Patent No. 4497065 Target recognition system enhanced by active signature measurements

 

Patent No. 4497065 Target recognition system enhanced by active signature measurements (Tisdale, et al., Jan 29, 1985)

Abstract

A passive electro-optical target recognition system which utilizes additional signature measurements of an active nature to enhance the discrimination of desired targets from detected candidate targets in the field of view thereof is disclosed. Candidate targets are detected by the passive target recognition system from passive image radiation received from the field of view of its one or more passive sensors. A laser beam generated substantially at a predetermined wavelength is positioned to illuminate individually each of the detected candidate targets. An active sensor tuned to the predetermined wavelength receives the active laser radiation separately reflected from the individually illuminated candidate targets. An active signature is derived for each of the detected candidate targets from its received active image radiation. Accordingly, a desired target is discriminated from the detected candidate targets in the field of view of the passive sensor based on the derived active target signatures thereof. To further enhance this discrimination process, polarizing properties of the laser beam may be used as an additional measurement for each candidate target beyond the derived active target signatures thereof.

 

Notes:

BACKGROUND OF THE INVENTION

The present invention relates to target recognition systems, in general, and more particularly to a passive electro-optical target recognition system which utilizes additional signature measurements of an active nature to enhance the discrimination of desired targets from detected candidate targets in the field of view thereof.

Generally, electro-optical (e/o) target recognition systems operate on the field of view of one or more passive imaging sensors, such as a television camera (TV) or foward-looking infrared detector (FLIR), for example, to isolate targets of interest and signal their presence to an operator by visual cues which may be displayed on a video display (CRT). Audible cueing may also be used. This cueing information may include target classification, location in the video display, and prioritization relative to other targets which may be present in the field of view of the imaging sensor. A measure of the performance of such systems is determined by how well they discriminate targets of interest from background objects so as to avoid false alarms or false omissions.

Some examples of passive e/o target recognition systems and related technology are disclosed in the following U.S. Pat. Nos.:

3,638,188; entitled "Classification Method And Apparatus For Pattern Recognition Systems"; issued to Glenn E. Tisdale et al on Jan. 25, 1972,

3,636,513; entitled "Preprocessing Method And Apparatus For Pattern Recognition"; issued to Glenn E. Tisdale on Jan. 18, 1972, and

3,748,644; entitled "Automatic Registration Of Points In Two Separate Images"; issued to Glenn E. Tisdale in July 24, 1973, all assigned to the same assignee as the instant patent application.

Target recognition systems of this type usually include one or more passive sensors, a preprocessor for converting the sensor image data into a condensed format for further processing, means for collecting or segmenting information associated with individual candidate targets, and means for classifying targets based on derived pertinent measurements or features from the candidate targets. In one known system, the segmentation operation is initiated by the detection of a specific image characteristic, such as a blob or edge, which may become the basis for collection of nearby image information of a scene. Classification is most often based upon the shapes of the candidate target images derived from the segmentation operation, and may also involve textures and intensity data of a candidate target image, or even context information thereof.

As would be expected, at long ranges (i.e. the distance the passive sensor is positioned from the candidate targets), say on the order of 3-5 kilometers, for example, the sensor image information available for a given target diminishes in the presence of background images and clutter. As a result, the ability to correctly classify a target is reduced, thus, the probability of false alarm or false omission occurrence is increased.

Some target recognition systems, like those disclosed in U.S. Pat. Nos. 3,261,014 and 3,076,961 issued July 12, 1966 and Feb. 5, 1963, respectively, have proposed a multi-sensor coordinated system by including a radar with a combination of TV and FLIR passive sensors to increase the amount of information for use in acquiring a target. These systems make use of a second sensor to provide useful information associated with the detection or range of potential targets, but they do not appear to assist in the classification process.

Other target recognition systems, like the one disclosed in U.S. Pat. No. 3,953,667; entitled "Passive And/Or Active Imaging Systems"; issued to Allen C. Layton et al on Apr. 27, 1976, increases its target image information by adding active laser illumination of the field of view in order to enhance the FLIR's capability to discern in a scene certain details, such as man made objects, especially under adverse environmental conditions, like thermal washout, for example. However, this system requires a highly complex mechanization combining the passive and active sensing elements. An advantage of the present invention is that it may be embodied with minor modification of an existing implementation as will become apparent from the description found herein below.

In view of the above, it is recognized that one way of improving both target recognition/classification and false alarm/omission rejection capabilities of a passive target recognition system is by introducing one or more additional image measurements of an active nature, especially when the sensor is at long ranges from the targets of interest so as to ameliorate the lower target resolutions and target to clutter contrast conditions which are prevalent in an all passive system.

Summary of Invention:

In accordance with the present invention, an electro-optical target recognition system operates on both active and passive radiation from the field of view thereof to discriminate desired targets from detected candidate targets. At least one passive sensor receives passive image radiation from the field of view thereof and converts the received image radiation into first electrical signals representative thereof. A passive target recognition means operates on the first electrical signals to detect candidate targets in the field of view of the passive sensor. In addition, a laser beam is generated substantially at a predetermined wavelength by an active source and is positioned to illuminate individually each of the candidate targets detected by the passive target recognition means. Furthermore, an active sensor tuned to the predetermined wavelength receives the active laser radiation separately reflected from the individually illuminated candidate targets and converts the separately received active laser radiation into respective second electrical signals representative thereof. An active signature is derived for each of the detected candidate targets from the respective second electrical signals. Accordingly, a desired target is discriminated from the detected candidate targets in the field of view of the passive sensor based on the derived active target signatures thereof.

More specifically, the laser beam may be positioned to traverse each detected candidate target region in the field of view individually to effect at least one narrow illumination sweep of each of the regions. The active sensor is operative to collect the active reflected radiation separately from each candidate target region concurrently with the respective active illumination traversal thereof. In one aspect of the invention, the collected active reflected radiation from a candidate target during the narrow illumination sweep thereof is converted into a series of radiation intensity values sequentially corresponding thereto. In this aspect, the series of radiation intensity values is used directly as the active signature of its corresponding candidate target. In this case, a desired target is discriminated from the detected candidate targets based on distinguishing characteristics between the series of radiation intensity values associated with each candidate target. In another aspect, an appropriate value is derived for each candidate target representative of the radiation intensity range of the values in the converted series correspondingly associated with each candidate target. In which case, a desired target is discriminated from the detected candidate targets based on the derived appropriate values correspondingly associated therewith.

A further aspect of the present invention includes using polarization properties of the active laser beam to further enhance the discrimination of a desired target from the detected candidate targets. For this aspect, the active source includes means for polarizing the generated laser beam and the active sensor includes means for rotating its direction of polarization relative to the transmitted laser radiation. A variation in the polarized radiation response from a detected candidate target corresponding to the rotation in the direction of polarization is measured and used as an additional characteristic for each candidate target to aid in the discrimination process.
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