Radar backscattering

McDonald (1962) has described the definition of the radar backscattering cross section as intrinsically awkward we heartily agree that it is awkward, but we... 

The traditional definition of the radar backscattering cross section can be stated clearly in a few words it is just 4it times what it ought to be. We therefore counsel the reader to mentally delete the factor 4it in (4.82) and only reintroduce it as a sop to convention when necessary. 

The extinction, scattering, and radar backscattering efficiencies are (to terms of order x4)... 

Glover D. M., Frew N. M., McCue S. J., and BockE. J. (2002) A multi-year rime series of global gas transfer velocity from the TOPEX dual frequency, normalised radar backscatter algorithm. In Gas Transfer at Water Surfaces, Geophysical Monographs (eds. M. A. Donelan, W. M. Drennan, E. S. Saltzman, and R. Wanninkhof). AGU, Washington, DC, vol. 127, pp. 325-333. 

Gade M, Alpers W, Hiihnerfuss H, Wismann VR, Lange PA (1998b) On the reduction of the radar backscatter by oceanic surface films scatterometter measurements and their theoretical interpretation. Rem Sens Environ 66 52-70... 

Gade M, Alpers W, Bao M and Huhnerfuss H (1996) Measurements of the radar backscattering over different oceanic surface films during the SIR-C/X-SAR campaigns. Proc... 

The Doppler spectra measured at HV-polarisation, without rain (dashed curves in Figure 6), show that no radar backscattering from a slick-covered water surface was measured up to wind speed of about 4 ms"1. Moreover, at 6-8 m s 1 only a very weak backscattered signal was measured. At 10 m s 1, where the surface film has already started to disperse, a significant radar Doppler signal was measured. 

The Doppler spectra measured at HV-polarisation at a rain rate of 160 mm h 1 (solid curves in Figure 6) are very similar in the whole wind-speed range used in the present investigation they show a symmetric, broad peak with its maximum at about -40 dB. The location of this maximum, however, depends on wind speed because of the wind-induced surface drift. From this finding we conclude that the radar backscattering at cross-polarisation from a slick-covered water surface agitated by rain is... 

The <7rei values measured without rain (crosses and pluses in Figure 7, corresponding to a slick-free and to a slick-covered water surface, respectively) show that the reduction of the radar backscattering by monomo-lecular slicks is lower at cross-polarisation than at co-polarisation. This finding is in contrast to the results presented by Braun et al. (2002) and we suspect that this is due to an insufficient signal-to-noise ratio encountered during the herein presented measurements. 

We also observed that, at wind speeds up to 8 m s 1, the radar backscattering at co- and cross-polarisation is mainly caused by the rain-induced effects. Since similar results were obtained at slick-lfee water surfaces, we conclude that the coverage of the water surface by a monomolecular slick has no significant influence on the measured radar backscatter. However, we expect larger differences are to be found at lower rain rates. 

Braun N, Gade M, Lange PA (2002) The effect of artificial rain on wave spectra and multi-polarisation X band radar backscatter. Int J Remote Sensing 23 4305-4322... 

During the two SIR-C/X-SAR missions in April and October, 1994, radar backscatter measurements were carried out with a 5-frequency/multi-polarisation scatterometer flown on a helicopter. This scatterometer, called HELISCAT, works at 1.25, 2.4, 5.3, 10.0, and 15.0 GHz (L-, S-, C-, X-, and Ku-band, respectively) and is capable of performing radar backscatter measurements at different incidence angles. 

Gade and Redondo (1999) used the same data set to derive areas of mean oil pollution by taking into account those dark patches in the SAR images that show a significant reduction of the radar backscatter. In all three regions they found more oil pollution during summer months (April - September) than during winter months (October - March), which they... 

In order to complement the results of the radar backscatter measurements on the open sea, laboratory measurements of the wave amplitude and slope and of the radar backscatter at X- and Ka-band were carried out in a wind-wave tank with mechanically generated gravity waves as well as with wind-generated waves on a slick-free and a slick-covered water surface. In this paper, we concentrate on the results of the radar measurements with wind-generated waves. For a full description of the obtained results the reader is referred to Gade et al. (1998c). 

The results from the wind-wave tank measurements show that at X- and Ka-band the radar backscattering from a slick-free water surface is caused by bound as well as by free propagating ripples. In the presence of a monomolecular surface film at certain wind speeds only bound or only free propagating ripples are responsible for the backscattering at X-band, which can explain higher measured damping ratios. 

A radar backscatter model that yields normalised radar cross section (NRCS), given a local ocean surface wave spectrum. 

On the basis of the extended VIERS wave spectral model presented above, it was attempted to calculate the radar backscatter along a transect through a slick on the sea surface. A slick of 50 m diameter of the same substance as used in the previous sections (oleic acid) was taken, and the radar backscatter was calculated along a 150 m section centred on the slick. A wind speed at 10 m height (t/10) of 5 m/s was used and the radar look direction was downwind. 

From these calculated wave spectra, the normalised radar cross sections (o °) were calculated using the VIERS-1 radar backscatter module (Janssen et al. 1998). This is a more or less standard composite model which includes the two-scale effect (Bragg backscatter influenced by longer wave... 

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