Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Patent
1993-09-27
1996-01-30
Sotomayor, John B.
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
342195, G01S 1390
Patent
active
054883733
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF INVENTION
A fundamental problem in synthetic aperture radar technique (SAR) is that, in its traditional form it, presupposes that an aircraft moves a relatively long distance along a straight path. When the SAR technique is driven to more extreme performances, this becomes a problem also in so far as it is not even possible to fly with the accuracy that is required. Moreover, military use may make it necessary to be able to make the SAR registrations while the aircraft is maneuvering.
In the wide band low frequency SAR called CARABAS, that has been patented under the Swedish patent no. 8406007-8, motion compensation is an important problem, as a registration is carried out during an approximately 10 km long distance, during which the accelerations must be less than 10.sup.-1 -10.sup.-3 ms.sup.-2 (depending upon reconnaissance distance and desired resolution), if a traditional SAR signal processing shall be used.
Methods have evolved in the SAR technique to compensate for movements. These are essential for the SAR technique as it is regularly used today in modern reconnaissance and attack aircrafts. These methods are, however, approximative and while they work for narrow band microwave SAR, having relatively short aperture distances, they can't be used for CARABAS.
The foregoing problem is solved by the present invention.
SUMMARY OF THE INVENTION
The invention provides a completely new method of processing SAR images, first of all for wide band SAR, that is carabas, but probably also capable of being used for narrow band SAR. While the conventional SAR processing is based upon Fourier transform techniques, or related convolution and correlation techniques, the method of the present invention is based on the numerical unfolding of differential equations along the flight path. The Fourier transform method is a special case (as a matter of fact an analytic solution to the differential equation in the case of a straight flight path). This method has several considerable advantages. For example, the method can be used along a curved flight path and irrespective of whether the aircraft accelerates. The form of the path must, however, be known in the form of accelerations, that are taken from the inertial navigation system.
Apart from motion compensation the method makes real time processing possible, where the SAR system functions as a so called line camera. In other words, real time imaging of the ground surface along a beam of desired direction from the aircraft is made possible. By means of the aircraft motion, a 2-dimensional image is gradually created.
The signal processing method can be divided into independent processes for each frequency in a wide band signal. It is extremely suitable for implementation in a massively parallel dedicated processor, based on perhaps 1000 identical ASIC circuits, (ASIC=Application Specific Integrated Circuit), each handling the process for one or a few of these frequencies.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows geometrical conventions for aperture position .alpha., bearing .psi. and range R.
FIGS. 2a-2d show successive steps of the numerical unfolding of the node equations for k=1 and with step size .DELTA..alpha.=1.75.
FIGS. 3a-3d show the numerical unfolding of the node equations for k=1 and the same point target, but for different step sizes with FIGS. 3a-3d respectively showing the solution at the fixed aircraft position .alpha.=105, for step sizes .DELTA..alpha.=2.1, .DELTA..alpha.=1.5 and .DELTA..alpha.=0.5, and wherein, for the purpose of comparison, the true solution C.sub.ideal/105 been included in FIG. 3d.
FIGS. 4a and 4b show the effect of k-integration with respect to 100 discrete k values within the interval 0.01.ltoreq.k.ltoreq.1, wherein FIGS. 4a and 4b respectively depict angular and range dependence for the previous point target at the aperture position .alpha.=175 and for .DELTA..alpha.=1.75.
FIG. 5 shows the definition of a stabilized image line.
FIG. 6 shows an architecture for a signal processing equipment according to the i
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