Location
University of Utah
Start Date
6-19-1998 2:30 PM
Description
Space-borne scatterometry has been used by NASA for several years to estimate the normalized radar cross-section (σ0) of the surface of the earth. The measured value of σ0 can then be used to study surface features such as vegetation, polar ice and ocean winds. Recently, size constraints have required NASA to switch the basic design of scatterometers from long antennas, which create a fan beam, to small parabolic dishes which create a narrow pencil beam. This change in antenna design requires new σ0 retrieval algorithms to be developed.
σ0 is calculated by dividing the power received by the conversion factor X, which is a function of the spacecraft and antenna positions. Because it is computationally expensive to calculate X for each data point the X factor algorithms proposes the use of a pre-computed table of nomimal X values for various scan angles and orbit positions. Unfortunately, the table does not take into account any variations in the orbit, or perturbations to the attitude of the spacecraft.
A perturbation correction algorithm is developed which uses the shift in baseband frequency (Δf) resulting from various perturbations to correct the nominal values of X. Using the combination of the X factor table and the Δf correction, σ0 can be retrieved rapidly and accurately. This algorithm will be used to calculate σ0 for the upcomming Quikscat and Sea Winds missions.
Measuring the Normalized Radar Cross-Section of the Earth Using Pencil Beam Scatterometers
University of Utah
Space-borne scatterometry has been used by NASA for several years to estimate the normalized radar cross-section (σ0) of the surface of the earth. The measured value of σ0 can then be used to study surface features such as vegetation, polar ice and ocean winds. Recently, size constraints have required NASA to switch the basic design of scatterometers from long antennas, which create a fan beam, to small parabolic dishes which create a narrow pencil beam. This change in antenna design requires new σ0 retrieval algorithms to be developed.
σ0 is calculated by dividing the power received by the conversion factor X, which is a function of the spacecraft and antenna positions. Because it is computationally expensive to calculate X for each data point the X factor algorithms proposes the use of a pre-computed table of nomimal X values for various scan angles and orbit positions. Unfortunately, the table does not take into account any variations in the orbit, or perturbations to the attitude of the spacecraft.
A perturbation correction algorithm is developed which uses the shift in baseband frequency (Δf) resulting from various perturbations to correct the nominal values of X. Using the combination of the X factor table and the Δf correction, σ0 can be retrieved rapidly and accurately. This algorithm will be used to calculate σ0 for the upcomming Quikscat and Sea Winds missions.