W. Grantham

The SeaSat-A satellite scatterometer

This paper describes the methods used to develop performance requirements and design characteristics for the microwave scatterometer (SASS) ocean-surface wind sensor on the NASA SeaSat-A satellite. Wind vector measurement requirements from the SeaSat user community such as wind speed and direction accuracy, resolution cell size, grid spacing, and swath width formed the basis for defining instrument characteristics. The resulting scatterometer is designed for 14.6 GHz using four fan beam antennas to measure wind speed and direction over a 1000-km swath width with a resolution cell size 50 \times 50 km. Results presented show scatterometer accuracy satisfies user requirements for wind speed from 4 m/s to greater than 24 m/s for the nominal SeaSat-A orbit of 790 km altitude, 108\deg inclination, and 0.001 eccentricity.

Seasat-A satellite scatterometer instrument evaluation

The Seasat-A satellite scatterometer (SASS) was designed to measure ocean surface wind speed and direction in twenty-four (24) independent cells over a 1000-km swath. It operated in the interrupted CW mode at a frequency of 14.6 GHz with four (4) fan beam antennas and used Doppler filtering in the receiver for resolving the cells on the surface. The instrument began operating in space on July 6, 1978, and gathered normalized radar cross section ( \sigma^{0} ) data for approximately 2290 h. The purpose of this paper is to describe the in-orbit evaluation of the SASS hardware and its compatibility with the spacecraft. It has been determined that the scatterometer operated flawlessly throughout the mission, met all design requirements, and established a good data base for geophysical processing.

The SASS^{1} scattering coefficient σ ° algorithm

This paper describes the algorithms used to convert engineering unit data obtained from the Seasat-A satellite scatterometer (SASS) to radar scattering coefficients ( \sigma\deg ) and associated supporting parameters. A description is given of the instrument receiver and related processing used by the scatterometer to measure signal power backscattered from the earths surface. The applicable radar equation used for determining \sigma\deg is derived. Sample results of SASS data processed through current algorithm development facility (ADF) \sigma\deg algorithms are presented which include \sigma\deg values for both water and land surfaces, \sigma\deg signatures for these two surface types are seen to have distinctly different characteristics. As expected, \sigma\deg values for water show strong dependence on both incidence angle and wind speed. For land, \sigma\deg values are relatively independent of incidence angle above 20\deg and have values in the range -14 dB. \sigma\deg measurements of the Amazon rain forest indicate the usefulness of this type of data as a stable calibration reference target. Using this Amazon data, relative biases between all four antennas and both polarizations are shown to be less than 0.4 dB.

Microwave Scattering from the Ocean Surface

This paper is a review of current aircraft and satellite microwave remote sensing programs concerned with the measurement of ocean wave and surface wind conditions. These particular measurements have been identified by the user community as offering significant economic and technological benefits. Active microwave remote sensing techniques for these applications have been described theoretically and verified experimentally. The results of recent aircraft and satellite experimental programs are presented herein along with plans for the SeaSat-A Satellite Scatterometer.

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.

SASS measurements of the Ku -band radar signature of the ocean

SeaSat-A Satellite Scatterometer (SASS) measurements of normalized radar cross section (NRCS) have been merged with high quality surface-wind fields based on in situ, to create a large data base of NRCS-wind signature data. These data are compared to the existing NRCS-wind model used by the SASS to infer winds. False-color maps of SASS NRCS and ocean winds from multiple orbits show important synoptic trends.

Performance of Airborne Microwave Remote Sensors in Hurricane Allen

In August 1980 two remote sensing instruments from the NASA Langley Research Center were flown through Hurricane Allen. These were the C-band Stepped Frequency Microwave Radiometer (SFMR) and the Ku-band Airborne Microwave Scatterometer (AMSCAT), carried on a NOAA aircraft at 3000 m altitude. An SFMR sea surface wind speed is calculated from the increase in antenna brightness temperature above the estimated calm-sea value, and an SFMR rain rate is calculated from the difference in antenna temperature measured at two frequencies. In the absence of severe beam attenuation caused by rain, an AMSCAT surface wind vector is determined from the seas normalized radar cross section measured at several azimuths. Comparison wind data were provided with the INS on board the aircraft at 3000 m and on a second, lower altitude, aircraft. The analysis shows that, once the rain free periods are selected with the SFMR data, the AMSCAT produces well behaved and consistent wind vectors.

The SEASAT-A Satellite Scatterometer

This paper describes the methods used to develop performance requirements and design characteristics for the microwave scatterometer (SASS) ocean-surface wind sensor on the NASA SeaSat-A satellite. Wind vector measurement requirements from the SeaSat user community such as wind speed and direction accuracy, resolution cell size, grid spacing, and swath width formed the basis for defining instrument characteristics. The resulting scatterometer is designed for 14.6 GHz using four fan beam antennas to measure wind speed and direction over a 1000-km swath width with a resolution cell size50 \times 50km. Results presented show scatterometer accuracy satisfies user requirements for wind speed from 4 m/s to greater than 24 m/s for the nominal SeaSat-A orbit of 790 km altitude,108\deginclination, and 0.001 eccentricity.

Intercomparison of the SEASAT, SASS, SMMR and Altimeter Derived Winds

The Seasat-A altimeter measures the ocean surface back-scatter coefficient, 0^{0} , using a 1.6\deg half-power radar beam width directed to the spacecraft nadir. The values obtained for 0^{0} can be used to infer the magnitude of the ocean surface wind. This investigation describes results obtained during the Gulf of Alaska Experiment which compare the altimeter derived surface winds with results obtained from the SASS side lobes evaluated at nadir and from the SMMR. For conditions characterized by significant wave heights of 4 m or less, the 0^{0} values obtained from the altimeter and the SASS agreed to within 1.5 db. The winds inferred using the GEOS-3 0^{0} -to-wind algorithm, developed by G. Brown, are compared with the results obtained from GOASEX surface wind fields derived from spot reports and from the GEOS-3 altimeter.

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.