Seasat-A Scatterometer Research Articles

A Preliminary Study of the Calibration for the Rotating Fan-Beam Scatterometer on CFOSAT

The first rotating fan-beam scatterometer (RFSCAT) will be launched onboard the Chinese-French Oceanic Satellite (CFOSAT) in 2018. It provides a set of radar cross-section (σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> ) measurements at different azimuth/incidence angles over a wind vector cell (WVC), in order to determine the near-surface wind field using the backscatter model, i.e., the so-called geophysical model function (GMF). The accuracy of the retrieved wind vector is a sensitive function of the radiometric accuracy of the σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> measurements. Therefore, in-flight calibration, including the loop-back (internal) calibration and the external calibration performed with natural extended-area targets, is studied in this paper. Several homogeneous areas over land are first analyzed to check the stability and azimuthal dependence of the σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> over these areas. A new calibration mask of the homogeneous land areas is generated and will be used by RFSCAT calibration. Then a simple method of external calibration is proposed to eliminate the azimuthal-dependent σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> errors induced by the insertion loss of the rotating joint, which can be applied to both the rotating pencil-beam scatterometers and the coming RFSCAT. The “observed” σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> of RFSCAT is simulated using the SeasatA scatterometer (SASS) measurements and the “perturbed” azimuthal-dependent σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> errors. The latter is then tracked by the proposed external calibration. The results show that the accuracy of gain corrections is up to 0.2 dB, ensuring consistency between different azimuthal measurements.

Sea ice identification using dual-polarized Ku-band scatterometer data

This paper describes a classification algorithm using dual-polarized scatterometer measurements to identify the edge of the sea ice cover. The distinct polarization scattering signatures of sea ice and open water are discussed and illustrated with the dual-polarized radar measurements from the Seasat-A scatterometer (SASS). The analysis of SASS data suggests that the ratio of vertical and horizontal polarization backscatter, denoted as the copol ratio, is a useful discriminator of sea ice and open ocean. A simple classification algorithm using the thresholds of the copol ratio and backscatter levels is proposed. The feasibility of this algorithm is demonstrated using the SASS data from the single-sided, dual-polarization mode. The results indicate that the dual-polarized measurements from the NASA scatterometer (NSCAT) can be used to produce routine maps of sea ice edges.

Use of ERS-1 wind scatterometer data over land surfaces

The capability of ERS-1 wind scatterometer (WSC) data for land surface studies at regional and global scales is investigated. Calibrated data, available since April 1992, consist of backscattering coefficient /spl sigma//spl deg/ values estimated over a 50 km ground resolution cell within a 18-59/spl deg/ incidence range. The WSC covers latitudes between 80/spl deg/S and 85/spl deg/N. Data quality is investigated. Results show a good radiometric stability throughout a two-year period (April 1992-March 1994), a good measurement accuracy and a very good intercalibration between antennas. A global backscattering coefficient image of the world is presented and statistics of /spl sigma//spl deg/ are extracted for the main land surfaces. A strong dependence between this image and global vegetation and elevation map is observed. Finally, WSC and Seasat-A Scatterometer System (SASS) data of typical regions are compared. Results indicate the high capability of ERS-1 WSC data for land surfaces monitoring at global and regional scales.

Integrating reconstructed scatterometer and advanced very high resolution radiometer data for tropical forest inventory

A scientific effort is currently underway to assess tropical forest degradation and its potential impact on Earth's climate. Because of the large continental regions involved, Advanced Very High Resolution Radiometer (AVHRR) imagery and its derivative vegetation index products with resolutions between 1 and 12 km are typically used to inventory the Earth's equatorial vegetation. Archival AVHRR imagery is also used to obtain a temporal baseline of historical forest extent. Recently however, 50-km Seasat-A Scatterometer (SASS) Ku-band imagery (acquired in 1978) has been reconstructed to ≈ 4-km resolution, making it a supplement to AVHRR imagery for historical vegetation assessment. In order to test the utility of reconstructed Ku-band scatterometer imagery for this purpose, seasonal AVHRR vegetation-index and SASS images of identical resolutions were constructed. Using the imagery, discrimination experiments involving 18 vegetation categories were conducted for a central South America study area. The results of these experiments indicate that AVHRR vegetation-index images are slightly superior to reconstructed SASS images for differentiating between equatorial vegetation classes when used alone. However, combining the scatterometer imagery with the vegetation-index images provides discrimination superior to any other combination of the data sets. Using the two data sets together, 90.3% of the test data could be correctly classified into broad classes of equatorial forest, degraded woodland/forest, woodland/savanna, and caatinga .

Vegetation studies of the Amazon basin using enhanced resolution Seasat scatterometer data

The Seasat-A scatterometer (SASS) was designed to measure the near-surface wind field over the ocean by inferring the wind from measurements of the surface radar backscatter. While backscatter measurements were also made over land, they have been primarily used for the calibration of the instrument. This has been due in part to the low resolution of the scatterometer measurements (nominally 50 km). In a separate paper the present authors introduced a new method for generating enhanced resolution radar measurements of the Earth's surface using spaceborne scatterometry. In the present paper, the method is used with SASS data to study vegetation classification over the extended Amazon basin using the resulting medium-scale radar images. The remarkable correlation between the Ku-band radar images and vegetation formations is explored, and the results of several successful experiments to classify the general vegetation classes using the image data are presented. The results demonstrate the utility of medium-scale radar imagery in the study of tropical vegetation and permit utilization of both historic and contemporary scatterometer data for studies of global change. Because the scatterometer provides frequent, wide-area coverage at a variety of incidence angles, it can supplement higher resolution instruments which often have narrow swaths with limited coverage and incidence angle diversity. >

Greenland ice-sheet surface properties observed by the Seasat-A scatterometer at enhanced resolution

AbstractFor 3 months in 1978, the 14.6 GHz Seasat-A scatterometer (SASS) measured the normalized microwave-radar back-scatter coefficient of the Earth’s surface for the purpose of estimating near-surface vector winds over the ocean. SASS also made back-scatter measurements over land and ice regions; however, the application of this data has been limited due to the low (50 km) resolution of the measurements. Using a new technique for generating 6 km enhanced-resolution SASS images of the radar back-scatter characteristics, we present a study of the 1978 condition of the Greenland ice sheet. We derive a time-series of back-scatter images spanning the period July–September 1978. These images show the extent of summer ablation along the ice-sheet periphery. Using the data and models relating firn structure and condition to radar back-scatter characteristics, we delineate and map the seasonal extent of zones which appear to correspond to dry-snow, percolation, wet-snow, and ablation facies, over virtually the entire ice sheet. The results provide a base line with which to compare current (ERS-1) and future Greenland radar maps of snow-and ice-surface conditions.

Greenland ice-sheet surface properties observed by the Seasat-A scatterometer at enhanced resolution

AbstractFor 3 months in 1978, the 14.6 GHz Seasat-A scatterometer (SASS) measured the normalized microwave-radar back-scatter coefficient of the Earth’s surface for the purpose of estimating near-surface vector winds over the ocean. SASS also made back-scatter measurements over land and ice regions; however, the application of this data has been limited due to the low (50 km) resolution of the measurements. Using a new technique for generating 6 km enhanced-resolution SASS images of the radar back-scatter characteristics, we present a study of the 1978 condition of the Greenland ice sheet. We derive a time-series of back-scatter images spanning the period July–September 1978. These images show the extent of summer ablation along the ice-sheet periphery. Using the data and models relating firn structure and condition to radar back-scatter characteristics, we delineate and map the seasonal extent of zones which appear to correspond to dry-snow, percolation, wet-snow, and ablation facies, over virtually the entire ice sheet. The results provide a base line with which to compare current (ERS-1) and future Greenland radar maps of snow-and ice-surface conditions.

Resolution enhancement of spaceborne scatterometer data

A method for generating enhanced resolution radar images of the Earth's surface using spaceborne scatterometry is presented. The technique is based on an image reconstruction technique that takes advantage of the spatial overlap in scatterometer measurements made at different times to provide enhanced imaging resolution. The reconstruction algorithm is described, and the technique is demonstrated using both simulated and actual Seasat-A Scatterometer (SASS) measurements. The technique can also be used with ERS-1 scatterometer data. The SASS-derived images, which have approximately 4-km resolution, illustrate the resolution enhancement capability of the technique, which permits utilization of both historic and contemporary scatterometer data for medium-scale monitoring of vegetation and polar ice. The tradeoff between imaging noise and resolution inherent in the technique is discussed. >

A Diagnostic Study of the Impact of SEASAT Scatterometer Winds on Numerical Weather Prediction

With the launch of ERS-1, observations of the wind close to the sea surface are expected to become available in near real time. To assess the effect of this new form of meteorological data an operational numerical weather prediction, the impact of similar data, from the SEASAT-A scatterometer in 1978, has been studied in the numerical forecasting system of the U.K. Meteorological Office. Some modifications to the system were necessary before the SEASAT data were assimilated into the model. The geographical distribution of the differences between the analyses in a control run (without SEASAT data) and a parallel run (including SEASAT data) showed the largest values in the Southern Hemisphere and in association with some disturbances in the tropics. Studies have been made of the influence of some of the internal features of the assimilation scheme, especially the vertical cutoff in the data selection and the vertical correlation function. Changes to the model's divergence and vorticity have also been investigated. Overall, the inclusion of the SEASAT data appears to have slightly improved the model analyses. The differences between the two versions of the forecasts to 24 hours have been reviewed. Assessment is difficult because of the lack of corroborating data, but a beneficial effect has been found in the meridional wind, and a change in the ageostrophic flow has been detected.

Beaufort-Chukchi seas summer and fall ice margin data from Seasat: conditions with similarities to the Labrador Sea

The margin of the sea-ice pack of the Beaufort and Chukchi seas is examined using the microwave data from Seasat taken during the summer to early fall, July 4 through October 8, 1978, and the observations are compared to the analogous observations taken in LIMEX'87. The sensors used are synthetic-aperture radar (SAR), the Seasat-A scatterometer system, and the Scanning Multichannel Microwave Radiometer. >

Assimilation of scatterometer winds into surface pressure fields using a variational method

A variational formulation was used to assimilate Seasat‐A scatterometer (SASS) surface wind measurements near and during a severe storm in the North Atlantic into conventional National Meteorological Center sea level pressure fields. An estimate of the relative vorticity at every point on a grid was calculated using each of these two data sets. A solution to a modified geostrophic stream function is found subject to the constraints that (1) the relative vorticities calculated from the data agree as closely as possible with the relative vorticities from the variational solution, and that (2) the average kinetic energy is a minimum. Results are obtained which support the idea that averaged satellite data can be treated as synoptic data. Direct substitution rather than a time‐weighted insertion made from SASS winds generally resulted in more accurate pressure analyses. In addition, this relatively simple model provides surface pressure fields which agree extremely well with surface truth and the results of other investigators who required additional sources of input data into more complex models. It will be possible to obtain improved wind field maps from future scatterometer pressure fields in mid‐latitudes.

Oceanic Wind and Balanced Pressure-Height Fields Derived from Satellite Measurements

Seasat-A scatterometer system (SASS) measurements of wind speed and direction and GOES-2 satellite measurements of cloud motion directions were used to analyze the surface wind field over part of the northern Pacific Ocean for 17 July and 3 October 1978. Surface pressure fields were computed from the nondivergent SASS-derived wind velocity fields using the balance equation and were compared to the National Meteorological Center surface pressure fields. Agreement between the balanced pressure-height fields and National Meteorological Center pressure fields was good. The linear correlation coefficient between the two fields was 0.91 for 17 July, and 0.84 for 3 October 1978. These results indicate that the surface wind and pressure fields of the global oceans (excluding a narrow equatorial zone where the balance equation is invalid) can be determined with acceptable accuracy by using satellite measurements exclusively.

Some Considerations in The Evaluation of Seasat-A Scatterometer (SASS) Measurements

A study is made of the geophysical algorithms relating the Seasat-A scatterometer (SASS) backscatter measurements σ° with a wind parameter. It is shown that although σ° is closely related to surface features, identification with surface layer parameters such as friction velocity u* or the roughness length z0 is difficult from both theoretical and practical considerations. It is shown how surface truth in the form of wind speeds and coincident stability can be used to derive either u* or the equivalent neutral wind uN at an arbitrary height. It is also shown that derived u* values are sensitive to contested formulations relating u* to z0, while derived uN values are not. Also, derived quantities are seen to be more sensitive to changes in positive stability than to change in unstable conditions. Examples of geophysical verification is demonstrated using values obtained from the Gulf of Alaska Seasat Experiment (GOASEX). Results of this verification show very little sensitivity to the type of wind parameter employed. It is suggested that this insensitivity is due mainly to large scatter in the SASS and surface truth data.

Some considerations in the evaluation of Seasat-A scatterometer (SASS) measurements : Halberstam, Isidore, 1980. J. phys. Oceanogr., 10(4): 623–632

Some considerations in the evaluation of Seasat-A scatterometer (SASS) measurements : Halberstam, Isidore, 1980. J. phys. Oceanogr., 10(4): 623–632