Scan Patter Syncronization in ENVISAT Wide Swath Mode

Ciro Cafforio(1), Davide D’Aria(2), Pietro Guccione(1), Andrea Monti-Guarnieri(3), Betlem Rosich(4) (1) DEE - Politecnico di Bari - Via E. Orabona, 4 - 70125 Bari - Italy

(2) Aresys – POLIMI spin-off - via Garofalo 39, 20133 Milano (ITALY) (3) DEI - Politecnico di Milano - Piazza Leonardo da Vinci 32 - 20133 Milano Italy

(4) ESA-ESRIN - Via Galileo Galilei - 00044 Frascati, ROMA, Italy

1. ABSTRACT The recent availability of single look complex ENVISAT ScanSAR product (WSS), opens new perspective in the field of interferometry for large scale, short revisit time applications. Several interferograms over large swaths, up to 400 km have already been generated demonstrating the feasibility of such techniques [1]. However, it is known that interferometry in ScanSAR mode is subject to a suitable synchronization between scan patterns. In this paper we investigate some properties of the scan pattern synchronization, and on constraints coming from the Doppler centroid and the baseline. We evaluate the occurrence of such synchronization for random scans, and we show preliminary statistics get from ENVISAT acquisitions. Finally, we provide examples of ScanSAR WSM/IM and WSM/WSM interferometry and applications.

2. INTRODUCTION: ENVISAT WSS PRODUCT The ASAR sensor, that was initially designed with interferometric capabilities, in full resolution (IM) and Alternating Polarization (AP), has provided outstanding achievement in its low-resolution-large-coverage ScanSAR mode, WSM. The ScanSAR acquisition scheme and the main features of WSM are summarized in Figure 1.

TR ~800 m

~ 405 km

4037332922Incidence angle

30 m (ground)Rg Resol (1-look)

120 mAz Res. (3-looks)

6171556550Echo smpls

910101314Bdwth (MHz)

SS5SS4SS3SS2SS1Swath

Figure 1 – Typical burst-mode acquisition and geometry for ENVISAT WSM mode, and a summary of the most relevant parameters..

In ScanSAR mode, bursts of 50-70 echoes are acquired cyclically out of each sub-swath, getting coverage for a strip that is 400 km wide on the ground (range). In ENVISAT WSM, each target in the scene is observed at least three times in three different bursts (two have been evidenced in Figure 1), by different view angles. The one-to-one angle-to-frequency correspondence of SAR ensures those different spectral portions of the target reflectivity are observed in the subsequent bursts. The processing required to get the complex focused product is summarized in Figure 2, referred to a single swath. The black stripes in the figure represent the data not acquired by the sensor (e.g. when the antenna was pointing in the other subswaths). The time-varying power spectrum (spectrogram) is superimposed as in color scale to the focused bursts, Figure 2b-d. The diagonal shape of the time-frequency domain support is typical of ScanSAR, and follows the Doppler rate.

Starting from 2005, ESA is providing phase preserving complex Wide Swath data as an added product, namely ASA_WSS_1P products (Wide-Swath-Single look), that can be generated, on users demand, from any Wide-Swath acquisition. The product covers a large scene of 400 x 400 km on the ground and stores data as a collection of complex focused bursts (or “looks”) for each of the 5 swaths. All the data are sampled on the same grid, in slant range, azimuth.

Azimuth sampling is quite coarse to save space, so the diagonal shaped power spectrum evidenced in Figure .b-d would be very hard to acknowledge to folding and tight spacing (in other word, the width of the power spectra in Figure 2b-d will be comparable with the whole sampling bandwidth). However, a Doppler Centroid grid is added to the product to allow users to reconstruct the absolute value of Doppler Centroid.

Azimuth

Ran

ge

Footprint (TF)

Burst n-1 Burst n Burst n+1

Raw data (rangecompressed)

Focused data

(a)

(b)

(c)

(d)

(e)

Figure 2 – ScanSAR acquisition and processing (within one of the 5 WSM subswath). Top to bottom: (a) range compressed data are organized in bursts of 50-70 samples, interleaved by zeros (that is when the antenna is switched to look the other subswaths), (b)-(d) Phase preserving focusing of the three bursts. Each burst:each imagette spans the whole antenna aperture,so it is ~30 times wider than the burst, but its resolution is also 30 times coarser. The colored overlay represtns the local iamge power spectrum: notice the space varying Doppler Centroid. (e) Focused data are stored in the WSS phase preserving product as a collection of imagettes (looks), notice that each target is observed in at least three looks.

3. ENVISAT WSM INTERFEROMETRY The complex, phase preserving WSS product can be used for interferometry in two different configurations [2], as

Figure 3 shows. The figure represents, on the left, an interferometric survey achieved by combing a full resolution IM-.mode and a WSM images. Such configuration allows for high quality fringes, where most of the geometric, baseline dependent decorrelation can be removed by exploiting a DEM [3]. However, the image size is limited to the IM capabilities, a strip 100 km wide on the ground.

The configuration shown in Figure 3 on the right combines two WSM-WSM images. In this case, geometric decorrelation may be significant, particularly in rough areas. However the most stringent constraint would come from synchronization of the scan-pattern in the two acquisitions that should be quite better than the burst extents (200 m) to get a good quality. The quality of the WSM/WSM interferogram would markedly depend upon the long term coherence of the scene and the topography, so it is expected low in vegetated areas, but in several cases (desert or rocky areas) coherences larger up to 0.95 have been measured. Examples of WSM-WSM interferograms in both vegetated and desert areas are shown in Figure 4. These interferograms can be generated by available commercial packages, starting from the WSS product, and exploited to estimate either subsidence of for DEM generation, an example of a DEM is show in Figure 5.

100 km

400 km

Figure 3 – Possible Wide-Swath Mode interferometric combinations, area of BAM, coseismic acqustions. Left: WSM, 2003/09/24, and IM, 2003/12/03, combination, baseline 70m. Right: WSM/WSM combination (2003/09/02, and 2004/06/08, Baseline = 100 m, ovelap 84%) .

Figure 4 – Large scale WSM-WSM interferograms (swath width 400 km), achieved on a highly vegetated area (northen Italy, date 2004/05/29 and 2005/07/23 on the left), and a desertic area (Caspio sea, date 2004/11/19 and 2005/03/04 on the right). Data courtesy of TRE (Tele-Rilevamento Europa).

4. SCAN PATTERN SYNCHRONIZATION The capabilities of producing combined WS/WS interferograms and their azimuth resolution is strongly dependent on the synchronization of the scanning pattern in the two acquisitions [4]. The fact is shown in Figure 6, that draws, on the right, the spectra of two repeat-pass focused datasets, acquired in case of slightly de-synchronized acquisitions. For example, let us assume that a specific target is observed at zero Doppler at the beginning of a burst in the first acquisition, and shifted in time of -Δt from the beginning of the burst in the second acquisition. The overlapped bandwidth (the darkest area in Figure 6, on the right), will be: RDc ftTB )( Δ−= , (fR being the Doppler rate), is the

inverse of the achievable azimuth resolution, and, for |Δt|<TD (the dwell time), no interferometric information can be extracted (in the case of homogenous targets). In Figure 6, the bell-shaped Power Spectra of the two acquisitions peak for different frequencies, corresponding to the Doppler Centroid. A Doppler Centroid mismatch, like the one assumed there, introduces a different illumination, thus an amplitude effect that has no significant impact as far the shift is comparable to the processed azimuth bandwidth (hundred of Hz).

Figure 5 – Generation of Digital Elevation Model form repeat-pass WSM interferometry, area of Senegal (image courtesy of SARMAP SA).

WS1 spectral support

WS2 spectral support

frequency [Hz]

fDC

spec

trum

ampl

itude

Frequency Time

Figure 6 – Left: time-frequency spectrogram of a typical ENVISAT WSM focused image. Right: local azimuth powers spectrum, a cut of the figure on the left at constant time, for two different repeat-pass aqcuistions. The two spectra should be overlapped to have interferometric capabilities. The bandwith of the overlapped contribution fixes the azimuth resolution.

5. SYNCHRONIZATION OF ENVISAT ASAR-WSM DATASETS The estimate of the scan pattern synchronization is fundamental to check the interferometric capability of a WS/WS

pair, a-posteriori, however, the a-priori achievement of such synchronization in fundamental. In Table 1, the achievable azimuth resolution is listed for different overlaps (in percentage), and the probability of such overlap, in case of randomly synchronized scanning, is show in the rightmost column. If we limit the azimuth resolution to 200 m, we get

that only 15% of the repeat pass acquisitions are cooperative, looking only at the overlap, but they may be much less if we request a small baseline, to mitigate for geometric decorrelation.

It seems that, in the case of non-synchronized acquisitions, the ASAR WSM interferometry would be unfeasible, in practice, due to the small probability of a consistent overlap. However there is a strong evidence, looking at the experience so far gained, to suggest the fact that ENVISAT ASAR-WSM acquisition are quite well synchronized in many cases, particularly when acquisitions are programmed in the Background Regional Mission (BRM). As an example, Table II lists the baselines and overlap for a set of 17 acquisitions over the rain forest. The dataset has not been selected with a particular criteria, it is just one with the largest number of acquisitions available in a repeat track. We notice that all the datasets are synchronized, with an overlap better than 40% (see also Figure 7), but for three datasets, two of these are however synchronized reciprocally (overlap 88%), whereas the third is asynchronous with all the other. Notice that there are many datasets with synchronization better than 95%, whose probability would be less than 2% in case of random scans.

In several other cases of acquisitions in BRM and also for repeated on-demand user acquisitions, a synchronization better than 30-40% has been observed. This confirms that the current planning strategy, especially for BRM acquisitions, which by default are planned to start systematically at the same locations, favors significantly the synchronization. However, the observed high percentages of synchronization are ultimately made possible by the accuracy of the a-priory orbit prediction, usually better (for one day in advance) than ±25 m, since predicted orbits are used in the final planning.

TABLE I. AZIMUTH RESOLUTION FOR DIFFERENT OVERLAPS

Overlap % Az. Resol (m) Probab % 5 2400 32 10 1200 30 20 600 27 30 400 24 50 240 17 70 171 10 90 133 3

100 120 0

TABLE II. OVERLAP AND BASELINES FOR A SET REPEAT TRACK ACQUSITION.

Baseline m Overlap % 15/06/2003 319 1046 855 1064 -104 -770 346 700 145 542 -80 146 -17 -40 136 232 20/07/2003 -1 -700 -600 -400 -414 -1079 -32 380 -164 233 -400 -500 -330 -349 -176 -78 28/09/2003 0 91 178 426 -1151 -1600 -700 -350 -900 504 -1100 -1200 -1080 -1000 -928 -826 02/11/2003 0 0 0 248 -973 -1390 -5122 -173 -720 -326 -950 -1000 -900 -900 -728 -633 08/08/2004 0 0 66 0 -725 -1800 -273 0 -475 -77 -700 -700 -650 -660 -490 -400 12/09/2004 0 -60 -70 0 98 665 -451 -1400 915 -647 -24 41 65 -64 218 -336 17/10/2004 0 -97 94 0 61 -63 -1116 -1400 -915 -1300 -689 -623 750 -730 900 -1081 21/11/2004 0 37 -41 0 -74 72 36 -350 201 152 426 842 -375 735 -222 114 26/12/2004 0 -43 -52 0 100 46 46 -90 550 775 775 842 -730 735 -580 464 30/01/2005 0 -69 -77 0 -90 72 72 -64 -75 -400 225 300 -169 185 -15 -86 06/03/2005 0 -56 -65 0 -98 96 60 -86 -81 88 -94 95 92 -40 -425 -311 10/04/2005 0 -62 -71 0 95 -98 65 -70 -80 93 624 66 58 -40 211 311 15/05/2005 0 -51 -60 0 -93 91 54 -92 -92 82 700 88 117 -106 -270 -271 19/06/2005 0 -64 -73 0 94 96 -67 69 80 95 -578 97 86 5 -152 -250 24/07/2005 0 -62 -70 0 96 -98 65 -81 -81 93 100 100 -90 98 -64 271 28/08/2005 0 -48 -56 0 -90 -90 -51 85 95 -80 -91 -86 -98 84 -98 -96 02/10/2005 0 0 0 -88 0 0 0 0 0 0 0 0 0 0 0 0 We furthermore notice that, although synchronization is planned at a certain position along the orbit (say, for example, at the ascending node), in practice this is not necessarily maintained along the orbit. As an example, the plot in Figure 8 shows an example of the overlap and baselines, measured on the average range interval (mid of subswath 3), estimated basing on a set of Doris state vectors and computed over the whole 100 min orbit. We notice that both baselines and synchronization vary, as expected, depending mainly on the orbit crossings. Finally, we cite that it may happen, that

two acquisitions over repeat tracks are made with different set of PRF, in that case the synchronization is impossible, as it will cyclically change according with the PRF differences. However this occurrence is limited to a minority of the acquisition over fixed location in the southern hemisphere. We remark that the ESA data catalogue software facility (EOLI SA) provides already the baseline information for ASAR interferometric pairs, including WSM-WSM pairs. A new version of this data catalogue software providing as well information about the percentage of burst overlap for WSM-WSM if expected by mid 2006.

-100 -80 -60 -40 -20 0 20 40 60 80 100%0

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Overlapdistribution

Figure 7 – Histogram of scan pattern overlap the repeat-track acquisitions in table II. The three non cooperative images have been discarded, for all the other, overlap better than 30% has been observed.

Figure 8 – Estimate of the scan pattern overlap and the baseline over a whole orbit (100’ min), achieved by exploiting a set of precise Doris orbit state vectors of a repeated track orbit pair. The variation is mainly due to orbit crossings.

6. CONCLUSIONS ENVISAT interferometric capabilities in WSM have been demonstrated in many cases, for application ranging form

DEM generation to the detection of large scale deformations like due to seismic effect. Users can get such interferograms by exploiting the Wide Swath Single look complex product and packages commercially available. The major constraints for such interferograms are provided by the baseline and the scan-pattern alignment. Regarding the first aspect, options to reduce the baseline values for interferometric pairs are being analysed. The second aspect is specific of WSM/WSM interferometry, and does not apply, for example, in WSM/IM combinations, that however have quite smaller bandwidth. In the past the scan pattern alignment constraint was assumed as the major aspect preventing such applications, as this would demand for a prediction of the orbit with accuracy along track better than say 70 m. However there are consistent evidences that the system has these capabilities, and planning of aligned acquisitions has been automatically implemented in many cases for the background regional missions. The current efforts are now in the sense of extending the synchronisation capabilities systematically to all the acquisitions and to provide scan pattern alignment as information to the user in the cataloguing facilities.

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for mapping and monitoring: preliminary results”, in proc. Fringe03 International Meeting ESA, (Frascati, Italy) 1-5 Dec, 2003. [2] A. Monti Guarnieri, P. Guccione, P. Pasquali, and Y. L. Desnos. Multi-mode ENVISAT ASAR interferometry: techniques and preliminary

results. IEE Proceedings Radar Sonar Navigation, 150(3):193-200, June 2003. [3] Monti Guarnieri and F Rocca. Combination of low- and high-resolution SAR images for differential interferometry. IEEE Transactions on

Geoscience and Remote Sensing, 37(4):2035-2049, July 1999. [4] Anrea Monti Guarnieri and Claudio Prati. ScanSAR focussing and interferometry. IEEE Transactions on Geoscience and Remote Sensing,

34(4):1029-1038, July 1996.