Lyle C. Schroeder

Radar backscatter from the ocean: Dependence on surface friction velocity

From the mid 1960s to the present, the normalized radar cross-section (NRCS) of the ocean has been measured using airborne radars operating over a frequency range of 0.4 to 14 GHz. Analyses of these data have shown that the NRCS was proportional to the ocean surface wind speed raised to some power, but the values of the exponent remained in dispute. This paper extends previous work and uses these NRCS measurements to demonstrate that to the first order, the NRCS is a function of only the friction velocity at the oceans surface. Further analyses characterize the dependence of the NRCS on radar variables such as frequency, incidence angle, polarization, etc. Finally, recommendations are made for using Ku-band radars at large incidence angles for remote sensing of the wind friction velocity vector.

Removal of Ambiguous Wind Directions for a Ku-Band Wind Scatterometer Using Three Different Azimuth Angles

The Seasat-A satellite scatterometer (SASS) sensor demonstrated very successfully that Ku-band scatterometers can make accurate synoptic measurements of surface wind speed over the ocean. Because SASS provided normalized radar cross section (NRCS) measurements from only two azimuths, however, the harmonic relationship of NRCS with azimuth results in up to four ambiguous wind directions. The primary improvement to be incorporated in a next-generation scatterometer design such as Navy Remote Ocean Sensing System (NROSS) is the addition of a third azimuth look at each sampled cell. With this and other instrument improvements, preliminary studies indicate that wind-direction ambiguities (aliases) could successfully be removed in at least 80 percent of the cases. Furthermore, these studies show that in over 90 percent of the wind solutions, the two most probable solutions correctly identify the wind streamlines. Methods were studied which could examine typical streamline patterns derived from scatterometers using continuity or pattern-recognition techniques to determine which of the possible two wind directions was correct. In addition, unambiguous solutions were sought for cases where streamlines were not correctly defined. This paper describes several approaches for such alias-removal algorithms. These algorithms were developed with the aid of simulated three-beam scatterometer ambiguous wind-solution data (based on NOSS conditions) over a known windfield. The resulting algorithms were evaluated using a different set of simulated orbital data, but withholding the true winds.

Inflatable antenna microwave radiometer for soil moisture measurement

Microwave measurements of soil moisture are not being obtained at the required spatial Earth resolution with current technology. The Low Earth Orbit Microwave Radiometry Workshop held in Hampton, Virginia, identified measurements of soil moisture at a resolution of 10 km as the general science driver. Recently, new novel designs for lightweight reflector systems have been developed using deployable inflatable antenna structures which could enable lightweight real-aperture radiometers. In consideration of this, a study was conducted at the NASA Langley Research Center to determine the feasibility of developing a microwave radiometer system using inflatable reflector antenna technology to obtain high spatial resolution radiometric measurements of soil moisture from low Earth orbit and which could be used with a small and cost effective launch vehicle. The required high resolution with reasonable swath width coupled with the L-band measurement frequency for soil moisture dictated the use of a large (30 meter class) real aperture antenna in conjunction with a pushbroom antenna beam configuration and noise-injection type radiometer designs at 1.4 and 4.3 GHz to produce a 370 kilometer cross-track swath with a 10 kilometer resolution that could be packaged for launch with a Titan II class vehicle. This study includes design of the inflatable structure, control analysis, structural and thermal analysis, antenna and feed design, radiometer design, payload packaging, orbital analysis, and electromagnetic losses in the thin membrane inflatable materials.<<ETX>>

Microwave Scatterometer Measurements of Oceanic Wind Vector

The remote sensing of oceanic wind vector using microwave scatterometry has come of age. During the I960’s when this technique was in its infancy, there were two major issues which cast doubt on its usefulness. First, there was serious debate on whether or not the normalized radar cross section of the ocean saturated beyond wind speeds of approximately 7–8 m/s. Secondly, considering the radar hardware technology, it was unknown whether an instrument could be developed which had good long term stability and which could be calibrated to measure σo better than ±2dB absolute.

Microwave radiometer sensor technology research for Earth science measurements

A research program has been initiated at NASA Langley Research Center to investigate the critical technologies for developing advanced microwave radiometers suitable for Earth science observations. A significant objective of this research is to enable microwave measurements with adequate spatial resolutions for a number of Earth science parameters, such as sea ice, precipitation, soil moisture, sea surface temperature, and wind speed over oceans. High spatial resolution microwave sensing from space with reasonable swath widths and revisit times favor large real aperture radiometer systems. However, the size requirements for such systems are in conflict with the need to emphasize small launch vehicles. This paper describes a tradeoff between the science requirements, basic operational parameters, test configurations, and expected sensor performance for a satellite radiometer concept. The preliminary designs of real aperture systems utilizing novel light-weight compact-packaging techniques are used as a means of demonstrating this technology.

Surface control adjustment experiments on a mesh reflector antenna

The authors present results of experiments performed for the purpose of determining the practicality of utilizing active surface control as a means of correcting for distortions in a mesh reflector antenna. A 15-m diameter hoop/column reflector antenna was reconfigured with surface-adjustment capability by installation of control motors on 28 surface shaping tension cords. Only one quadrant of the surface was configured with control motors; all other surface cords had preset tensions. After the motors were installed, the antenna was deployed and the surface distortion measured. The initial deployed surface had a root-mean-square (RMS) distortion of 0.164 in. determined by measuring 239 photogrametric targets over the surface of the controllable quadrant. A finite-element structural model was used to determine the amount of adjustment required for each surface shaping cord to minimize the deviation from a paraboloid.<<ETX>>

Performance Evaluation of Space-borne Scatterometer

Study results are presented showing performance capability of a spaceborne scatterometer to operationally measure ocean surface wind speed and direction. In addition, a research mode is described which will allow development of improved radar signatures for ocean, sea ice, and land targets. The study results show that a scatterometer can meet the operational requirements of \pm 2 m/s wind speed accuracy (or \pm 10% , whichever is greater) and \pm 20\deg wind direction accuracy over most of the expected ocean surface conditions. The six beam scatterometer design evaluated is shown to be skillful (> 90% correct) in specifying the correct wind vector solution (with a 180\deg ambiguity) from the multiple solutions derived; further improvement must rely on meteological and pattern recognition techniques now under study.