A Geosat altimeter wind speed algorithm and a method for altimeter wind speed algorithm development

Witter, Donna L.; Chelton, Dudley B.

doi:10.1029/91jc00414

Cited by 189 publications

(109 citation statements)

References 9 publications

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“…With the correction of the tilting effect in the cross section measurement, the calculated wind speed is found to be in much better agreement with the surface buoy measurement. The improvement is of the order of 40% when compared to the results derived from other statistical or empirical algorithms including Brown et al [1981 ], Witter and Chelton [1991 ], and Freilich and Challenor [1994]. This result suggests that the theoretical framework relating the backscattering cross section and the surface roughness is fundamentally sound when the tilting effect that modifies the local incident angle is taken into account.…”

Section: Discussionsupporting

confidence: 57%

A statistical comparison of wind speed, wave height, and wave period derived from satellite altimeters and ocean buoys in the Gulf of Mexico region

Hwang¹,

Teague²,

Jacobs³

et al. 1998

J. Geophys. Res.

157

113

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Abstract. The capability of spaceborne altimeters to provide precise measurement of significant wave height and wind speed has been demonstrated repeatedly. It is shown in this paper that in addition to the significant wave height and wind speed, the wave period can be calculated from the semiempirical functions established from earlier wave research. The calculated characteristic wave period using the altimeter-derived wind speed and wave height are found to be in excellent agreement with the peak wave period and average wave period from the ocean buoy measurements in the Gulf of Mexico. Also, with the long time series of collocated data set, it is possible to compare altimeter output of wind and wave parameters with ocean buoy measurements taking into consideration the spatial lags between the buoy locations and the altimeter footprints, and the temporal lags between the two sensor systems. It is found that when the spatial lags are less than 10 km, the RMS difference of the significant wave height is approximately 0.1 m, which is the digitization resolution of the output from both altimeter and ocean buoy. For the wind speed, the RMS difference approaches 1.2 m/s in the Gulf of Mexico using the empirical algorithms. The wind speed agreement is significantly improved to 0.8 m/s when the tilting effect on the altimeter cross section is accounted for. In contrast to the spatial lags, temporal lags of up to 1 hour do not appear to produce significant difference in the statistics of comparison based on this study.

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Section: Discussionsupporting

confidence: 57%

A statistical comparison of wind speed, wave height, and wave period derived from satellite altimeters and ocean buoys in the Gulf of Mexico region

Hwang¹,

Teague²,

Jacobs³

et al. 1998

J. Geophys. Res.

157

113

View full text Add to dashboard Cite

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“…In general, the radar altimeter gives wind speed with an accuracy of 2 m/s in the range of 2 to 15 m/s. The lower and upper limits are due to the inherent measurement limitations associated with specular reflections [19].…”

Section: Independent Sources Of Data For Comparisonmentioning

confidence: 99%

“…The wind speed measurements are derived using the model function provided by Witter and Chelton [19] (see also [20]). This function is an empirical relationship between wind speed and TOPEX-derived values derived from some 240 000 Seasat scatterometer measurements.…”

Section: Independent Sources Of Data For Comparisonmentioning

confidence: 99%

Wind speed measurement using forward scattered GPS signals

Garrison

Komjáthy

Zavorotny

et al. 2002

IEEE Trans. Geosci. Remote Sensing

295

137

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Abstract-Instrumentation and retrieval algorithms are described which use the forward scattered range-coded signals from the global positioning system (GPS) radio navigation system for the measurement of sea surface roughness. This roughness has long been known to be dependent upon the surface wind speed. Experiments were conducted from aircraft along the TOPEX ground track and over experimental surface truth buoys. These flights used a receiver capable of recording the cross-correlation power in the reflected signal. The shape of this power distribution was then compared against analytical models, which employ a geometric optics approach. Two techniques for matching these functions were studied. The first recognized the most significant information content in the reflected signal is contained in the trailing edge slope of the waveform. The second attempted to match the complete shape of the waveform by approximating it as a series expansion and obtaining the nonlinear least squares estimate. Discussion is also presented on anomalies in the receiver operation and their identification and correction.

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“…These eliminate a systematic bias in the Witter & Chelton (1991) algorithm. The best results are obtained when using the altimeter Ku-band backscatter coefficient together with the altimeter significant wave height measured at C band (Figure 2).…”

Section: Model Developmentmentioning

confidence: 99%

Towards a unified theoretical model of ocean backscatter for wind speed retrieval from SAR, scatterometer, and altimeter

Anderson¹,

Macklin²,

Gommenginger³

et al. 2002

Canadian Journal of Remote Sensing

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INTRODUCTIONEmpirical models of ocean radar backscatter have shown that spaceborne radars can provide quantitative information about wind fields. Examples include the CMOD and IFREMER-2 models developed for the ERS Wind Scatterometer. However, such models are limited because they do not include all the relevant physical effects, such as the influence of swell, rain, surface slicks and currents. Recent theoretical developments have addressed these aspects, and here we consider the construction of a 'unified' theoretical model, that is, one with a common set of parameters applicable to SAR, scatterometer and altimeter observations. MODEL DEVELOPMENTThe model requires a scattering theory to be applied to a theoretical description of the ocean waveheight spectra. We compare predictions from the composite-surface and IEM (integral equation method) scattering models, using ocean spectra described by the models of Apel (1994), Elfouhaily et al. (1997) and Lemaire et al. (1999). We also incorporate a swell component and limited fetch, and we use published models of the perturbations to the wave spectrum due to the presence of rain, surface slicks or currents. The resulting model shows the sensitivity of the radar backscatter to all these parameters. It also predicts dependences on radar parameters which are relevant for future missions such as Envisat.Finally, it predicts the circumstances when the factors neglected by the empirical models such as CMOD will be significant. For example, it is found that the greatest effect on C-band, VV-polarised backscatter from swell is at low incidence angles, while from rain it is at high incidence angles (Figure 1).Note that the atmospheric attenuation from rain is not included in the modelling.New empirical algorithms for the recovery of wind speeds from altimeter data are developed by applying nonlinear functionfitting procedures to an extensive set of Topex data collocated with buoy measurements. These eliminate a systematic bias in the Witter & Chelton (1991) algorithm. The best results are obtained when using the altimeter Ku-band backscatter coefficient together with the altimeter significant wave height measured at C band (Figure 2). MODEL TESTSThe theoretical model reproduces the empirical dependence on wind speed at nadir very well, but it under-predicts the empirical dependence on swell properties. This discrepancy is reduced if an influence of swell on the relation between wind stress and wind speed is incorporated. This model is also tested by comparison with empirical scatterometer models at C and Ku band. The results indicate that the accuracy of the theoretical model is limited by the present understanding of the description of ocean wave spectra. For example, the IFREMER-2 model for C band, VV polarisation shows best agreement with the results from the Lemaire et al. (1999) spectrum, although there is a systematic difference at high wind speeds and high incidence angles (Figure 3). Also, the directional spread of waves is not well understood; this leads to a systemat...

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A Geosat altimeter wind speed algorithm and a method for altimeter wind speed algorithm development

Cited by 189 publications

References 9 publications

A statistical comparison of wind speed, wave height, and wave period derived from satellite altimeters and ocean buoys in the Gulf of Mexico region

A statistical comparison of wind speed, wave height, and wave period derived from satellite altimeters and ocean buoys in the Gulf of Mexico region

Wind speed measurement using forward scattered GPS signals

Towards a unified theoretical model of ocean backscatter for wind speed retrieval from SAR, scatterometer, and altimeter

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