PERFORMANCE OF ENVISAT/ ASAR INTERFEROMETRIC PRODUCTS

Jürgen Holzner

German Aerospace Center (DLR) Oberpfaffenhofen, D-82234 Wessling, Germany

Phone: +49-8153-28-2650 Fax: +49-8153-28-1444, E-mail: juergen.holzner@dlr.de

1. INTRODUCTION ENVISAT/ ASAR interferometry is a useful tool for the retrieval of geo- and bio-physical parameters. In this paper the aim is show the quality of interferometric ENVISAT/ ASAR products. This objective will be ap-proached in two ways. First, the timing accuracy of various ENVISAT/ ASAR products is evaluated. The geo-metric calibration and validation includes the determination of slant-range and azimuth timing offsets for various orbit types, beams, and modes. Second, based on a high coherence interferogram the phase quality and the tim-ing accuracy of ENVISAT/ ASAR data is explored. From this data a first ENVISAT/ ASAR DEM is generated and evaluated. This DEM proves the high quality that may be obtained from ENVISAT/ ASAR data in case of sufficiently high coherence. As a test site serves the area around Las Vegas (36.2° N, 115.2° W) which is quite a dry region and remains stable even over the 70 day interval in-between the SAR acquisitions.

2. TIMING ANALYSIS OF IMS AND APS PRODUCTS BASED ON RESTITUTED AND PREDICTED ORBIT DATA

The results discussed in this section are based on beam IS2 data delivered with restituted orbit. Data sets from beams IS3, IS6, and IS7 are also evaluated. These data were provided with predicted orbit information. The timing offsets are obtained using strong point-like targets like transponders or corner reflectors. Their precisely known position is mapped into the azimuth slant-range plane and compared to the SAR signature’s position [1]. This chapter provides the results in two major parts. These two sections refer to the results from the Netherlands (52.52455 N, 5.04933 E) and the Oberpfaffenhofen (48.04668 N, 11.27885 E) test site, respectively. Finally, the preliminary average timing offsets is determined.

From the available data sets for the Netherlands test site conclusions on the:

§ determination accuracy of the timing offsets,

§ transponder positioning accuracy, and

§ dependency of the range timing offset on the beam and the supplied orbit

can be drawn.

First we set up a reference which is a time series containing data from beam IS2 and IS4 supplied with restituted/ precise orbit information. Then we cross-checked the results with the results for other beams and orbits.

Within the first step we obtained very consis tent results for the transponders Swifterbant and Edam. The mean of the individual time series and the standard deviation are given in figure 2.1. The very good standard deviation of about 0.1 slant range samples (0.78 m) for both transponders corresponds well to the orbit accuracy of the resti-tuted orbit data (better 1m across track). When the results of all transponders are compared based on this avail-able accuracy, we find that the position differences of the transponders deviate significantly. This suggests inac-curacies in the transponder position vector of up to 8 m (swifterbant - aalsmeer).

The evaluation of data sets of other beam modes indicates a beam dependent electronic delay. Table 2.1 shows the results for beams IS3, IS6, and IS7. The range timing offset of the Swifterbant transponder for the beams IS2/ IS4 in the first row of the table is used as a reference. The standard deviation of the time series serves as the criterion for the interpretation of the results. Values inside this error margin around the IS2/ IS4 mean are con-sidered valid. In this sense, the obtained range values are significantly different. The maximum deviation amounts to 7.6 m (IS2 to IS7).

In azimuth the timing offsets also deviate considerably, however, this errors can be assigned to the inaccuracies of the predicted orbit (equivalent to 6.2 samples). In this case, the consistency in terms of mean azimuth dis-placement has to be verified. (The cell which is indicated with ‘XX’ refers to an extraordinary value for the azimuth displacement.)

__________________________________________________________________________________________________________Proc. of Envisat Validation Workshop, Frascati, Italy, 9 – 13 December 2002 (ESA SP-531, August 2003)

Figure 2.1 Results for the Netherlands Transponders. The timing offsets are given in meters. One range and azimuth sample correspond to 7.80 m and 4.04 m, respectively.

beam mode orbit az

(mean, sigma)

[samples]

rg

(mean, sigma)

[samples]

IS2/ IS4 IM restituted/ precise ( 4.10, 0.11) ( 5.42, 0.30)

IS3 AP predicted 3.30 - 2.13

IS6 AP predicted 3.83 - 0.86

IS7 IM predicted 3.13 XX

Table 2.1 Results for the timing offsets for the Swifterbant transponder for various beams, orbits, and modes.

The results for the Oberpfaffenhofen test site confirm the results obtained for the Netherlands transponders. Similarly, the corner reflector show significant deviations in their position, if the standard deviation of 0.1 sam-ples (previous section) is considered as the error bound. For the Oberpfaffenhofen test site two acquisitions were available. Figure 2.2 shows the results for the corner reflectors. Both were acquired descending with beam IS2; the scenes are supplied with restituted orbit data. Each of the corner reflectors were imaged only once.

Finally, from the previous results a preliminary value for the timing offsets is determined. In order to obtain an electronic delay value for beam IS2 and IS4, the results for both test sites are integrated in order to averaged out the position inaccuracies. Finally, for the slant range and azimuth timing offsets values of (29 ± 3) m and (18 ± 4) m are obtained (mean ± standard deviation). In terms of accuracy this is a very stable result.

edam (rg x az) mean : 29.3 x 16.1 sigma : 0.70 x 1.17

aalsmeer (rg x az) mean : 23.2 x 16.6 diff. : 2.11 x 1.58

zwolle (rg x az) mean : 27.0 x 11.7 sigma : -

swifterbant (rg x az) mean : 32.0 x 21.9 sigma : 0.86 x 1.21

91 km

48 km

Figure 2.2: Results for the Oberpfaffenhofen (near Munich, Germany) test site. The timing offsets are given in meters. One range and azimuth sample correspond to 7.80 m and 4.04 m, respectively.

3. INTERFEROMETRY RESULTS Table 3.1 shows the data set parameters. It has a small height conversion factor corresponding to the baseline which is about half the critical baseline. This means half the range spectrum of the original SAR image is dis-carded and only half the resolution is obtained for the interferogram when compared to the SAR image. The remaining resolution amounts to about 16 m in range. From the difference of the Doppler centroid values there is no dramatic bandwidth reduction and the remaining resolution is in the order of the SAR image azimuth resolu-tion (~ 5 m). After establishing a common bandwidth for the data sets coherence values of up to an average of 0.83 are obtained. The amplitude, the interferogram phase and the terrain corrected coherence are shown in fig-ure 3.1.

orbits 2092 3094beammodeorbit typefDC [Hz] -518 -560temporal baseline [days]spatial perp. baseline [m]critical baseline [m]height conversion factor [m/cycle]coherence [1]

25up to 0.83

1100

IS2IM

70420

restituted

Table 3.1: Data set parameters.

CR6 (rg x az) 26.2 x 18.0 CR3 (rg x az)

29.2 x 19.7

CR2 (rg x az) 26.5 x 17.9

a) amplitude

b) interferogram phase

c) coherence

Figure 3.1: Master amplitude, interferogram phase and terrain corrected coherence of the data set with parame-ters listed in table 3.1.

Since no precisely known ground control points are available for the Las Vegas test site, a DEM based method was used to verify range and azimuth timing offsets. For the test site a high quality DTED-2 DEM was available. Based on this terrain information a differential interferogram was generated for timing offset evaluation. (figure 3.2). The timing offsets introduce the visible high frequency perturbations. This perturbations occur in moun-tainous terrain where a mis -alignment of the measured and the simulated phase will generate phase errors. After correction of the timing parameters as obtained from the measurements in section 2, the high frequency distur-bances vanish. After timing correction a DEM was generated. The data set covers an area of 90 x 90 km (figure 3.3).

a) differential interferogram before timing correction

b) differential interferogram after timing correction

Figure 3.2: Differential interferogram before a) and after b) timing correction of 4 samples in range and 5 sam-ples in azimuth.

Figure 3.3: DEM result obtained from the ENVISAT/ ASAR data set with parameters listed in table 3.1. The DEM covers an area of 90 km x 90 km.

4. CONCLUSION

Based on the FOS restituted and DORIS precise orbit data ( beams IS2 / IS4, IMS products ) a range and azi-muth timing offset of 29 m and 18 m was obtained with a standard deviation of 3 m and 4 m, respectively. This is a very stable result. The evaluation of complex products from beams IS3, IS6, and IS7 suggested slightly dif-ferent values. Furthermore, the azimuth timing strongly depend on the accuracy of the provided orbit (FOS pre-dicted/ restituted, DORIS precise/ preliminary).

The first ENVISAT/ ASAR DEM from interferometric data was presented. It was obtained from a data set with a temporal baseline of 70 days acquired over the very stable Las Vegas (36.2 ° N, 115.2° W) test site. The gener-ated DEM proves the high quality of the interferometric data that can be obtained from ENVISAT/ ASAR image mode products. From a comparison of a high quality DEM with the obtained interferometric phase the timing offsets obtained from the transponder and corner reflector measurements could be verified.

5. REFERENCES [1] Holzner, J., Eineder, M., Schättler, B. (2002): FIRST ANALYSIS OF ENVISAT/ ASAR IMAGE MODE PRODUCTS FOR INTERFEROMETRY , Envisat Calibration Review, ESTEC, Nordwijk, The Netherlands, 9-13 September 2002.

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