Planetary Data System NASA/Jet Propulsion Laboratory Mail Stop 202-101 4800 Oak Grove Drive Pasadena, CA 91109 (818) 354-4321 pds_operator@jpl.nasa.gov
PSPG-00644
GxDRs
Global Emissivity Data Record (GEDR): The emissivity of a surface is defined as the thermal power emitted by that surface divided by the power emitted by a black body of the same size and at the same physical temperature. The polarization direction is horizontal (E-field tangential to surface), and the emissivity is not corrected for incidence angle or surface roughness or tilt. To make the GEDR images in the form of readings of antenna noise temperature, Magellan radiometer data are corrected for antenna sidelobe effects and for emission and absorption by the Venutian atmosphere. The component reflected from the surface is modeled as a specular reflection with power reflection coefficient equal to (1 - emissivity). The location and planetary radius at the antenna boresight intercept point are derived from GTDR images, and the physical surface temperature is calculated using the adiabatic model of Kliore et al., 1985. The data are then filtered to remove low-frequency instrument calibration errors, and re-sampled into sinusoidal, Mercator, and north and south polar stereographic projections at a resolution of about 5 x 5 kilometers per pixel.
Global Reflectivity Data Record (GREDR): The power reflection coefficients are derived by fitting altimeter echoes from the fan-beam altimetry antenna as a function of time to Hagfors' radar backscatter model templates. The values are then corrected for the presence of multiple echo peaks and further corrected for diffuse scattering using backscatter data obtained by the side-looking mode of the Magellan radar. The reflectivities are then filtered to remove low-frequency instrument calibration errors and re-sampled into sinusoidal, Mercator, and north and south polar stereographic projections at a resolution of about 5 x 5 kilometers per pixel.
Global Slope Data Record (GSDR): The surface meter-scale slopes are derived by fitting altimeter echoes from the fan-beam altimetry antenna as a function of time to Hagfors' radar backscatter model templates. The data are then re-sampled into sinusoidal, Mercator, and north and south polar stereographic projections at a resolution of about 5 x 5 kilometers per pixel.
Global Topographic Data Record (GTDR): The range to surface is derived by fitting altimeter echoes from the fan-beam altimetry antenna as a function of time to Hagfors' radar backscatter model templates. The ranges are subtracted from the spacecraft radial coordinate (derived from Doppler tracking), yielding measurements of planetary radius. The data are filtered to remove low-frequency errors in the in-plane elements of each spacecraft orbit, and then re-sampled into sinusoidal, Mercator, and north and south polar stereographic projections at a resolution of about 5 x 5 kilometers per pixel. A fifth image contains estimates of the absolute accuracy of the radius values in each pixel of the sinusoidal projection. This is derived by taking the root mean square of (a) the statistical accuracy anticipated from the template fitting and (b) the systematic errors estimated from the in-plane element filtration. Note that this is the absolute topographic accuracy--the relative errors are much smaller (about 10-15 meters in relatively flat areas).