Principles of Modern Radar - Volume 2 1891121537

8.5 InSAR Processing Steps 373While it can be argued that the goal of phase unwrapping should be to find the L 0solution, it can also be shown that the L 0 solution of the phase unwrapping problemis NP-hard, in effect stating that it is theoretically intractable. This merely means thatunwrapping algorithms should focus on developing efficient heuristic approximations tothe solution. This observation also motivates the use of L 1 solutions, since it is well-knownin optimization that L 0 and L 1 solutions are often similar but that L 1 solutions can usuallybe found much more efficiently.8.5.5 Multibaseline InSARAn alternative approach to resolving phase ambiguities uses a three phase-center systemto provide two different interferometric baselines and therefore two different ambiguityintervals. This approach is very similar in concept to the multiple pulse repetitioninterval (PRI) techniques commonly used to resolve range and Doppler ambiguities inradar [12]. The longer baseline provides precise but ambiguous elevation estimates, asdiscussed earlier, while the short baseline, while imprecise, provides a large ambiguouselevation interval that can be used to resolve ambiguities. A particular implementation inthe RTV system uses two antennas, one a conventional antenna and the other an amplitudemonopulse antenna [7]. A baseline of 0.33 m is formed between the conventionalantenna and the monopulse sum port, while a second short effective baseline of 0.038m is formed by the elevation monopulse antenna. With an RF of 16.7 GHz, p = 1,β = 90 ◦ , and nominal range of 5 km and depression angle of 30 ◦ , the 0.33 m baselinehas an ambiguous elevation interval h amb of about 363 m (equation (8.28)), while theshort 0.038 m baseline gives h amb of about 3.15 km, more than enough to represent anyexpected elevation variation for most terrains. This system requires no phase unwrappingalgorithm at all; a very simple algorithm suffices to remove the phase ambiguity inthe conventional InSAR image using the short-baseline monopulse angle of arrival information.An additional advantage is that the IPD ambiguity removal is performed ona pixel-by-pixel basis, so errors, if they occur, do not propagate throughout the scene.The cost of this improvement is that three receiver channels and image formers mustbe implemented, one each for the monopulse sum, monopulse elevation difference, andsecond antenna channels. Figure 8-23 gives both an orthorectified SAR image of thePentagon and the corresponding DTED level 4 DEM generated by the RTV system.FIGURE 8-23DEM of thePentagon generatedby the RTV system.(a) OrthorectifiedSAR image, 0.75 mposts. (b) DTED level4 digital elevationmap. (Courtesy ofSandia NationalLaboratories.)(a)(b)