Television – Special applications – Observation of or from a specific location
Reexamination Certificate
1999-02-22
2001-11-20
Britton, Howard (Department: 2713)
Television
Special applications
Observation of or from a specific location
C348S036000, C348S218100, C701S223000
Reexamination Certificate
active
06320611
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to the field of day
ight reconnaissance, and more particularly to air/ground reconnaissance carried out by airborne optronic equipment. The invention applies in particular to the taking of satellite/ground snapshots or reconnaissance.
DISCUSSION OF THE BACKGROUND
Currently existing airborne reconnaissance equipment is based on the use of a photosensitive photographic film in equipment of very high focal length, or else on the use of a linear array of elementary electronic sensors read by charge transfer CCD (initials of “Charge Coupled Device”) or by CMOS circuit (initials of “Complementary Metal Oxide Semiconductor”). The latter technique is integrated into optronic equipment. Only this type of equipment allows high-resolution day
ight reconnaissance.
Since sighting is carried out sideways with respect to the displacement of the carrier, the analysis of the image is performed conventionally according to two modes:
in so-called “push-broom” mode: in the case of optronic equipment, the linear array is oriented in such a way that the main axis of its image on the ground is substantially perpendicular to the velocity vector of the carrier aircraft. Thus, it is the displacement of the aeroplane which performs the scanning of the image of the linear array of detectors;
in so-called panoramic mode: the image of the linear array on the ground, projected parallel to the velocity vector of the aeroplane, is scanned sideways by rotating the line of sight of the equipment about an axis parallel to the velocity vector of the aeroplane; the scanning means usually consist of a movable mirror serving also in the stabilizing of the line of sight by virtue of gyroscopic slaving loops. They may also consist of a “two-axis” platform carrying all the equipment.
Comparison of these two modes leads to the following pros and cons.
The “push-broom” mode is simpler to implement and supplies a directly exploitable image; however, it is more exposed to the defects in stabilization of the line of sight which tend to be larger in this mode.
Moreover, the output signals from a detection linear array are conventionally processed by post-integration, for example by so-called TDI or Time and Delay Integration technology, which makes it possible to increase the sensitivity of the linear array electronically both in visible light by day and in infrared so as to operate at night. Post-integration raises difficulties of exploitation in “push-broom” mode owing to the fact that, since the snapshot cannot usually be taken vertically, the inclined projection of the linear array and of its elementary sensors on the ground has variable dimension from one extremity of the linear array to the other.
For its part, the panoramic mode leads to an exploitation of the image which is more complex since the latter is formed of trapeziums which, being obtained by integration in the course of outward and return scans, partially overlap one another. Motorization of the line of sight of the equipment is then required in order to perform this scanning in addition to stabilization. However, the use of a TDI technology detector is compatible with this mode of analysis.
In both these modes, the defects engendered by the stabilization dynamics, and referred to hereafter as the “stabilization residuals”, manifest themselves as losses of optical resolution and as geometric defects in the image which may make it unexploitable.
More recently, two-dimensional matrix detectors have been used instead of linear arrays. The mode of analysis implemented with this type of detector then consists in covering the area observed by a scan of the panoramic type, with the aid of several successive exposures. Marginal overlapping of the snapshots conventionally allows electronic realignment of the images obtained.
This solution demands that the line of sight be immobilized successively over each position during the time of exposure of the matrix (typically 10 ms). High performance with regard to the quality of stabilization of the line of sights is therefore necessary. Such performance is difficult to achieve and maintain.
Moreover, capturing an entire image, through successive exposures of the matrix detector, does not make it possible to obtain better than the spatial resolution demanded by the sampling of the sensor. It is not, in effect, possible to carry out interlacing and hence to comply with Shannon's sampling criterion.
SUMMARY OF THE INVENTION
The invention proposes the use of detector matrices in aerial reconnaissance together with a particular mode of analysis which makes it possible to avoid the above drawbacks, in particular the image defects due to stabilization and to sampling.
To this end, the invention optimally exploits the simultaneous availability of a large number of sensors, through the use of a detection matrix in quasi-continuous panoramic scan mode, together with multiple and particularly fast snapshot capture. The optimal approach then consists in carrying out image reconstruction, using the stabilization residual to perform an oversampling of the image, and thus to improve the resolution of the equipment. Thus, the proposed solution identifies and uses to its advantage a drawback of the prior art, namely the scanning residual, to alleviate the optical defects arising from this scanning residual.
More precisely, the subject of the invention is an air/ground reconnaissance process for optronic equipment fitted with a high-speed detection matrix having elementary sensors which are sensitive in the infrared spectrum and supply values of luminance of an observation area on the ground by panoramic scanning of the area in outward and return patches, characterized in that it consists in performing a two-dimensional correlation of elementary image offset by pinpointing areas of overlap of like luminance level in successive elementary images supplied by a panoramic scanning of this matrix, in storing these elementary-image correlations in succession, and in positioning the luminance values of each correlated elementary image in a memory for a reconstructed image patch, the patch memory possessing an elementary memory spacing equal to a fraction of the image-point spacing of each elementary image.
According to a particular embodiment, the number of overlap areas is at least equal to 4, the elementary memory spacing of the patch memory is equal to half the image-point spacing of each elementary image, and the luminance values addressed to one and the same area of the patch memory are accumulated by successive summation.
The invention also relates to a device for implementing this process. This device is based on the use of a so-called “fast” detector matrix, that is to say one with a high reading speed, for example 400 Hz, to form elementary images with a high overlap factor.
According to particular embodiments, there is provided a stabilizing device for maintaining the elementary images of the detector perpendicularly to the path of the aircraft, an apparatus for performing a compensation scan making it possible to obtain immobility of the global scan during the exposure times, two image memories for integrating the luminance values of two successive elementary images, or else a single current image memory associated with a reference image memory. In the latter case, the luminance values of the reference image are obtained by particular processing on a set of values obtained in the course of the scan.
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"Thomson-CSF"
Britton Howard
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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