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Discriminating Seeker

The gamma-ray homing sensor/seeker technique patented in 1997 for the US Army employs a combination of devices which are effective at short distances and longer distances. The method of the system is accomplished at short distances of about 5 to 10 kilometers without the use of any activation methods. At longer ranges of more than 100 Km, an activation method (such as a Neutral Particle Beam Weapon) must be used in conjunction with the gamma seeker discriminator, in order to obtain the gamma signal required by the gamma sensor. The gamma sensor will be aided by a conventional infrared sensor, in order to assure that the interceptor detects the targets at large distances, and to perform the tracking of the target after the gamma seeker discriminator has identified a RV. In other words, the IR sensor will perform the tracking, while the gamma seeker discriminator will perform the discrimination function.

The gamma seeker discriminator also provides the interceptor with a means of performing sensor fusion of the gamma signatures with the infrared data, which should further enhance performance. The means for collecting gamma radiation over a large area and focusing the radiation onto a relatively small detector includes the use of a key element which is a large crystal diffraction lens. This lens uses "Variable-Metric" (V-M) crystals. The V-M lens is obtained by either creating a thermal gradient across the lens, or by changing the chemical composition of the lens in order to obtain the change in lattice spacing, which results in greater focusing capability.

The gamma-ray homing sensor/seeker technique of this invention employs a combination of devices which are effective at short distances and longer distances. The method of the system is accomplished at short distances of about 5 to 10 kilometers without the use of any activation methods. At longer ranges of more than 100 Km, an activation method (such as a Neutral Particle Beam Weapon) must be used in conjunction with the gamma seeker discriminator, in order to obtain the gamma signal required by the gamma sensor. The gamma sensor will be aided by a conventional infrared sensor, in order to assure that the interceptor detects the targets at large distances, and to perform the tracking of the target after the gamma seeker discriminator has identified a RV. In other words, the IR sensor will perform the tracking, while the gamma seeker discriminator will perform the discrimination function. The gamma seeker discriminator also provides the interceptor with a means of performing sensor fusion of the gamma signatures with the infrared data on the same interceptor platform to enhance performance.

Edlin, et al. were granted United States Patent 5,611,502 on March 18, 1997, assigned to the United States Army. The key element in this system is a crystal diffraction lens that collects radiation over a large area and focuses it on to a relatively small detector. This system uses "Variable-Metric" (V-M) crystals. A V-M crystal is formed by varying the crystal lattice spacing with position in the crystal, allowing for greater focusing strength and imaging properties, when compared to conventional condensing lenses (flat crystals). The V-M crystals can focus gamma-rays onto detectors of diameters of less than one millimeter, while conventional state-of-the-art flat mirrors are limited to about one square centimeter. The V-M lens is obtained by either creating a thermal gradient across the lens, or by changing the chemical composition of the lens in order to obtain the change in lattice spacing, which results in greater focusing strength.

From a systems point of view, the V-M crystals provide a factor of better than 100 signal gain (i.e. 1 cm.sup.2 /1 mm.sup.2), which also results in a factor of better than 100 improvement in the signal to noise ratio of the sensor, when compared to "conventional" gamma sensors. This allows for adequate gamma detection of an RV at about 10 kilometers with no activation techniques. When used in conjunction with a Neutral Particle Beam (i.e.,activation of the nuclear material), detection at ranges of more than 100 kilometers can be obtained. Additionally, by choosing to look at only a couple of narrow energy band widths, the V-M crystal lens can be designed to limit the sensor to a field-of-view of a few arc seconds which is well within the field of view, required to eliminate off axis nuclear detonations.

Edlin et al identified and employed a very promising gamma seeker that discriminates RV's from decoys by measuring the fission gammas emitted by a RV containing a nuclear warhead. This measuring of fission gammas is viable because of a relatively new gamma detection device which has been invented at Argonne National Laboratories by Robert K. Smither (U.S. Pat. No. 4,429,411, entitled "Instrument and Method for Focusing X-rays, Gamma-Rays, and Neutrons" (issued 31 Jan. 1984)). This instrument and method provide orders of magnitude better signal to noise ratio than conventional gamma-ray sensors while also providing better instantaneous field-of-view capabilities. The signal to noise improvement has resulted in improving the sensors detection range from a few kilometers to hundreds of kilometers. The small instantaneous field of view (a few arc seconds) provides the system with substantial background noise reduction (about two orders of magnitude better than conventional gamma-ray sensors), which is required in the presence of nuclear detonations.