Principles of Modern Radar - Volume 2 1891121537

8.4 InSAR Operational Considerations 359(a)FIGURE 8-12 Images of Mount Vesuvius. (a) ERS-2 SAR image showing foreshorteningand brightening of the slopes of the volcano facing the radar (upper slopes in this image).Compare with Google Earth satellite image in (b). (SAR image courtesy of the EuropeanSpace Agency (ESA). Google Earth image c○ 2011 Google and c○ 2011 GeoEye.)tomographic aperture at a fixed time and slant range for scatterers of different elevationsshows that it consists of a term that is linear in scatterer elevation, and a quadratic termthat does not depend on scatterer elevation. Applying an appropriate set of weights acrossthe tomographic aperture outputs for a given pixel creates a beam steered to a specifiedelevation. Varying the weights to vary the focus elevation allows creation of a verticalprofile of the scattering scene. The angular resolution obtained is on the order of λ / D t⊥ ,where D t⊥ is the tomographic aperture size orthogonal to the line of sight. For pixels at arange R, this becomes a vertical resolution on the order of λR / D t⊥ meters. As with anysampled aperture, attention is needed to aperture spacing and weight tapering to controlgrating lobes and sidelobe levels [12].The phase model above leads to an alternative vertical resolving method using Fouriertransforms. If the vertical aperture samples are evenly spaced, then the phase slope acrossthe aperture is identical to that of a sampled sinusoid whose frequency depends on thescatterer elevation. A discrete Fourier transform (DFT) of the vertical data then producesa vertical image for a given pixel. In practice, interpolation in the vertical dimension isneeded to produce data on an evenly sampled vertical grid prior to the DFT.(b)8.4 INSAR OPERATIONAL CONSIDERATIONS8.4.1 One-Pass versus Repeat-Pass OperationInSAR data collection operations can be characterized as one-pass or repeat-pass. Figure8-13 illustrates these two cases. In one-pass processing, a platform with two physicalreceive apertures, each with an independent coherent receiver, collects all of the radar dataneeded in a single pass by a scenario of interest. As discussed earlier, either one or twotransmitters can be used and the receivers can be displaced horizontally as shown, vertically,or along any baseline orthogonal to the velocity vector. The two SAR images a(x,y)