Missile Defense Infrared Sensors

Strategic and tactical missile detection and target detection require basically two functions. First, is the detection of the missile launch and the burning of the propellant during acceleration into the ballistic trajectory. Secondly, the missile must be tracked to determine where it is targeted. In the event of multiple launches, the number of missiles need to be counted and tracked at substantially the same time.

Remote sensing of visual, ultraviolet, and infrared sources currently employ electro-optical detection systems. The detection systems are mounted on platforms supported by satellites, airplanes, ships or land based vehicles. The detection systems include an electro-optical or optical sensor, typically a telescope, for collecting photons representative of a target and a detector for converting these collected photons into electrons and hence an electronic current. This enables information to be gathered about the target.

Traditionally, strategic and tactical missile launch detection and tracking systems use infrared surveillance techniques employed by satellite sensors. The satellite sensors are required to view the whole earth at one time because the missiles could be launched from any location. It is necessary to determine where the missile is going and, from the trajectory, to find where the missile is likely to hit with a high probability of detection. For example, it is usually required to have a probability of detection of at least 99% and to avoid a false detection to a probability of less than once per month.

Based on these requirements, satellite sensing systems have been developed that contain many individual picture elements or pixels so that the systems can detect, count and track missiles anywhere on the earth. In order to track missiles to the accuracy required, it is generally necessary that the pixels have an extremely high resolution so that they can observe about a 1.5 kilometer square grid on the earth every few seconds. To do this with a staring array several millions of pixels are often needed. Surveillance systems using these large staring arrays are quite heavy (for example, about 6,000 to 8,000 pounds) due, in large part, to the multiplicity of pixel elements employed.

To launch a satellite using such large staring arrays, expensive and heavy spacecraft launch vehicles, such as the Titan IV, must be used. This, of course, undesirably increases the cost of the project. The number of pixels can be reduced by scanning sensors but the pixels must be scanned at such a high rate that it produces an exceedingly large number of operations per second for the on-board satellite computer. Alternatively, all of the sensor data can be transmitted to the ground with a wide band data link. However, this is less desirable than on-board processing because it is less robust and radiation hard when compared with the narrow band data link that is used with on-board processing.

A prior electro-optical remote detection system for intelligence gathering applications which senses infrared radiation from a target is disclosed, for example, in a paper entitled "Defense Support Program: Support to a Changing World" by Kidd et al, given at the AIAA Space Programs and Technologies Conference, May 24-27, 1992 and the Infrared Handbook, published by ERIM for the U.S. Navy, 1989. Generally such systems seek a target signal in the presence of an overwhelming background signal. For example, these systems attempt to detect a missile from a down looking satellite against an earth background using a satellite sensor which covers a single band for scanning the exhaust signature of the missile. Such a single band detection system is described with reference to FIG. 2 which shows plots of relative signal power or intensity versus wavelength for a background signal 10 and a composite signal 11 of background and target. The background signal is almost always present in the band of interest. Typically, the background signal has undesirable "clutter" interference due, for example, to contiguous areas of high contrast. Severe detection problems often occur when, for example, the satellite scans a target having a sunlit earth behind it due to "glint" reflections of sunlight.

The background signal closely approximates the composite signal over the infrared (IR) spectrum between 2.0 and 5.0 micrometers, with the largest variation occurring between about 2.6 and 3.2 micrometers. Another significant difference exists between about 4.1 and 4.8 micrometers. The detection band having a range of about 2.7 micrometers to 3.0 micrometers (see, e.g., page 2-76 of the Infrared Handbook) is typically chosen by the detection system for target recognition or extraction from the background. This corresponds to the water band exhaust signature which is prominent in a missile.

Conventional detection occurs when the intensity of the composite signal containing the missile exhaust within the band exceeds the threshold. The limitation in this approach is that the detected intensity of the composite signal must be perceptively larger than the background within the detection band or false alarms will occur as a result of background variances. Texts in this field (see, e.g., Chapter 15 of Radar Handbook, M. Skolnik editor, 1970) have recognized the problem with false alarms in previous approaches. One attempt to solve the problem involves raising the threshold level of the detector so that false detection of a target would occur infrequently. However, as shown, the composite signal contains actual, but weak, target signals which are less than the threshold over a portion of the detection band. Therefore, if the threshold level is raised too high, many actual targets would remain undetected.

Many known missile detection systems sense infrared radiation in a single band. This band contains information about the missile as well as unwanted background. In another prior art approach, as used in the Defense Support Program (DSP), two discrete radiometers are used to cover two detection bands which are independently processed. However, such approaches can be defeated by a strong background signal that approximates or exceeds the target within its two relatively narrow detection bands. Hence it is also subject to a large number of false alarm indications when the detection threshold level is too close to the background clutter or to undesirably lower detection rates when the threshold is raised too high.

When an intercontinental ballistic missile (ICBM) is launched into an orbit toward a target, it can be readily detected by radar as it approaches a target area. Several decoy missiles, which are designed to appear to a radar system as though they are ICBM's, may accompany one or more ICBM's so that a cluster of missiles will appear to a tracking radar, thereby camouflaging the identity of the true ICBM's. However, this cluster of missiles is also being observed by detection systems other than radar. For example, passive infrared detection systems such as forward looking infrared systems (FLIRS) in the 3-5 micron band and the 8-14 micron band are used. These systems sense the temperature of an object by observing the thermal energy which it radiates. Thus, when an ICBM and accompanying decoys are launched into a polar orbit they will pass from a sun lit region into a region shaded by the earth's shadow. When this happens, both the massive ICBM's and any decoys which accompany them begin to radiate energy to the cold regions of outer space and begin to cool down. Since each radiates energy at about the same rate, the ICBM's with their large thermal mass tend to change temperature very slowly. However, the light weight decoys, because of their small thermal mass, change temperature very rapidly by comparison, allowing the FLIRS to quickly discriminate between the ICBM's and the decoys.