Principles of Modern Radar - Volume 2 1891121537

386 CHAPTER 8 Interferometric SAR and Coherent Exploitationmultibaseline InSAR processing, eliminating the need for explicit phase unwrapping. Thesecond is real-time onboard generation of mosaiced InSAR data products at peak areamapping rates of 10 km 2 /minute (DTED level 3) or 3.5 km 2 /minute (DTED level 4).Additional information is available online (http://www.sandia.gov/radar/rtv.html).8.8 OTHER COHERENT EXPLOITATIONTECHNIQUESInSAR is a form of coherent exploitation of SAR imagery. This term refers to the useof multiple complex SAR images, collected from different vantage points, times, or bothto form new data products. 15 The distance between vantage points is called the spatialbaseline, while the difference in vantage times is called the temporal baseline. Spatialbaselines are further subdivided into along-track (collinear with the radar velocity vector)and cross-track (orthogonal to the velocity vector) components.The InSAR technique for measuring elevation variations discussed so far in this chapteruses images displaced along a cross-track spatial baseline and is sometimes referredto as cross-track interferometry (CTI). In this section, we briefly introduce three other coherentSAR exploitation techniques. Terrain motion mapping and change detection bothemploy images taken from the same vantage point but at significantly different timesand thus use a temporal baseline but no spatial baseline. Change detection generally impliestime baselines of minutes to perhaps days, while terrain motion mapping typicallyimplies time periods of days to years. Because the data are obtained by repeat-pass operationalong the same flight path, these are both examples of repeat-pass interferometry.The third exploitation method introduced here uses images generated from aperturesseparated by an along-track spatial baseline and is referred to as along-track interferometry.Due to the platform motion, ATI can also be viewed as short (less than 1 second,typically) temporal baseline processing. It is used for measuring time-varying elements inthe SAR scene, such as mapping of ocean wave fields, glacier movements, and detectionof moving surface targets. A significant extension of this concept enables the combinationof SAR imaging with interference cancellation using space-time adaptive processingtechniques. A good introduction and comparison of spatial, temporal, and mixed baselineradar applications is given in [51].8.8.1 Terrain Motion MappingInSAR presumes that there is no change in the imaged scene between the two image datacollections, so phase differences are due only to elevation variations viewed from slightlydifferent aspect angles. This assumption is clearly true in one-pass systems but may notbe in repeat-pass systems due to temporal decorrelation. This problem can also be anopportunity: methods similar to InSAR can be used to detect changes in terrain elevationover significant time periods.Terrain motion mapping examines the change in phase due to a change in scattererelevation at a fixed location on the ground between two different times [20]. As with15 The effective vantage point and time for a SAR image are, respectively, the position and time of theradar platform at the center of the synthetic aperture from which the image is formed.