ScanSAR interferometric monitoring using the PS technique. A. Monti Guarnieri Dipartimento di Elettronica e Informazione - Politecnico di Milano Piazza L. da Vinci, 32 - 20133 Milano - Italy fax: +39-2-23993585, e-mail:
[email protected]
Abstract A technique to provide quite accurate monitoring of small displacements by means of ASAR ScanSAR acquisition is here presented. This technique merges the advantages provided by Permanent Scatterers, namely the high accuracy, and the correlation over wide temporal and spatial baselines, with the advantages provided by WSM, in terms of short revisiting time and global coverage. It is then possible to avoid the marked decorrelation that would arise from the large ENVISAT orbit repeat time-interval and the large ScanSAR resolution cell. The feasibility study here presented has been performing by exploiting a large set of ERS data takes to simulate repeat pass ScanSAR acquisitions. The porting of the PS technique to this case led then to promising results in terms of accuracy and density of PS in the low resolution WS image. Basing on those results, the major limitations and possibilities of the technique have been identified.
1 Introduction ENVISAT ASAR sensor and products have been designed to get good interferometric capability. The long orbit repeat interval (35 days), makes the system much more suitable for monitoring than for DEM production, and the former is indeed the most demanding interferometric application of the future. Besides the temporal decorrelation that will affect anyway ENVISAT interferometry, the capability of getting frequent revisiting on a certain area could be limited by the acquisition planning, that has to select one of the possible 37 different operative modes of the ASAR sensor, to meet the desires of the potential users. The ENVISAT ASAR operative modes are summarized in Fig. 1. Among these modes, the WSM is indeed an interesting choice, since it covers the whole 400 km swath and it has an acceptable azimuth resolution of 150 m, whereas the range resolution can be programmed by tuning the chirp rate, up to the full resolution case (Image Mode). In this paper we propose a monitoring application that exploits Permanent Scatterers [1] and Wide Swath Mode acquisitions. The synergy of these two techniques will lead to several advantages:
the centimetric accuracy by PS (implied in their intrinsic stability) will be conserved; the temporal decorrelation expected by the large ENVISAT repeat time will be avoided, as PS are by definition correlated in long time intervals; decorrelation due to non-synchronized ScanSAR acquisition [2] will be avoided, since PS are correlated over wide angular aperture; the small revisiting time allowed by the large ScanSAR swath (< 7 days [3]) will be conserved, once that a suitable number of images would be acquired, at different orbit positions; the volume scattering decorrelation, that is expected to be strong in urban areas, enhanced by the large WSM resolution cell and by the larger baseline, will be prevented
All these advantages will be achieved provided that a suitable number of PS exists. PS existence is due to the fact that a stable and strong point-like scatterer dominates in the resolution cell. The PS should be enough to provide an estimate of the atmospheric contribution, that is correlated over tenths or hundred of meters. In this paper, we will simulate ScanSAR acquisition starting from full resolution ERS dataset, and provide statistics and results about the survivals of ERS - PS in the degraded WSM resolution cell. 1
c European Space Agency (ESA). Figure 1: ENVISAT ASAR operative modes.
2 Permanent Scatterers Interferometry Getting accurate “Displacement Fields” out of a set of PS spread over large areas entails a quite complicated processing, that is summarized in the block diagram of Fig. 2. The identification of (1) PS, (2) the Atmospheric Phase Screen (in each image) and, (3), of the Displacement Field are carried out jointly by exploiting the different characteristic of the three phase fields in the 3D space formed by baseline, time and “scene”. Topography is estimated first, by a regression over the baseline. Then, a phase unwrapping over the sparse grid of PS is carried out to identify the APS and DF phase fields: these two fields are then separated basing on the incorrelation of APS with time. The capability of estimating the PS DF is subject to the correct estimate of topography and APS, and this requires a suitable sampling in both the baseline and the space domain. This is usually fulfilled in full resolution acquisitions over urban areas, like in Pomona, where the measured density of 1000 PS /km 2 (with < 0:7 rad) was dense compared to APS field correlation (a few hundred of meters). The extension to ScanSAR case is however to be checked.
3 ENVISAT ScanSAR mode Let us assume the ENVISAT WSM ScanSAR mode. Here, an azimuth resolution of 150 m is achieved with 3-4 looks (the inter-burst interval being 800 ms) for each of the 5 subswaths (range swath is 400 km). The range bandwidth is programmable, and it can be as high as the full resolution one (16 MHz), as it will be assumed in the following. It can be shown that ScanSAR acquisition and phase preserving focusing of one burst results in an azimuth varying bandwidth filtering (further details are given in literature), that can be approximated by the following (azimuth varying) transfer function:
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Figure 2: Schematic block diagram of the PS technique. centroid frequency, T F the footprint time and the target zero-Doppler time. The central frequency, f x0 , sweeps with azimuth resulting in a non-stationary spectrum: f x0 = fR ( 0 ) (where 0 is the origin for target time coordinate), typical of ScanSAR, and this causes decorrelation for interferometry over non-synchronized scans [2]. The small available bandwidth, f R TF ' 50 Hz causes the loss of azimuth resolution, typical of ScanSAR. In the study here reported, the ScanSAR mode has been simply simulated by windowing ERS raw data according to the ”burst” timelining. Such simulation is quite good for WSM subswaths SS1/SS2, that share the same ERS geometry. The scenes resolution loss is evident in Fig. 3, that shows the incoherent superposition of 40 full resolution focused image of the data-set of Pomona (LA) and the same superposition of 40 simulated ScanSAR images (3 azimuth looks each). Even if the WSM resolution does not allow to distinguish the blocks in the urban areas, the major and brightest features are still evident.
4 ScanSAR PS interferometry The ScanSAR modeling here introduced shows that the overall acquisition and focusing makes ScanSAR system close to a low resolution SAR. In this sense, the porting of PS technique to ScanSAR seems a straightforward modification of the schematic diagram of Fig. 2. However, it is necessary to revisit the basic assumptions of PS interferometry, and to include the specifics feature of ScanSAR acquisition to guarantee the success of the technique.
4.1 PS visibility in full resolution SAR The PS visibility in SAR interferometry is subject to the condition that the stable scatterer is the dominant one in the resolution cell. This implies that the PS 0 is than all the other scatterers in the neighboring area. A typical histogram of PS reflectivity measured in an urban area is shown in Fig. 4: the average PS has a Radar Cross Section '500 m 2 ; corresponding to an 80 cm trihedral, that is 10 dB better than the typical mean reflectivity of an urban area (at least for the data set tested). This RCS corresponds roughly to a backscattering 0 = 5 dB that, compared with the Noise Equivalent 0 of ERS (-24 dB), we get a very good SNR of 30 dB for the “average” PS, that for some PS could be much higher. Clearly, the high SNR is the reason (necessary, not sufficient) of the good phase accuracy provided by PS.
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Figure 3: (a) Superposition of 40 SAR images, area of Pomona (8 km azimuth 16 km round range), resolution 5 20 m. (b) Simulated ASAR WSM ScanSAR multiple images superposition, resolution 150 20 m. In the area there are 71000 PS. PS non PS
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Figure 4: Histogram of the reflectivity of PS and that of all the other scatterers in an urban area. Test site: Milano (Italy). Reflectivities have been averaged on 60 images. 4
4.2 PS visibility in ScanSAR The availability of PS in ScanSAR is expected to be quite lower than in full resolution SAR, since the PS reflection is here compared with the superposition of reflections over a much larger area (30 times for ENVISAT ScanSAR / full resolution). This has been checked by simulating the ScanSAR acquisition in the data set of Pomona - where the “full-resolution” PS location was known. We assumed 220000 PS in an area of 400 km 2 , selected for a phase dispersion < 0:65 rad (SN R >10 dB). Two ScanSAR images have the been simulated starting from the full resolution one, by means of the proper azimuth-varying bandpass filter after: (a) applying the PS mask and (b) by canceling the PS. The scatter plot of the signal (the PS 0 ) versus “noise” (defined as the combined reflectivity of all the “non-ps”), measured in correspondence of each PS is in Fig. 5: it tells the expected SNR of each target in the ScanSAR image. In the figure, the SNR=0 dB level is marked by a line: 1/3 of the PS “survive” above that level, however many PS experience a SNR of 20 dB. Occurrence (%) 0.16
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Figure 5: Scatter plots of PS reflectivity vs. “background” (intended as the contribute of non-PS) in the simulated ScanSAR image. Only one PS per resolution cell has been assumed.
4.2.1 Super-PS In deriving the PS versus “non-PS” SNR in the ScanSAR image, we have assumed that the eventual contribution of multiple PS to a same ScanSAR resolution cells results in a destructive interference. In that case, only the “strongest” PS has been accounted for “signal”, whereas the others adds up to noise. It corresponds to the “volume scattering” decorrelation, that is commonly experienced in SAR interferometry over urban areas, where PS can be distributed at very different heights over the reference “flattened” DEM. In the study we assumed also that this decorrelation could be avoided if the location of all PS in the resolution cell would be known, e.g. by assuming that PS map and heights are known from full resolution processing. The percentage of PS that survive in the ScanSAR acquisition has been evaluated in both cases discussed here, just by integrating the histogram in Fig. 5. The result, in Fig 6, refers to the single look case: the final SNR would however benefit of the contribution of the 4 overlapping looks in ASAR WSM mode. The number of PS that “survive” exponentially reduces with the accuracy. If we impose that the phase noise contribution due to the non-PS is equated to the maximum PS (0:65 in case assumed), we get a “survive rate” of 30% for SAR-ScanSAR combination and 20% for the ScanSARScanSAR combination (we have assumed that noise is incorrelated in the different looks). If we accept a reasonable loss of quality, like SNR=0 dB (coherence = 0:7) we would achieve a “survival rate” of 30% (50%) of the PS. This turns out to be a density of 170 PS/km 2 in the urban are tested, and this value reasonably allows for compensating the APS (whose correlation extends for a few hundreds of m).
4.3 Results from simulations The results achievable by ScanSAR PS interferometry are visible in Fig. 7.a. In the example, the ScanSAR interferogram has been compensated for the precise PS topography and masked on the PS location. The conventional interferometry 5
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Figure 6: Percentage of PS whose SNR (in the ScanSAR image) is above a certain thresholds. The two curves differs for the presence of multiple PS in the ScanSAR cell, that has been assumed destructive (continuous line) or constructive (dot line). would result in pure noise, even in the case of perfectly aligned scans, since the high baseline ( 800 m) would decorrelate a distribution of scatterers within a height of 10 m in 15020 m 2 resolution cell. The residual fringe cycle detectable in the figure is mainly due to the terrain sinking, caused by water pumping in the110 days temporal baseline, and much less to atmosphere (that, however, can be retrieved as explained in [1]). The average phase field is consistent with the one detectable by the PS full resolution interferogram in Fig. 7.b: the histogram of the difference between the two phase fields is in Fig. 8.a. The phase standard deviation is rather low, 0.3 rad, and has been achieved by selecting in the ScanSAR image only the PS that “survive” with a quite good SNR (+3 dB). This choice is quite suitable to track the low frequency motion due to subsidence. On the other hand, conventional ScanSAR interferometry would completely Fail in this case, since the volume scattering decorrelation prevails, even in the case of synchronized acquisition, like Fig. 8.b shows. PS phase, full resolution 0 3
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Figure 7: PS ScanSAR (a) and full resolution (b) interferometry (area of Pomona). Despite the significant reduction in the number of PS in the ScanSAR resolution cell fringe due to motion are still visible.
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5 Conclusions The capability of ENVISAT WSM ScanSAR mode to provide interferometry by exploiting Permanent Scatterers in urban areas have been validated. Compared with the full resolution case, the number of “PS survival” is strongly reduced: up to 5% of the PS still maintain an SNR > 10 dB ( < 0:65 rad). However, the PS density achieved by relaxing that SNR constraint still allows for the proper detection of Atmospheric Phase Screen and separation from Displacement Field. Clearly, ScanSAR resolution does not allow to detect motion of single buildings; however, it is quite suited for monitoring “smooth” terrain swelling. The unique impulsive feature of PS allows for removing decorrelation term due to volume scattering and asynchronous scanning that, otherwise, would prevent interferometry. The major limitation of the technique is in the amount of images required to identify PS, correspondent to a two years time period (in the ENVISAT case). ScanSAR interferogram
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Figure 8: (a) Histogram of phase differences between full res. and PS ScanSAR interferometry. (b) Conventional ScanSAR interferometry (synchronized acquisitions). To be compared with the Fig. 7.b. In this case, the volume scattering decorrelation, enhanced by the large 800 m baseline dominates and no information can be retrieved.
References [1] A Ferretti, C Prati, and F Rocca. Non-uniform motion monitoring using the permanent scatterers technique. In Second International Workshop on ERS SAR Interferometry, ‘FRINGE99’, Li e` ge, Belgium, 10–12 Nov 1999, page 6. ESA, 1999. [2] Andrea Monti Guarnieri and Claudio Prati. ScanSAR focussing and interferometry. IEEE Trans. on Geoscience and Remote Sensing, 34(4):1029–1038, July 1996. [3] A Monti Guarnieri and F Rocca. Combination of low- and high-resolution SAR images for differential interferometry. IEEE Trans. on Geoscience and Remote Sensing, 37(4):2035–2049, July 1999.
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