Study On Spaceborne Rotating, Range-gated, Fanbeam Scatterometer System

Spaceborne radar scatterometers are designed to measure backscatter coefficient (σ~0) from the ocean surface for the retrieval of near surface wind vector field. Any scatterometers operating in the future should attempt to meet the next two essential requirements: firstly, high quality winds data, which would enable positive impacts on the Numerical Weather Prediction (NWP) models; and secondly, frequent global coverage, that requires a large swath. A Rotating Fanbeam SCATterometer (RFSCAT) produces large overlaps on the pixel within the total swath by successive sweeps and provides a large number ofσ~0 acquisitions with diverse azimuth and elevation look angles for a single surface resolution cell. Furthermore, RFSCAT accommodates a non-nadir-gap swath. So RFSCAT has the indispensable conditions for meeting the above requirements.An end-to-end simulation model is adopted to analyze and to optimize the performances of the RFSCAT system, taking into account the radar observation conditions of CFOSAT scatterometer. The simulation results indicate that the current configuration of the discussed scatterometer satisfies the requirements of wind retrieval, which performance is roughly comparable to the pencil-beam radar scatterometers (e.g. SeaWinds on QuikSCAT). The wind retrieval quality of CFOSAT is worse than SeaWinds when the true wind speed is less than 6 m/s. However, it is better than SeaWinds when the true wind speed is higher than 15 m/s, especially the retrieval quality of wind direction.This paper analyzes both the resolution of RFSCAT and the Doppler effects based on a signal analysis model. It shows that RFSCAT should be designed to transmit linear chirped pulse for better. In the receiver, the echo can be digitally deramped and an FFT is used to extract the range information. The signal analysis model can also be used to derive the communication error of theσ~0 of both slice and combined wind vector cell (WVC). Furthermore, the calibration error and the modeling error are also briefly discussed in this paper. Equations of these error models may be used to improve the end-to-end simulation model introduced previously. System performances of the CFOSAT scatterometer assessed by the new simulation model are almost equal to those of the old model.Then this paper proposes a dual-frequency RFSCAT, which swath is large enough for quickly recovering the ocean surface. The proposed RFSCAT should accommodate the main requirements of operational application and science research in the future. The improved simulation model is adopted to assess the dual-frequency RFSCAT. And the simulation results indicate that it performs much better than CFOSAT on wind retrieval.Finally, this paper discusses the scatterometer calibration methodology, and presents a new mask of Amazon rain forest for external calibration. Meanwhile, model ofσ~0 response and correction algorithms are studied for calibration using land targets.