Postlaunch sensor verification and calibration of the NASA Scatterometer

Scatterometer instruments are active microwave sensors that transmit a seri es of microwave pulses and measure the returned echo power to determine the normalized radar backscattering cross section (sigma-0) of the ocean surfa ce from which the speed and direction of near-surface ocean winds are deriv ed, The NASA Scatterometer (NSCAT) was launched on board the ADEOS spacecra ft in August 1996 and returned ten months of high-quality data before the f ailure of the ADEOS spacecraft terminated the data stream in June 1997, The purpose of this paper is to provide an overview of the NSCAT instrument and sigma-0 computation and to describe the process and the results of an intensive postlaunch verification, calibration, and validation effort, This process encompassed the functional and performance verification of the fli ght instrument, the sigma-0 computation algorithms, the science data proces sing system, and the analysis of the sigma-0 and wind products, The calibra tion process included the radiometric calibration of NSCAT using both engin eering telemetry and science data and the radiometric beam balance of all e ight antenna beams using both open ocean and uniform land targets, Finally, brief summaries of the construction of the NSCAT geophysical model functio n and the verification and validation of the wind products will be presente d. The key results of this paper are as follows: The NSCAT instrument was show n to function properly and all functional parameters were within their pred icted ranges. The instrument electronics subsystems were very stable and al l of the key parameters, such as transmit power, receiver gain, and bandpas s filter responses, were shown to be stable to within 0.1 dB. The science d ata processing system was thoroughly verified and the sigma-0 computation e rror was shown to be less than 0.1 dB, All eight antenna beams were radiome trically balanced, using natural targets, to an estimated accuracy of about 0.3 dB. Finally, a new model function, called NSCAT-1, was constructed and used to produce wind products. The wind products were statistically verifi ed using ECMWF wind fields and were validated using NDBC buoy measurements. Overall, we believe that NSCAT generated high-quality wind products with w ind speed and direction accuracies that met the science requirements.