Image analysis – Applications – Seismic or geological sample measuring
Reexamination Certificate
1998-12-21
2001-04-24
Bella, Matthew C. (Department: 2721)
Image analysis
Applications
Seismic or geological sample measuring
C382S172000, C382S293000, C342S192000
Reexamination Certificate
active
06222933
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of processing spotlight SAR raw data.
REVIEW OF RELATED TECHNOLOGY
A short introduction to SAR and spotlight SAR systems is given below. Radar having a synthetic aperture SAR (Synthetic Aperture Radar) is an instrument for remote sensing that is being used increasingly in imaging, monitoring and investigating the earth's surface. A system of this nature has a platform that moves at a constant speed, an antenna that points in a direction orthogonal to the direction of motion, and a coherent radar system that periodically transmits electromagnetic pulses.
The direction of platform motion is referred to as the azimuth direction, and the direction orthogonal thereto, which is oriented diagonally downward, is referred to as the range direction. In the normal SAR operating mode, which is called strip mode, during a flight over a region to be observed, a strip having the length of the segment that is flown over is imaged. The strip width is dependent on, among other things, the length of the time window in which the backscattered radar echoes from the transmitted pulse are received. The received echoes are converted (mixed) in frequency, quadrature-demodulated, digitized and stored in a two-dimensional echo memory.
Each SAR processing essentially comprises a range compression, a correction of the range migration and an azimuth compression:
The geometrical resolution in the range direction is a function of the bandwidth of the transmitted radar pulses. To improve this resolution, the transmitted pulses are modulated, for which, in most cases, a linear frequency modulation is used that will always be assumed hereinafter. Range compression means a filtering of the received signals in the range direction according to the matched filter theory, whereby the temporally-expanded signals are compressed into pulses.
The range migration is a consequence of the variation in the distance between the antenna and the point target in the formation of the synthetic aperture. The correction of the range migration describes the range variation of the echoes associated with a point target in the two-dimensional echo memory.
The pulse compression in the azimuth in accordance with the matched filter theory corresponds to the formation of the synthetic aperture. The azimuth modulation is likewise a consequence of the change in distance between the antenna and the target as the platform passes. The matched filtering means a coherent summation of all echoes associated with an azimuth position. If the range migration is corrected prior to the azimuth compression, the azimuth compression can be realized by a one-dimensional filter function.
Spotlight SAR is an operating mode of SAR systems that permits a significantly-higher geometrical resolution.
FIG. 7
illustrates the imaging geometry of a spotlight SAR system. In the spotlight mode, the antenna is directed at the center of the imaged scene during the entire pass. Because of this antenna steering, the scene is illuminated far longer than would be the case in the normal strip mode with an immovable antenna. The longer illumination time permits the formation of a longer synthetic aperture, or spotlight aperture.
FIG. 7
shows the synthetic aperture for the strip mode (SAR aperture) in the scene center, and the longer spotlight aperture over the azimuth axis. The long spotlight aperture produces azimuth signals that have a large bandwidth, and signify a high geometrical resolution in the azimuth direction. In spotlight mode, because of the antenna steering, a scene that is limited in the azimuth direction is imaged, whereas in strip mode, a scene theoretically having an unlimited azimuth expansion can be imaged.
To attain as high a geometrical resolution in the range direction as in the azimuth direction, a linearly frequency-modulated radar signal having a very large bandwidth is transmitted. To reduce the signal processing, particularly the requirements on sampling in the A/D conversion, the chirp modulation is often compensated during reception. In the process, prior to the A/D conversion, the received echoes are multiplied by a linear frequency-modulated signal that is centered on the center of the scene and has the inverted modulation rate of the transmitted signal. The result of the multiplication is a superpositioning of sinusoidal signals having frequencies that are linearly dependent on the range difference between the point target and the center.
The bandwidth of this signal is considerably smaller than the bandwidth of the received echoes for the case of a small size of the scene in the range direction. This is practically always a given in the spotlight mode.
FIG. 8
illustrates the compensation of the chirp modulation during reception. Here the range time t
r
is shown on the abscissa, and the range frequency f
r
is shown on the ordinate. The signals from targets in the near range (A), in the center of the scene (B) and in the far range(C) possess the modulation rate k
r
, while the compensation function possesses the modulation rate −k
r
. The reduced bandwidth B
wr2
following the compensation of the chirp modulation during reception is a function of the range dimension &Dgr;
r
of the scene and the bandwidth of the transmitted signal B
wr1
. The speed of light is represented by c
0
.
The polar-format method, the range-migration method or an adapted strip mode, for example the chirp-scaling method, can be used for spotlight SAR processing. The azimuth scaling used in the present case was used in a similar form for ScanSAR processing. (See German Patent Application 196 09 728.2-35.) An SAR system that operates according to the spotlight principle is also known from U.S. Pat. No. 5,546,084.
The polar-format method (see the publication by G. Carrara, R. S. Goodman and R. M. Majewski: “Spotlight Synthetic Aperture Radar,” Artech House Boston, London 1995, pp. 80 through 115) operates with compensation of the chirp modulation during reception, in which the motion compensation is likewise performed during reception for the center of the scene. This motion compensation is effected while the raw data are being recorded, by means of a shifting of the reception window as a function of the range variation of the echoes of the scene center.
This method begins with polar-format operation, which represents a conversion of the raw data from the polar format into Cartesian format. This conversion is effected with the aid of a two-dimensional interpolation. In addition to numerous correction steps, which also include the correction of the change in target range for the entire scene, the compression is effected in both the azimuth and range directions through FFT (Fast Fourier Transformation).
The range-migration method (see the publication by C. Prati, A. M. Guarnieri and F. Rocca, “Spot Modus SAR Focusing with the &ohgr;-k Technique,” Proceedings of the 1991 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), Espoo, Finland, Jun. 3-6, 1991, pp. 631 through 634) is derived from the method of processing seismic data. The first processing step is a one-dimensional Fourier transformation in the azimuth. Subsequently, the signal is multiplied by a two-dimensional phase function that precisely corrects the range migration for all targets in the range of the scene center. The range migration is corrected for the entire scene with the subsequent Stolt interpolation. Finally, a two-dimensional inverse Fourier transformation yields the completely-processed image.
In principle, each method used to process SAR data in the strip mode can also be used for the spotlight mode. An example of this is the chirp-scaling method (see the publication by Raney R. K., Runge H., Bamler R., Cumming I. and Wong F.: “Precision SAR Processing Using Chirp Scaling,” IEEE Transactions on Geoscience and Remote Sensing, Vol. 32, pp. 786 through 799, July 1994, and A. Moreira, J. Mittermayer and R. Scheiber, “Extended Chirp Scaling Algorithm for Air- and Spaceborne SAR Data Processing in Stripmap and ScanSAR
Mittermayer Josef
Moreira Alberto
Bella Matthew C.
Browdy and Neimark
Choobin M.
Deutsches Zentrum fur Luft-und Raumfahrt e.V.
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