2 - SAR Imaging using Coherent Modes of Diversity: SAR Polarimetry, Interferometry and Tomography
Résumé
Abstract: Spectral and spatial diversity measurements, Synthetic Apertureă Radar (SAR) imaging uses modes to focus 2D images of electromagnetică reflectivity of environments. Even though this information is veryă useful for detecting objects or to evaluate some of their geophysicală properties, it has some limitations for further applications or characterizations.ă In fact, very diverse objects, such as a vehicle or a forest parcel,ă may provide responses with a similar energetic level. As a resultă their differentiation must necessarily involve additional information,ă among which polarization diversity is often considered. Classic SAR imaging uses a system of antennas polarized in the same way, andă so measures 2D scalar information. The use of antennas with differentă polarizations allows the measurement, for each image pixel, of aă multi-variate polarimetric quantity which provides information onă the intrinsic geophysical properties of the imaged objects. Anotheră significant limitation of SAR imaging comes from the naturală ambiguity linked to the 2D mapping of 3D environments. This dimensionă reduction causes a direct loss of height related information becauseă of height and, in particular, mixes the contributions from scatterersă located at different altitudes. The solution usually considered usesă additional spatial diversity to determine, for each 2D SAR image pixel, either the elevation position of the main scatterer, or theă vertical distribution of the imaged reflectivity, for tomographică applications. It should be noted that \SAR\ interferometry (InSAR)ă requires the coherent combination of N = 2 \SAR\ measurements,ă compared with N ? 3 for tomography and 2 ? N ? 4 for polarimetrică measurements. These different measurement modes, which can be combinedă with each other, are illustrated. The use of wave polarization oră the measurement of an object?s position through interferometry areă widely known techniques used since the post-war period in the fieldă of optics. During the 1950s and 1960s, a group of researchers developedă the theoretical tools necessary to handle coherent multi-variateă data, using wave polarization to differentiate radar obstacles oră to identify them from their unique polarimetric behaviors. In theă 1970s, deeply modified the approach of polarimetric data processingă by combining a rigorous mathematical analysis with a phenomenologicală interpretation of electromagnetic scattering mechanisms, paving theă way for sophisticated techniques such as the ones presented in thisă chapter. \SAR\ interferometry, which can determine the elevationă position of the different \SAR\ image pixels, appeared in the 1970s?1980să with airborne applications, and then satellite applications withă SEASAT-A measurements. The introduction of a nearly uninterruptedă series of satellite missions adapted to the InSAR mode since theă ERS-1 launch in 1991 has resulted in a boom in \SAR\ interferometryă techniques, which have now reached a nearly industrial level of maturityă and which provide, through differential \SAR\ interferometry, aă technique with unique performances. \SAR\ tomography was born ată the beginning of the 2000s and has made it possible to image a sceneă in 3D by combining several inSAR images. To date, its applicationă is limited to airborne data, but adjustments are currently beingă studied to extend this approach to satellite measurements.