High Resolution Surface Backscatter Measurements with the Sea Winds Pencil - Beam

To continue and expand upon the foundation provided by the recent flight of the NASA Scatterometer (NSCAT), NASA is developing the SeaWinds instrument which is scheduled to fly on the Quikscat mission in November 1998, and on the Second Japanese Earth Observation Satellite (ADEOS-11) to be launched in early 2000. In a significant design departure from previous “fan-beam” scatterometer systems, Sea Winds will be a “pencil-beam” system. Pencil-beam systems employ a single, approximately 1 m parabolic dish which is conically scanned about the nadir axis to provide multiple azimuth measurements [3] [4] (see Fig. 1). A key advantage to pencil-beam systems is that, because of their more compact design, they are much easier to accommodate on spacecraft without the necessity of complex deployment schemes or severe fieldof-view constraints. In an era where smaller space missions with faster development times are often mandated as is the case with the Quikscat mission, for example such a reduction in payload size is highly desirable. An additional advantage to pencil-beam systems is that because they measure ocean backscatter at a constant incidence angle suitable for wind retrieval, there is no “nadir gap” in swath coverage as there is for fan-beam systems. The resulting contiguous swath offers a significant improvement in Earth coverage. For these reasons, the pencil-beam design has been adopted for NASA scatterometers into the next century. The adoption of the pencil-beam approach, however, has presented system designers with new challenges as well. One design challenge, largely unanticipated until the success of the recent NSCAT mission, is the coarser spatial resolution of the pencil-beam footprint obtainable with a 1 m parabolic dish as compared to the smaller 80 cells collected by NSCAT. As originally envisioned, the Figure 1: Fig. I : Sea Winds Measurement Geometry