraft.mgn is_instrument_host Magellan Spacecraft Radar System Instrument urn:nasa:pds:context:instrument:mgn.rdrs is_instrument Venus Planet urn:nasa:pds:context:target:planet.venus data_to_target GVDR 376 2599 Stanford Center for Radar Astronomy gvdr_0001 Table cartography_parameters_to_image_object 0.00 360.00 90.00 -90.00 Sinusoidal 0 Coordinate Pair 18.225 18.225 5.79478 5.79478 -9.1125 9504.3375 Planetocentric 6051.0 6051.0 6051.0 Positive East urn:nasa:pds:magellan_gvdr:document:gvdrsis data_to_document MGN-V-RDRS-5-SCVDR-V1.0 data_to_derived_source_product 10.17189/1522514 GEO gvanf.tab 1994-05-10T23:12:06.000 Table 0 1303573 The GVANF file contains estimates of the large-scale surface statistics derived from the altimeter echoes by a linear inversion procedure. The ANF acronym stands for 'Altimetry iNversion File', one of the data files in the SCVDR data product. This file also contains least-square fits of certain scattering models to the empirical scattering function produced by the inversion procedure. This file is a table; each row in the table summarizes ANF results in a particular altimetry viewing geometry. The location and number of rows associated with each pixel are given by the corresponding values of 'ANF Start' and 'ANF Samples' in GVPIDX.TAB. See the descriptions in the GVTIDX.LBL and GVPIDX.LBL files for the overall organization of the GVDR. These files describe the division of the planet into pixels and organization of pixels into rectangular tiles. 7 3 102 Reclen 1 1 UnsignedMSB2 2 This field will removed in the final product. It is unused in this version. Sample Count 2 3 UnsignedMSB2 2 The total number of SCVDR ANF altimeter footprints used to compute the estimates of radar scattering properties given in this row of the table. The quantities given in later columns pertain only these footprints. Any altimeter footprint that is partially or completely contained inside this pixel is included. The distance from pixel center to footprint center is used to weight the averaging process. The extent of the altimeter footprint is defined by the first-order range-aliasing and frequency-aliasing points, or the antenna pattern's 3 dB beamwidth, whichever is smaller. Note that this is much larger than the definition used for the ARCDR ADF files, and near the poles results in each footprint overlapping hundreds of GVDR pixels. Scattering Angle Count 3 5 UnsignedByte 1 The number of scattering angles for which estimates of scattering cross-section are given in the field 'Cross Section Container'. The maximum scattering angle is limited by geometric factors, and decreases toward the poles. Only the first 'Scattering Angle Count' angles are valid. Scattering Fit Count 4 6 UnsignedByte 1 The number of analytic scattering models which have been fitted to the observed scattering law. Only the first 'Scattering Fit Count' fits in 'Scattering Law Fits Container' are valid. Doppler Centroid 5 7 UnsignedMSB2 2 Hz 0.183122 -6000 The average observed centroid of the received altimetry echo spectrum. Nadir Track Azimuth Angle 6 9 UnsignedByte 1 degree 1.44 The average azimuthal angle of the spacecraft nadir track. The nadir point is the point on the surface intersected by the line between the spacecraft and the center of the planet. The nadir track is the locus of that point as it moves along the surface. The surface is assumed to be perfectly spherical for the purposes of calculating the azimuthal angle. The azimuthal angle is defined in two ways, depending on the image map projection in use. For the Sinusoidal and Mercator map projections, the direction of the nadir track is expressed as a heading on a compass, in degrees clockwise from North. For example, if the nadir point is moving due east, the azimuthal angle is defined to be 90 degrees. Since this definition becomes useless near the poles, this field is set to zero above 85 degrees of latitude for the Sinusoidal projection. The Mercator projection does not extend to such high latitudes. For the Polar Stereographic map projection, the azimuth angle is expressed in the cartesian map coordinates rather than in geographic coordinates. This makes the azimuth angle more useful for interpretation because its meaning no longer varies with position. First, the azimuth direction is computed as above. Then, this direction is transformed to a direction in map coordinates; the transformed vector is parallel to the vector originating at the nadir point and pointing in the azimuth direction. This vector is expressed in degrees clockwise from the +Y (up) direction on the map. For example, if the north polar projection has 0 degrees of longitude at the bottom, then a footprint at 90 degrees of longitude with a true azimuth of 90 degrees (nadir point moving due east) has a transformed azimuth of 0 degrees. The nadir point appears to be moving in the +Y direction on the map. The relationship between true azimuth and transformed azimuth is simple. For the north polar projection with 0 degrees longitude at the bottom, CARTESIAN_AZIMUTH = GEOGRAPHIC_AZIMUTH - LONGITUDE and for the south polar projection with 0 degrees longitude at the top, CARTESIAN_AZIMUTH = GEOGRAPHIC_AZIMUTH + LONGITUDE As discussed in the GVHDR.XML file, the group of altimetry observations used in this row forms a 'cohort'. Within a cohort, the azimuth angle of each observation falls within a single interval of size 360/N, where N is the value of 'ANF Cohort Azimuth Count' from the GVHDR file. We can reconstruct that interval by noting that the average azimuth angle of all the observations (given in this column) falls within the same interval. Specifically, we find a value of I that satisfies I * 360.0 / N less than or = AZIMUTH_ANGLE less than (I+1) * 360.0 / N where N = 'ANF Cohort Azimuth Count' I = integer between 0 and N-1 inclusive The azimuth angles of all observations in the cohort lie in the interval [I*360 , (I+1)*360], and their average value is given in this column. Pad 7 10 UnsignedByte 1 This field will be removed in the final data product and 'Nadir Track Azimuth Angle' will be increased to two bytes. Cross Section Container 1 21 1 0 A vector of specific radar cross section measurements, indexed by incidence angle. Only the first 'Scattering Angle Count' elements of this vector are valid. The values were obtained by linear inversion of a discretized integral equation for scattering from a sphere. 11 21 Specific Radar Cross Section 1 UnsignedByte 1 0.024 -3 The observed specific radar cross-section at a given angle of incidence. The units of this measurement are dimensionless, meters-squared per meters-squared, and are recorded here logarithmically. To obtain the actual cross-section, apply the scaling and offset given above and raise 10 to this power. SAMPLING_PARAMETER_NAME = INCIDENCE_ANGLE SAMPLING_PARAMETER_UNIT = DEGREE SAMPLING_PARAMETER_INTERVAL = 0.5 MINIMUM_SAMPLING_PARAMETER = 0.25 MAXIMUM_SAMPLING_PARAMETER = 10.25 Cross Section Variance Container 2 21 1 0 A vector of the formal variance of the above specific radar cross section measurements, indexed by incidence angle. Only the first 'Scattering Angle Count' elements of this vector are valid. The values were obtained by linear propagation of errors, based on physical estimates for the thermal and speckle noise in the received radio signal. 32 21 Specific Radar Cross Section Variance 1 UnsignedByte 1 0.032 -3 The formal variance of the corresponding entry in the object 'Specific Radar Cross Section'. The units of this value are dimensionless, meters-fourth per meters-fourth, and are recorded here logarithmically. To obtain the actual variance, raise 10 to the value given here. SAMPLING_PARAMETER_NAME = INCIDENCE_ANGLE SAMPLING_PARAMETER_UNIT = DEGREE SAMPLING_PARAMETER_INTERVAL = 0.5 MINIMUM_SAMPLING_PARAMETER = 0.25 MAXIMUM_SAMPLING_PARAMETER = 10.25 Scattering Law Fits Container 3 5 10 0 A collection of best fit scattering models to the observed scattering law in 'Specific Radar Cross Section'. The fits are made using the formal variance in the object 'Specific Radar Cross Section Variance'. Each repetition contains the results of a single least-squares fit of the empirical scattering function to a particular scattering model. 53 50 Scattering Law ID 1 UnsignedByte 1 Identification number that specifies the analytic scattering law used to fit the observed scattering law. The possible ID numbers and scattering models are 0 Hagfors 1 Exponential 2 Gaussian 3 Rayleigh 4 Muhleman The scattering model determines the interpretation of the remaining fields. Fit Flag Group 2 UnsignedByte 1 A collection of bit flags describing the results of fitting the observed scattering law to the analytic model. If the corresponding bit is 1, the flag is set. The bit values are 0x01 Fit failed: 'Fit Parameter 1' too large 0x02 Fit failed: 'Fit Parameter 1' too small 0x80 Fit failed: unknown error The other bit positions are unused. Fit Parameter 1 3 UnsignedByte 1 The first parameter solved for in the fit to the observed scattering law. The interpretation of this parameter varies with the scattering model, but for most of them is a measure of the rms roughness of the surface. See the GVDR SIS for a complete description of this field. Fit Parameter 1 Variance 4 UnsignedByte 1 The formal variance of the first fitted parameter, computed using linear propagation of errors through the least squares solution. Since the variance on the observed scattering law estimate is sometimes questionable, this field should be used with caution. Fit Parameter 2 5 UnsignedByte 1 0.012 -3 The second parameter solved for in the fit to the observed scattering law. The interpretation of this parameter varies with the scattering model, but for most of them is a measure of the bulk reflectivity of the surface. See the GVDR SIS for a complete description of this field. Fit Parameter 2 Variance 6 UnsignedByte 1 0.036 -9 The formal variance of the second fitted parameter, computed using linear propagation of errors through the least squares solution. Since the variance on the observed scattering law estimate is sometimes questionable, this field should be used with caution. Fit RMS Slope 7 UnsignedByte 1 radian 0.08 The rms surface slope associated with the fit parameters above. This field is computed from analytic expressions for the rms slope implied by a given model, and is quite dependent on that model. Fit RMS Slope Variance 8 UnsignedByte 1 0.028 -6 The formal variance of the rms slope given above, computed using linear propagation of errors through the least squares solution and analytic expression for rms slope. Since the variance on the observed scattering law estimate is sometimes questionable, this field should be used with caution. The units of this field are log10(radian-squared). To obtain the actual variance, raise 10 to the value reported here. The result will be in radians-squared. Fit Residual 9 UnsignedByte 1 2 The formal residual of the model's fit to the observed scattering law. It is the sum of squares of the difference between the observed scattering law and the model's prediction, summed over the incidence angles reported in 'Scattering Angle Count'. When interpreting the residual, the number of degrees of freedom is two less than the number of angles, because of the two fitted parameters. Since the variance on the observed scattering law estimate is sometimes questionable, this field should be used with caution. Spare 10 UnsignedByte 1 Unused space to pad the row to an even number of bytes. gvanf.lbl 0 PDS3 Original PDS3 label Carriage-Return Line-Feed gvanf.fmt 0 PDS3 Original PDS3 format file Carriage-Return Line-Feed gvnff.fmt 0 PDS3 Original PDS3 format file Carriage-Return Line-Feed