A.&~uct - This paper proposes a new approach for high resolution airbome SAR data processing, which uses a modified chirp scaling algorithm in order to ...
AIRBORNE SAR PROCESSING USING THE CHIRP SCALING AND A TIME DOMAIN SUBAPERTURE ALGORITHM Yonghong Huang, A l k r t o Moreira German Aerospace Research Establishment (DLR) Institute for Radio Frequency Technology D-8031 Oberpfaffenhofen, Germany
A.&~uct
- This paper proposes a new approach for high resolution airbome SAR data processing, which uses a modified chirp scaling algorithm in order t o incorporate the correction of strong motion errors a s well as large the variations of the Doppler centroid in range and azimuth. Additionally, data acquired with squint angle greater than 40" can be processed with less than 3.5 % degradation of geometric resolution. A time domain subaperture approach can be used in range compression for accommodating the variation of the frequency modulation of the chirp signal and also the vanation of the secondary range migration, which are caused by the modified chirp scaling algorithm. Further, the time domain subaperture approach can be used in azimuth for incorporating the Doppler centroid variations in azimuth.
Keywords : Synthetic Aperture Radar (SAR), Airborne SAR Processing, Chirp Scaling, Squint SAR. 1. INTRODUCTION
SAR processing is based on a two-dimensional correlation of the backscattered signal with a space-variant reference function. The range-Doppler and the wave-number algorithm (ref. I . 2, 3, 4) have been commonly used for erficient SAR processing. However, both algorithms need interpolation for Correction of the rangc cell migration (range-Doppler) or for Stolt change of variahles (wavenumber). The chirp scaling algorithm has been recently pmposcd for high quality SAR processing (ref. 5 and 6). This algorithm avoids any interpolation in the SAR processing chain and is suitable for the high quality processing of several SAR systems (e.g. SEASAT. ERS-I, RADARSAT). It consists basically of mutiplying the SAR data in the range-Doppler domain with a quadratic phase function (chirp scaling) in order 10 cqualize the range cell migration, followed by an azimuth and range compression in the wave-number domain. After transforming the simal back to the range-Doppler domain, a rcsidual phase correction is carried out. Finally. rzimuth IFFTs a!? performed to generate the focussed image. It has been shown in ref. 6 that the image quality and the phase accuracy of the chirp scaling algorithm is equal or better than that of thc range-Doppler algorithm. However, airborne SAR pmccssing requires an update o f the Doppler centmid as a function of the range distance and an accurate time domain motion compensation. The above mentioned requirements can not be included in the original chirp scaling algorithm. since i t is basically a frequency domain focussing approach. Some alternative methods can be implemented in the original chirp scaling algorithm to incorporate the Doppler centroid variations (eg. Block processing. increase of azimuth FFT sizc ref. 7). In this case, the computation efficiency of the chirp scaling algorithm decmases considerably. ~
The new algorithm (denoted as the extended chirp scaling algorithm) has been developed for the processing of airborne data with
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strong motion errors (e.g. E-SAR system) or satellite data with large variations of Doppler centroid in range or azimuth. One essential characteristic of thc extended chirp scaling algorilhm is that the azimuth compression is performed in one dimension (all range migration is removed before this step), so that the compression is not only rcstrictcd to the wave-number algorithm. Other time domain compression appmachs can be used for increasing flexibility in the correlation pmcess. Next section describes the extended chilp scaling algorithm. The simulation results in Scction 111, using the parameters of the E-SAR system. show that the quality of the impulse response function (IRF) is excellent even for squint angle values greater than 4V. The rcsults of the image processing and analysis are presented in Section IV. Currently, the processing with Doppler centroid variations in range and azimuth is being incorporated into the extended chirp scaling algorithm. 11. EXTENDED CHIRP SCALING ALGORITHM
The extended chirp scaling algorithm has a new structure with the necessary phase correction functions in order to accommodate the motion error compensation required by airborne SAR as well as the Doppler centroid variations in range and uimuth. The pmcessing steps are described in thc following (see fig. I): The first step includes a multiplication with first order motion compensation and removal of Doppler centroid. which is assumed in fig. 1 lo be constant. The first order motion compensation accounts for compensating the motion e r m n in onc reference range. This operation ensures an optimal signal-to-noise ratio for thc presumming. After presumming, the signal spectrum is shifted to ib original position to prcserve the phase information during the pmccssing chain. The data is transformed from signal domain (T, r) to the range-Dopplcr domain (z. f.) by means of an azimuth FFT. The multiplication with I$,(T, f.) performs the chirp scaling in a reference range r., to equalize the targets migration trajectory and to avoid any interpolation in the processing. This step is identical to the original chirp scaling algorithm if there is no variation of Doppler centroid in rangc or azimuth. Range F F T s are carried out to map data from the rangeDoppler domain to the two-dimensional frequency domain (wave number domain). Thc multiplication with Q,,(f;.f.) carries out an accurate range comprcssion with modulation rate and secondary range migration corrections, which are dependent on the azimuth frequency. Additionaly. the phase @2, includes a linear phase term in range. which removes all the range migration. This comction can not be included in the original chirp scaling phase, since it would lead to a strong degradation in the range IRF duc to the large frequency shift required for this operation. No azimuth comprcssion is performed in the wave-numbcr domain. Range FFTs are performed to transform the data into the
From the above description, the additional flexibility of the pmposed algorithm is justified in the following: By introducing an azimuth FFT and IFFT and changing the phase $21V;,L),the motion compensation can be included in the processing chain. Removing the azimuth processing from the original wavenumber approach increases the flexibility of the algorithm. Time-domain subaperture processing (ref. 8) can be used to reduce the computation requirement in the airborne case. An additional linear phase term, which is included in the range compression. removes the range migration completely, so that the following azimuth compression becomes an onedimensional operation. Time domain subaperture processing can be used for accommodating Doppler centroid variations in azimuth. The subapenures are efficiently generated by short azimuth FFTs (e.g. 256 points). For each azimuth FFT, a constant Doppler centroid is assumed. After the range compression step, the Doppler rate and the accurate phase correction in line of sight for motion compensation can be extracted using the RDM(reflec1ivity displacement method - ref. IO) approach by means of a cross correlation between the short azimuth spectra.
Figure 1. Rlock diagram of the extended chirp-scaling algorithm for high precision airborne SAR processing. In this diagram, a constanl Doppler cenlroid value is assumed for processing. range-Doppler domain. The multiplication with Q Z 2 ( ~ , X ) compensates the slowly varying azimuth frequency. which is a quadratic function with range. By means of azimuth IFFTs. the data is transformed from the range-Doppler domain into the range and azimuth timedomain. At [his step. all targcts are range compressed and have their azimuth trajectories without any range migration. The second order motion compensation accounts for the accurate varying phase correction in range and compensates the residual azimuth phase error. The azimuth FFTs transform the motion compensated data into the range-Doppler domain. The multiplication with &(T, f;) performs the azimuth compression. This term leads to an accurate azimuth IRF, since it is a hyperbolic function of the azimuth frequency. The final image is obtained after azimuth IFFT.
For variations of the Doppler centroid with range, a time domain subaperture approach can be used for carrying out the range compression, since the range modulation rate will be changing in range after the chirp scaling operation.
111. SIMULATION RESULTS
The following simulation parameters were used according to the specifications of the E-SAR system or DLR (ref. 9): Radar wavelength: h = 0.0566 m Sensor velocity: v = 74.6 mls Incidence Angle: 8, = 37' Squint Angle: variable Chirp frequency modulation rate: k = 2 10" HZ/S Radar PRF: 1100 Hz.
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The impulse response functions for a squint angle of 16O and 40" are shown in fig. 3.a and 3.b. respectively. No weighting function was used in the processing and a rectangular antenna pattern was assumed for range and azimuth. so that a sin(x)/x-function is expected for an error free compression.
R C 4 E = 1.89375m AZJLR=
-13.25lldb
R C S L R c -13.4848db
AZJROADEN-
1.685311
ROBROADEN= 2 . l J l 8 4 X
4
Range
(sample. axpandad by 4)-
Figure 2. (a) IRF for 16 squint angle. (h) IRF for 40" squint angle. Sensor and processing parameters of the E-SAR system are used for the simulation. One-Look processing was selected for analysis of phase accuracy.
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azimuth 4
a7imuth
4
b
Figure 3. Processed E-SAR image using the extended chirp scaling algorithm. (a) Wilhout correclion of motion errors. (b) With correction oCthe motion errors. Sensor and prmssing parameters are: 2205 m flight altitude, 74 m/s ground speed, 7.80 squint angle, C-band, 8 looks with 50 5% overlap, VV polarisation, 2.5 mx 4.0 m resolution (range x azimuth). For 16" squint angle. the simulation The deterioration of the azimuth and and 2.1 90,respectively. In the case of resolution is slightly deteriorated (3.3 grated sildclotx ratios mantain almost
results arc almost perfect. range resolution arc 1.7 % 40 ' squint angle, the range %). The peak and the inteunchanged.
V. RESULTS O F I M A G E PROCESSING
A flight of the E-SAR system (ref. 9) over the airfield of Otxrpfaffenhofcn in Germany was used to test the new approach. The selected data set for pmcessing has a squint angle of 7.8" and very strong motion errors. The phase correction for motion compensation was obtained by the approach described in ref. 11. Fig. 4.a and 4.b show the pmcessed image (laser printer output) wihout and with motion compensation. The image sizes are 2100 m x 3220 m (range x azimuth). By evaluating the image quality parameters and by comparing it with the results obtained by the range-Doppler algorithm. we conclude that the proposed algorithm can perfectly accommodate the motion error correc~on.Additionally, the focusing quality of thc extended chirp scaling algorithm is superior to the range-Dopplcr algorithm due to the more accurate secondary range migration comction.
VI. FINAL REMARKS Cumntly, the processing software is being extended for incorporating variable Doppler centmid in range and a7.imuth. The lime domain subapenure approach will be included in this step. Future work includes also the image processing of satellite data with the extended chirp scaling algorithm.
VII. REFERENCES Jin, M.Y. and Wu, C.: "A SAR Comlation Algorithm which Accommodates Large Range Migration". IEEE Trans. on Geosci. and Remote Sensing, Vol. 22, No. 6, 1984. pp. 592597. R. Bamler: "A Comparison of range-Doppler and wave-number Domain SAR Focussing Algorithms". IEEE Trans. Gcosci. and Remote Sensing, Vol. 30. No. 4, July 1992. pp. Calforio. C.. Prati. C. and Rocca, F.: "SAR Data Focussing Using Seismic Migration Techniques". IEEE Trans.Aerosp.
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Electron. Syst.. Vol. 27, 1991, pp. 199-207. Raney. K. and Vachon. P.: "A Phase Preserving SAR Pmcessor". Pmc. of IGARSS 1989, pp. 2588-2591. Runge, H. and Bamler, R.: " A Novel High Precision SAR Focussing Algorithm Based on Chiip Scaling". Roc. of IGARSS, Houston, 1992, pp. 372-375. Cumming, I., Wong. F. and Raney, K.: " A SAR Processing Algorithm With No Interpolation". Proc. of IGARSS. 1993. pp. 376-379. Prati, C . and Rocca. F.: "Focusing SAR Data with Time-Valying Doppler Centroid. IEEE Trans. on Geosc. and Remote Sensing, Vol. 30, No. 3, May, 1992, pp. 550-559. Moreira, Albeno: "Real-time Synthetic Aperture Radar Processing with a New Suabpenure Approach". IEEE Trans. on Geosci. and Remote Sensing, Vol. 30, No. 4, July 1992. Horn, R.: "E-SAR - The Expenmental Airbome L/C-Band SAR System of DFVLR". Proc. of IGARSS, Sept. 1988. pp. 1025- 1026. [IO\-Moreira. I.: " A New Method of Aircrafi Motion Error Extraction from Radar Raw Data for Real-Time Motion Compensation". IEEE Trans. Geosci. and Rcmote Sensing. Vol. 28, No. 4, 1990, pp. 620-626. I1 1 I Buckreuss. S.: "Motion Errors in an Airbome Synthetic Aperture Radar System". European Trans.on Telecommunications and Related Technologies, Vol. 2. No. 6. 1991, pp. 55-64.
