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Anti-Missile Sensors

Sensors and their associated systems will function as the "eyes and ears" of a ballistic missile defense system, providing early warning of attack, target identification, target tracking, and kill determination. New and innovative approaches to these requirements using unconventional techniques are encouraged across a broad band of the electromagnetic spectrum, from radar to gamma-rays. Passive, active, and interactive techniques for discriminating targets from decoys and other sensor-related device technology is also needed, with the intended goal of producing either a specific product or process.

BMDS sensors would provide the relevant incoming data for threat ballistic missiles. The data from these sensors would travel through the communication systems of the proposed BMDS to Command and Control (C2) where a decision would be made to employ a defensive weapon such as launching an interceptor. The BMDS sensors would provide the information needed to determine the origin and path of a threat missile to support coordinated and effective decisionmaking against the threat. Additionally, these sensors would provide data on the effectiveness of the defense employed, that is, whether the threat has been negated.

BMDS sensors would be developed or enhanced to acquire, record, and process data on threat missiles and interceptor missiles; detect and track threat missiles; direct interceptor missiles or other defenses (e.g., lasers); and assess whether a threat missile has been destroyed. These sensors (i.e., radar, infrared, optical, and laser) would include signal processing subcomponents, which receive raw data and use hardware and software to process these data to determine the threat missile's location, direction, velocity, and altitude. This and other relevant information would then be integrated into planning and controlling intercept engagements through the C2BMC component of the BMDS.

The operating environments of the existing and proposed BMDS sensors can be considered in four general categories. Land-based sensors may be fixed, located in or on a building, or mobile, located on a vehicle or trailer. Air-based sensors are located on platforms that can travel through the air such as airplanes, balloons, and airships. Seabased sensors are located on platforms that travel on water (e.g., ships or a floating platform) or are fixed in water (e.g., a man-made island or platform like an oil platform that is fixed to the seafloor). Space-based sensors are located on satellites, which travel in circular or elliptical orbits around the Earth. These satellites can be in several different types of orbits including GEO, which is an orbit at approximately 36,000 kilometers (21,700 miles), synchronized with the Earth's rotation, and LEO, which is an orbit at an altitude of approximately 160 to 1,600 kilometers (100 to 1,000 miles). Weather, communications, and some military satellites, such as DSP satellites, typically use GEO orbits.

Radar, which stands for RAdio Detection And Ranging, typically is an active sensor that emits radio frequency energy toward an object and measures the energy of radio waves reflected from the object. Radars are currently based in land and sea operating environments. Most modern radars operate in a frequency range of about 300 megahertz (MHz) to 30 gigahertz (GHz), which corresponds to a wavelength range of one meter to one centimeter. The time delay in the return signal or echo allows the determination of distance to the object and the change in the frequency of the echo through the Doppler Effect allows the determination of the object's speed. The Doppler Effect is the shift in frequency resulting from relative motion of an object in relation to, in this case, the radar. Most current radars are mono-static because the transmitter and receiver are collocated. There are also radars with multiple transmitters and multiple receivers in different locations that are called bi-static and multi-static radars based on the number of transmitters and receivers.

Infrared sensors detect the heat energy or infrared radiation from an object. Infrared electromagnetic radiation (EMR) has wavelengths longer than the red end of visible light and shorter than microwaves (roughly between one and 100 microns). The Defense Support Program (DSP) satellite, is an example of a space-based infrared sensor (SBIRS) that can detect the heat signature or plume from the launch of a ballistic missile.

Optical sensors operate in the visible range and are generally passive sensors that detect objects or missiles by collecting light energy or radiation emitted from the target in wavelengths visible to the human eye. Specifically, the human eye perceives this radiation as colors ranging from red (longer wavelengths, approximately 700 nanometers) to violet (shorter wavelengths, approximately 400 nanometers). The planned Space Tracking and Surveillance System (STSS) satellites, for example, would have both infrared and optical sensors.

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Laser sensors use laser energy of various energy levels and frequencies (ultraviolet, visible) to illuminate an object to detect the object's motion. Like radar, a laser-based sensor is an active sensor that sends out laser energy toward an object and then receives a return echo from the object. The time delay in the return signal or echo allows the determination of distance to the object and the change in the frequency of the echo through the Doppler Effect allows the determination of the object's speed. The ABL aircraft uses passive infrared sensors to detect, and laser sensors to illuminate and track threat ballistic missiles.

Examples of some of the specific areas to be addressed are: cryogenic coolers (open and closed systems), superconducting focal plane detector arrays (for both the IR and sub-mm spectral regions), signal and data processing algorithms (for both conventional focal plane and interferometric imaging systems), low-power optical and sub-mm wave beam steering, range-doppler lidar and radar, passive focal plane imaging (long wavelength infrared to ultra-violet; novel information processing to maximize resolution while minimizing detector element densities) interferometry (both passive and with active illumination), gamma-ray detection, neutron detection, intermediate power frequency agile lasers for diffractive beam steering and remote laser induced emission spectroscopy, lightweight compact efficient fixed frequency radiation sources for space-based SDI application (uv-sub-mm wave).

Missile, aircraft and RSO surveillance systems have been utilized on satellites and/or aircraft for some time now. Such systems are used to detect and track missiles, aircraft, satellites and other objects of interest by processing electromagnetic signals, such as those in the optical or infrared spectra, emitted or reflected by the objects of interest.

A use for such surveillance systems is for National Missile Defense, for example, for defending the United States against a missile attack from a rogue country. In order to perform such a function a surveillance system must be capable of monitoring a large portion of the earth. Accordingly, such systems need a number of sensors on multiple satellites that can perform the functions of detection, acquisition, tracking and booster typing and discrimination.

Another closely related use for surveillance systems is tactical missile defense. This is basically the same use as national missile defense but on a smaller scale. For example, instead of defending the United States from a long range missile from a remote country such as Russia, tactical missile defense is used in situation such as that encountered during the Gulf War, when dealing with missiles having a range of only several hundred miles. An example of tactical missile defense would be defending Israel from missiles launched from Iran or another country relatively close to Israel.

In addition, surveillance systems have many other uses, even in peacetime. For example, surveillance systems can observe satellites in space (space surveillance). The North American Air Defense command (NORAD)currently keeps track of approximately 7,500 space objects using ground-based radar tracking. There are probably at least about 20,000 space objects that are not tracked by NORAD. Observing military satellites is an important function as these satellites occasionally maneuver and therefore do not remain in a stable, predictable orbit. Another use for surveillance systems is intelligence gathering. For example, surveillance systems can be used to monitor troop movements. Surveillance systems can also be used for supporting tests of "friendly" missiles by gathering data on missile performance (range support).

Missile detection and tracking systems enable a user to identify missile types and launch points, estimate missile impact point, and track the position of the missile throughout its trajectory. Information gathered by these systems can be used for handoff to another sensor or to a weapon system to destroy the missile or divert the missile from its intended target.

Onboard signal and data processing is a key technology in missile detection and tracking systems. A first generation processor was developed for the Forward Acquisition System (FAS) in the 1980's. FAS never was completed, but much design work was done before the program was canceled. FAS was a scanning system that used, for the first time, a mosaic array of detectors instead of a single line array of detectors. FAS used 28,000 detectors, an enormous number of detectors in comparison with prior systems. A breakthrough of the FAS program was to develop simple techniques for data processing so that this enormous amount of data could be processed.

A second generation processor built for the Airborne Surveillance Testbed (AST) Program, formerly known as Airborne Optical Adjunct, was introduced in 1984 as a follow-on to FAS. AST was similar to FAS, but the AST system is carried on an airplane instead of on a satellite. A major innovation of AST is the use of some very sophisticated signal processing which makes corrections for focal plane jitter and uses adaptive filtering methods. The AST program was a success and it is still operating and gathering data.

In 1996, a third generation processor built for a satellite-based surveillance system, called the Midcourse Space Experiment (MSX), was launched into a 900 km orbit. The MSX program is a military program funded by the U.S. Ballistic Missile Defense Organization. MSX uses an onboard signal and data processor (OSDP) to acquire and track objects [Pfeiffer and Masson, "Technology Demonstration by the Onboard Signal and Data Processor," Johns Hopkins APL Technical Digest, Volume 17, Number 2 (1996)]. A major innovation of MSX is the ability to package software and hardware capable of handling the huge processing load, similar to that of the AST program, into a small package, capable of operating onboard a satellite. In the AST program, the programmable processor demands were on the order of 50 million operations per second (MIPS). The OSDP in the MSX program performs essentially the same functions as the AST programmable processor, but requires only about 1 MIP and combines both signal processing and data processing into one unit so that the processing required for tracking is actually done onboard. MSX has been a great success, producing results better than predicted.

Another sophisticated satellite-based missile surveillance system uses a first staring sensor for missile detection and a second staring sensor for missile tracking. A rotating color filter wheel is used to project multiple selected bands of radiation onto the first staring sensor. Missile detection is separated from missile counting and tracking in dual subsystems that are coordinated by a multispectral signal processor.

Fourth generation processors for missile and aircraft surveillance systems such as the Space and Missile Tracking System (SMTS, also known as Space-Based Infrared System--Low, or SBIRS-Low) will have to meet requirements exceeding the capabilities of any existing surveillance system. The SMTS will use both scanning and staring electro-optical sensors to detect and track missiles, aircraft and resident space objects (RSO's). The scanning sensor onboard the SMTS will be required to detect events of interest over a wide field of regard and then hand over the line-of-sight (LOS) positions and velocities of these events to an onboard staring sensor and to staring sensors on other platforms. Staring sensor performance over a large area is thereby achieved with small field of view staring sensors. Resulting single-platform (mono) LOS tracks are then combined with LOS tracks from other platforms to produce interceptor-quality stereo tracks.

To achieve such results, the scanning sensor must provide very effective below-the-horizon (BTH) clutter suppression to insure that the staring sensors are not spending most of their time servicing false alarms. The staring sensors must provide very effective suppression of BTH and above-the-horizon (ATH) clutter and focal plane pattern noise in order to detect dim targets.

Functional requirements which must be met by such a system are:

1. Operation of scanning and staring sensors in a radiation environment. Algorithms for detecting and circumventing radiation-induced spike events in the data must therefore be implemented.

2. Very low false alarm rate operation of a scanning sensor by clutter-adaptive detection, so that the staring sensors have time to handle the handover tasks. The targets of interest are boosting vehicles, RSO's and jet aircraft ("slow walkers").

3. Accurate, high speed calculation of scanning sensor LOS tracks. Toward this end, the accurate typing of boosting vehicles and aircraft is needed.

4. Attack-assessment-quality midcourse calculation of launch points, impact points and parameters from a single platform (mono, three-dimensional (3-D) tracking), where the initial position and velocity of the tracking filter is obtained with booster typing information and an initial orbit determination algorithm.

5. Accurate handover of mono 3-D tracks to a second platform to produce interceptor-quality state vectors by multi-platform (stereo) processing.

6. Staring sensor streak detection which adapts to background clutter characteristics in the presence of focal plane pattern noise and jitter. The targets of interest are boosting vehicles, RSO's and jet aircraft.

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