Airborne Polarimetric SAR Interferometry K.P. Papathanassiou ' , A. Reigber', R. Scheiber', R. Horn', A. Moreira', and S.R Cloude2

1: Deutsches Zentrum fur Luft- und Raumfahrt e.V., (DLR) P.O. Eox 11 16, D-82230 Oberpfaffenhofen, Germany. Tel/Fax: $49-8153-28-2367/-1135

2: Applied Electromagnetics 11 Bell :Street, St. Andrews Fife, KY16 SUR, U.K. Tel./Fax. ++44-(0)1334-477598/475570

Email: Kostas.Papathanassiou@dlr.de, scloude@fges.demon.co.uk

Abstract - - I n this paper we show for first time the applicability of polarimetric interferometry techniques on high resolut,ion interferometric scattering matrix airborne dat,a. We point out t,he main requirements considering syst,em hardware as well as data processing and calibration algorithms which are important for the polarimetric int,erferomet,ric data evaluation and describe their imple- iiient,a.tion in the case of the DLR's experimental SAR system (E-SAR).

INTRODUCTION

C'ompared t,o conventional interferometry the application of polarimetric SAR interferometry [1],[2],promises not only significant improvements of the performance of SAR int,rrferomet,ry but. also new application possibilities. Until now only t,he Space Shuttle-borne SIR-C radar system provided int,erferomet,ric fully polarimetric data in L-, and (:'-hand acquired in a repeat-pass mode over only a few test sit,rs. Because of this lack of useful data it is important to t,ransfer a.nd adapt t'he new techniques to the more flexible airborne case. Polarimetric and interferometric evaluations place high requirements on the phase preservation and acciiracy of t,he niotic'n compensation and data processing algorit.lini, especially in the case of airborne SAR sys- tems. This paper considers the basic requirements on hardwa.re and processing algorithms for the application of pola.riniebric int.erferometry in airborne SAR systems and describes t.lieir implementation in the case of the E-SAR syst,em. Finally we demonstrate the potential of the new technique by showing first results from the polarimetric int,erferomet,ric analy.& of airborne SAR data acquired wit'li t,lie E-SAR system which operates, in addition to the convent,ional modes, interferometric (repeat-pass mode) and fully polarimetric in L- and P-band [ 3 ] .

SYSTEM REQUIREMENTS

System hardware parameters play an essential role in the cpiality of the acquired data. Sufficiently high SNR level, which allows accurate phase measurements, combined with phase stability over i i high bandwidth to achieve a high

0-7803-4403-0/98/$10.00 0 1998 IEEE

spatial resolution are required. High cross-talk isolation is important for the measurement of scattering matrix data. For systems operating at low frequencies a high signal-to-interference noise ratio and a high instantaneous system dynamic range are required to reduce or eliminate RF interference jamming effects. Table 1 summarizes the most important E-SAR system parameters for the L- and P-band. For interferometric applications the exact knowledge of the antenna phase center positions of the an- tennas in the order of 1/30 wavelength or better is needed. Interferometric processing requires a precise reconstruction of the raw-data, as it would have been acquired on ideal straight tracks with a constant forward velocity as well as the positioning of the different tracks within a global coordinate system. Consequently accurate measurement of the absolute and relative sensor position is essential. In the E-SAR case, position, velocity and the position angles of the aircraft are recorded by a CCNS4/Aerocontrol navigation system coupled with DGPS and a LTR-88 INS with an update rate of 50Hz. The absolute position accuracy of the CCNS system is specified O.lm, the relative accuracy 0.01m. The position angles are specified with an accuracy better than 0.03 degrees. Additionally the CCNS system gives an online control about the flight path during data acquisition. In that way the devia- tions from the planned flight track can strongly be reduced.

DATA PROCESSING

The deviation of the real flight track from an ideal straight track causes errors in the range delay and in the phase of the raw-data [4]. These errors have to be corrected during processing. In the repeat-pass case each track has its own motion errors, making interferometry much more sensitive to the performance of the motion compensation. All the acquired position data is utilized to compensate motion errors off-line in an interferometric processing algorithm. This interferometric algorithm is implemented in that way, that it processes the raw-data of two different tracks with respect to each other. A first step here is the exact synchronization of the position data with the raw-data

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I I L-Band I P-Band I

Range Bandwidth [MHz] Chirp Dnrat,ion [pec] Sampling Rate [MHz]

50 or 100 5 or 10

60 or 100

1 0perat.ing Frequency I 1.3 GHz I 450 MHz I 20 or 60 5 or 10

60 or 100

Dynamic Range [dB] Syst.em Noise Level [dB]

Cross Isolation [dB]

I PRF [Hz/channell I 400 I 400 I 55 dB 55 dB

-88 -88 -35 -25

Residual impreciseness in the position data leading to additionally azimuth misregistration. But also topography causes due to the different look-angles in each image a different slant-range shift depending from the local height. All this reasons makes an additional coregistration and resampling of the processed SAR images necessary.

POLARIMETRIC CALIBRATION

Accurate calibration of both amplitude and phase of quad-polarized SAR data is essential for polarimetric interferometry. Polarimetric basis transformations require

Table 1: E-SAR system parameters accurate co-polar and cross-polar complex calibration. wit.11 help of the GpS second, which is recorded in both Therefore the cros:-talk calibration, the symetrisation d a h set.s. Then follows the calculation of the reference scheme and the estimation of the Co-Polar Phase Play a t,racks. This is clone by calculating two parallel linear key role in the polarimetric post-processing. Especially t,racks which are minimizing the total deviation from the for interferometric applications, where not only the phase real flight tracks, using a Ieast-square method. Since in the between elements of one scattering matrix is important E-SAR systelll the PRF is fixed and the forward velocity but also the relative phase between two scattering matrices during dat,a acquisition is not constant, the raw-data is required, the requirements for a phase preserving pixels are equidista.nt in tilne but not in space. In order to polarimetric calibration procedure are more stringent than get, a constant pixel-spacing in azimuth direction, the raw in conventional polarimetric applications. (]at,a a.nd posit,ion data of both tracks is resampled to an For the cross-talk calibration we use a modification of the avera.ge velocity before SAR processing starts. Quegan calibration algorithm which allows the separation

Aft,er resampling the data the cross track deviation of of cross-talk removal and symetrisation operations. The t,he sellsor posit,ion from the calculated reference track polarimetric calibration is then implemented in a four-step is calculat,ed in order t,o generate phase- and rangedelay procedure with i) cross-talk removal, ii) symetrisation, iii) correct,ion files for later use in the SAR processor. These complex channel imbalance correction, and iv) absolute correct,iolls are range dependent, because the incidence calibration. A detailed description of the calibration angle is varying from 2.5 to 60 degrees from near to far procedure is given in [ 5 ] . range. To a.void any disturbation of the range chirps, only Here we like to outline only a new possibility for the a.11 average coilstant correction can be done as a first step estimation of the HH-VV channel phase imbalance arising before compressing t,he data in range. After this the rest from the use of polarimetric interferometric data sets. of t,lie motion compensation follows. The phase of the differential interferogram between the The incidence angles for corresponding pixels are different two co-~olarised channels (i.e. the phase between the in bot,li t~racks, this effect is also range dependent. As interferogram formed using the HH-channel from the a. result. t.he pisel-spacing in ground range is different first track and HH-channel from the second track and for bot.11 pict,ures, and also their ratio is not constant the interferogram formed using the HH-channel from over range. As we are using an extended chirp scaling the first track and the VV-channel from the second a.lgorit,hni for processing, a first order compensation of track) represents the co-channel phase offset, and can t,his can be done by using a scaling factor not equal to be estimated with a accuracy on the order of the sensor One [l]. This causes a linear scaling in range for one phase measurement resolution. AS both interferograms pict,ure. The devia.tion from the linear scaling has to be have the same baseline, their differential phase results conipensat,ed after processing. Another effect of different directly by phase subtraction in the complex domain. In incidence angles is the spectral shift in range [3]. TO obtain this way the HH-VV phase imbalance can be estimated an opt,imal interferometric result, the range spectra are using distributed surface scatterers with a accuracy on the filkred during processing. For the filter width we used order of the sensor phase measurement resolution. t,he highest, frequency shift, which is present in near range

EXPERIMENTAL RESULTS where t,he baseline perpendicular to the line of sight B l has t,lie biggest value. .Aft,er processing a misregistration in range still remains The first E-SAR experiment with the new polarimetric hecause of the fact that the processor performs only a interferometric repeat-pass mode was performed on the first order correct,ion of the pixel-spacing in slant-range. test site Kuettigkohen, Switzerland. Full-polarimetric

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da.t,a was acquired in L-, and P-band, within two passes separat,ed by a t,iiiie interval of 30 min. The mean effective haseline was 41111 for the L-band and 21m for the P-band. The L-band a.nd P-band coherence maps generated by using t,he VV-channels are shown on the left side of Figure 1 a.nd 2. The main decorrelation contribution in both frequencies is baseline decorrelation over areas where the effedive scat#t,erers have i t height-distribution. The effect is much st,ronger in L-band - due to the large applied baseline - where t,he forest,ed areas are totally decorrelated (mean coherence less than 0.3). O n the left side of Figure 1 and 2 t,lie opt,iiiiised coherence maps are shown. The drastical iniprovenieiit8 of the interferometric coherence especially over t8he forested and vegetated areas demonstrates the rsseiit,ial role that, pohrimetry can play for airborne repea.t,-pass interferometric systems with its typically large baselines. Figure 3 shows one of the first results of the application of t,lie polarimetric interferometric decomposition algorithm on E-SAR dat,a. A perspective view of height difference bei8ween scatt,ering mechanisms extracted a t L-band over tmlie corii-fields superpos,ed with the SAR interferogram i n t,he HV-channel. The mean height difference is about 2.2m. which is in good agreement with the height of the prcwnt, veget,at#iori.

CONCLUSIONS

The great, pot,ent,ial of polarimetric interferometry is related t,o t,lie coherent interferometric decomposition algorithms wit,li respect^ t.0 t,he improvement of coherence and to the tlerivat,ioii of interferograms from the different scattering mrdianisms. The coherence optimisation algorithm can help t.0 solve t,he h a d e off between the high height resolu- t,ioii achieved with a large baseline and the resulting prob- lem of high volunie decorrelation. The main limitations are posed by t,he precision of t,he flight navigation system and, in t,he case of multi-pass interferometry, by the scene clecorrelat,ion. Fut,ure airborne SAR systems should inves- tiga.t.e t,he potential of polarimetric SAR interferometry a t different, frequency bands (e.g., C-, S-, L-, and P-band). The est,rnsion of polarimetric interferometry t o wide-band syst,rms is a challenge and especially interesting for appli- cat,ions relat,ed t<o ground penetration imaging.

REFERENCES

[l] S R. C'loude and K P. Papathanassiou, "Polarimetric SAR Interferometry", Submitted to IEEE Trans. GRS, Nolembrr 1997. ['L] S. R . Cloude and E;. P Papathanassiou, "Coherence Optiiiiisatioii 111 Polariinatric SAR Interferometry", Pro- Cftr( / l ly5 of IGAR.CS'97 Singapore, 1997. [:31 R Horn, "DLR Airborne SAR Project, Objectives

Figure 1: L-band coherence maps. Right: VV-channel, Left: Optimised

Figure 2: P-band coherence maps. Right: VV-channel, Left: Optimised

and Status" , Proceedings of the First International Air- borne Remote Sensing Conference, Strasbourg, France, 11- 15 September 1994. [4] A. Moreira, Y. Huang, "Airborne SAR Processing of Highly Squinted Data usi ng a Chirp Scaling Approach with integrated Motion Compensation" , IEEE Trans. GRS, Vol. 32 No. 5, pp. 1029-1040, 1994. [5] K.P. Papathanassiou and M. Zink, "Polarimetric Cal- ibration of the Airborne Experimental SAR System of D L R , Proceedings of the European Conference on Syn- thetic Apertur Radar, EUSAR'98, Friedrichshafen, Ger- many, 25-27 Mai 1998.

Figure 3: Perspective view of height difference between scatter- ing mechanisms extracted at L-band superposed with the SAR HV-Image

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