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    01-Oct-2014
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Radar Course III
37. Bragg scattering
43. Texture and image analysis
42. Temporal averaging
12. Synthetic Aperture Radar (SAR)
34. Space, time and processing constraints
15. Slant range / ground range
8. Side-looking radars
19. Shadow
10. Real Aperture Radar: Range resolution
11. Real Aperture Radar: Azimuth resolution
9. Real Aperture Radar (RAR)
7. Radar principles
38. Radar image interpretation
35. The radar equation
36. Parameters affecting radar backscatter
16. Optical vs. microwave image geometry
25. Method
18. Layover
32. Landers Earthquake in South California
23. Introduction
27. Interferogramme of Naples (Italy)
29. Interferogramme and DEM of Gennargentu (Italy)
2. Independence of clouds coverage
40. Image interpretation: Speckle
41. Image interpretation: Speckle filters
39. Image interpretation: Tone
20. Geometric effects for image interpretation
22. Geocoding: Geometry
17. Foreshortening
26. First ERS-1/ERS-2 tandem interferogramme
6. Electromagnetic spectrum
30. Differential interferometry
45. Data reduction: 16 to 8 bit, blockaverage vs incrementing
4. Control of imaging geometry
3. Control of emitted electromagnetic radiation
24. Concept
28. Coherence image of Bonn area (Germany)
44. Classification of ERS-1 SAR images with Neural Networks
5. Access to different parameters compared to optical systems
13. SAR processing
33. SAR interferometric products
21. SAR image geocoding
14. ERS SAR geometric configuration
31. The Bonn experiment
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Bragg scattering

As the incidence angle of the ERS SAR is oblique (23) to the local mean angle of the ocean surface, there is almost no direct specular reflection except at very high sea states.

It is therefore assumed that at first approximation Bragg resonance is the primary mechanism for backscattering radar pulses.

The Bragg equation defines the ocean wavelengths for Bragg scattering as a function of radar wavelength and incidence angle:

The short Bragg-scale waves are formed in response to wind stress. If the sea surface is rippled by a light breeze with no long waves present, the radar backscatter is due to the component of the wave spectrum which resonates with the radar wavelength.

The threshold windspeed value for the C-band waves is estimated to be at about 3.25 m/s at 10 meters above the surface. The Bragg resonant wave has its crest nominally at right angles to the range direction.

For surface waves with crests at an angle to the radar line-of-sight (see this figure ) the Bragg scattering criterion is

where:
is the wavelength of the surface waves propagating at angle to the radar line- of sight.

The SAR directly images the spatial distribution of the Bragg-scale waves. The spatial distribution may be effected by longer gravity waves, through tilt modulation, hydrodynamic modulation and velocity bunching.

Moreover, variable wind speed, changes in stratification in the atmospheric boundary layer, and variable currents associated with upper ocean circulation features such as fronts, eddies, internal waves and bottom topography effect the Bragg waves.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry