E. M. Bracalente

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.

σ ° Signature of the Amazon Rain Forest Obtained from the Seasat Scatterometer

The radar response from the Amazon rain forest has been studied to determine the suitability of this region for use as a standard target to calibrate spaceborne scatterometers. Backscattering observations made by the Seasat-1 scatterometer system (SASS) show the Amazon rain forest to be a homogeneous, azimuthally isotropic radar target which is insensitive to polarization. The variation with angle of incidence may be adequately modeled as σ°(dB) = aθ + b. Typical values for the incidence-angle coefficient a are 0.07-0.15 dB/degree. No change in the relations was observed over the 99 days of the Seasat mission. A small diurnal effect occurs, with measurements at sunrise being 0.5-1 dB higher than the rest of the day. For a fixed incidence angle and time of day the standard deviations ranged from 0.6 dB at the extremes to 0.1 dB at the antenna pointing angle of 44°. Because of its consistent response the Amazon forest appears to be a suitable target for calibrating spaceborne scatterometers. Further research is needed to check for seasonal effects at other times of year.

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.

Seasat Scatterometer: Results of the Gulf of Alaska Workshop

The Seasat microwave scatterometer was designed to measure, globally and in nearly all weather, wind speed to an accuracy of � 2 meters per second and wind direction to � 20� in two swaths 500 kilometers wide on either side of the spacecraft. For two operating modes in rain-free conditions, a limited number of comparisons to high-quality surface truth indicates that these specifications may have been met.

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.

Airborne Doppler radar detection of low-altitude wind shear

As part of an integrated windshear program, the Federal Aviation Administration, jointly with NASA, is sponsoring a research effort to develop airborne sensor technology for the detection of low altitude windshear during aircraft take-off and landing. One sensor being considered is microwave Doppler radar operating at X-band or above. Using a Microburst/Clutter/Radar simulation program, a preliminary feasibility study was conducted to assess the performance of Doppler radars for this application. Preliminary results from this study are presented. Analysis show, that using bin-to-bin Automatic Gain Control (AGC), clutter filtering, limited detection range, and suitable antenna tilt management, windshear from a wet microburst can be accurately detected 10 to 65 seconds (.75 to 5 km) in front of the aircraft. Although a performance improvement can be obtained at higher frequency, the baseline X-band system that was simulated detected the presence of a windshear hazard for the dry microburst. Although this study indicates the feasibility of using an airborne Doppler radar to detect low altitude microburst windshear, further detailed studies, including future flight experiments, will be required to completely characterize the capabilities and limitations.

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.

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.

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.