Space


Exo-Atmospheric

The missile defense battle space is divided between intercepts in the atmosphere, what is called endo-atmospheric, and out of the atmosphere, what is called exo-atmospheric. Exoatmospheric missiles are capable of completing missions outside of the earth's atmosphere. Exoatmospheric missiles may be used to provide an orbital defense layer against hostile ballistic missiles. Typically, desired sensor characteristics for exoatmospheric missions include a narrow field of regard and LWIR detection. The field of regard for the sensor of an exoatmospheric missile is less sensitive to the angle of attack. However, providing sufficient cooling or a thermal operation is often more important than for endoatmospheric missiles.

The exoatmospheric interceptors use IR sensors operating outside of the Earth's atmosphere which see much lower space noise backgrounds, and thus are expected to be more effective. However, while it has been demonstrated that exoatmospheric sensors can home in on and destroy moving targets in the presence of "crude penaids", it is still questionable whether passive IR sensors can discriminate threats that contain credible decoys (i.e., weights of a few percent, same size and shape, reflectivity, etc. of RV's).

Since only a limited number of these systems are available for early deployment, the addition of large mumbers of credible decoys could quickly erode the ability of these systems to accomplish their missions. Therefore, it is necessary to provide a system having the capability to discriminate the decoys from the real Reentry Vehicles (RV's). Since passive and active sensors have difficulties discriminating decoys from RV's with decoys whose weights are just a few percent of the RV's, the value of an interactive system is recognized.

An example of an exoatmospheric missile is the kinetic kill vehicle (KKV) used to provide antisatellite capability. The KKV is a light-weight missile having on-board propulsion and guidance systems. The KKV differs from conventional missiles in that the vehicle is not equipped with a warhead, but instead delivers sufficient kinetic energy upon striking the target to disable or destroy the target. As described in Beam, the conventional seeker is an infrared sensor that receives energy from a target to determine a target angle relative to a boresight of the KKV. The measurements from the seeker are combined with body orientation sensed by a gyro-based altitude reference system, producing a target line-of-sight measurement.

In comparison with the conventional approach, the kinetic kill vehicle guidance system preferably uses a visible or ultra-violet sensor to resolve both stars and targets. The seeker generates two signals. The first signal is indicative of the angle between a longitudinal axis extending through the missile and the line of sight of the missile to the target. The second signal is indicative of the angle between the longitudinal axis extending through the missile and a line of sight between the missile and a celestial body, such as a star. From these two signals, a velocity vector for the missile propulsion system is calculated. The calculation is executed in a manner to guide the KKV so as to complete the intended mission.

In order to maximize the range of a missile having a given energy supply, designers attempt to minimize the weight of the various systems that comprise the missile. Regulating the weight is particularly important for exoatmospheric missiles, such as the KKV described in Beam. Another concern is the space requirement of each system within the missile. Consequently, the mass and the number of components required for target acquisition, tracking and ranging are important. Nevertheless, seekers having more than one mode of operation are known. The different modes of operation may be distinguishable by frequencies of interest, such as a system capable of sensing both in the millimeter wave spectrum and the infrared spectrum.




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