Endoatmospheric/Exoatmospheric Interceptor (E2I)

During the summer of 1990, SDIO Director Ambassador Cooper approved the Endoatmospheric/Exoatmospheric Interceptor (E2I) program as a "logical follow-on to the HEDI KITE program." At the same time, HEDI was identified as a "viable candidate for the lead ground-based interceptor for the SDS [Strategic Defense System] architecture." Using both a medium wavelength and an LWIR seeker, the E²I would expand the range of SDI's terminal defense interceptor from "tens of kilometers to hundreds of kilometers."

The overall objective of the Endoatmospheric/Exoatmospheric Interceptor (E2I) Program was to develop an interceptor to support the endoatmospheric mission of SDI Organization National Missile Defense, while its primary technical objective is to develop the hardware and software required to demonstrate the deployable interceptor that can perform onboard target cluster track and target selection.

The Global Protection against Limited Strikes architecture for ground-based defense against strategic ballistic missiles consisted of a command center and a combination of Brilliant Eyes satellites, terminal phase ground-based radar trackers (GBRT), and high-speed accurate interceptors. These interceptors would be terminal phase endo-exoatmospheric interceptors (E2I) and/or midcourse phase exoatmospheric ground-based interceptors (GBI). An option also existed to add the ground-based surveillance and tracking system (GSTS) to the architecture. BE satellites are derivatives of the previously planned space-based surveillance and tracking system satellites.

For planning purposes, the ground-based defense tier of a GPALS system includes the following:

1. approximately 750 ground-based interceptors,

2. six ground-based radars,

3. approximately 60 BE satellites, and

4. the appropriate command and control for the ground-based tier.

If E2Is were used, BE would provide postboost and midcourse surveillance and GBRTs would support terminal intercepts. The BE satellite would track the PBVs, clusters of RVs, and, in some cases, individual RVs to provide the data to commit the E2I. The GBRT would acquire, track, and discriminate between RVs and decoys in the late midcourse and terminal portions of their trajectories, providing kill assessment and additional target selections to the E2I. If GBIs were used, either BE, GSTS, or some combination of each will be used to provide cluster tracks for the GBIs. GBIs may require GBRT for commitment against short time of flight SLBM, but this requirement remained to be validated as the program matures.

The choice between E2I and GBI, or possibly whether to continue with both, will be made before full-scale development and will depend on the resolution of several issues at that time. Terminal defense could benefit from the easier discrimination of RVs from decoys by atmospheric slowdown, but only, at the expense of requiring a more complicated interceptor that could withstand the heating and mechanical stress caused by operating in the upper atmosphere. Midcourse interceptors are inherently simpler and could be used much more flexibly throughout the long midcourse portion of the RVs' flight trajectory. However, the defense must have confidence in its ability to discriminate RVs in midcourse in the expected threat environment.

These same elements would counter SLBM attacks. The functions of the elements would be very similar to those performed in defending against ICBMs. However, an SLBM attack launched from a submarine very close to the US coast would constitute a more stressing threat, especially if flown on a depressed trajectory. Although BP would be effective against such an attack, those RVs not engaged by BP would have to be intercepted by ground-based interceptors to completely counter such an attack.

On October 28, 1997 Anthony D. Vandersteen (Cupertino, CA) and William C. Lynch (Los Altos, CA) were granted United States Patent # 5,681,009 for "Missile having endoatmospheric and exoatmospheric seeker capability" which was assigned to Lockheed Missiles and Space Company (Sunnyvale, CA). A seeker device has an exoatmospheric mode of operation in which a first optical assembly forms a narrow and preferably fixed field of view for receiving incoming energy via a first path through an optical opening. The seeker device also includes a second optical assembly for providing a wide field of regard and for receiving incoming energy via a second path through the optical opening. The incoming energy from the wide field of regard is directed into the first optical assembly, so that components for collecting, focusing and sensing optical energy are not duplicated.

In the preferred embodiment, the first optical assembly is an infrared telescope mounted within the housing of a missile. The infrared telescope is mounted to have a direct line of sight through the optical opening, with the line of sight being off-axis of the missile housing, suitable for exoatmospheric flight. The second optical assembly is preferably a mirror that is pivoted to define the wide field of regard. At one extent of the field of regard, the line of sight of a pivotable mirror provides a low angle of attack potential, as measured with respect to the axis of the missile housing, which is required for endoatmospheric flight.

In the preferred embodiment, the seeker device includes a second mirror which is a dichroic mirror that is selective with respect to transmission of bands of frequencies. The dichroic mirror is preferably fixed in place. In the fixed embodiment, the dichroic mirror has a high transmissivity with respect to long wave infrared (LWIR) energy within the narrow field of view of the telescope. The dichroic mirror is in the direct line-of-sight axis of the telescope through the optical opening of the housing. The rear surface of the dichroic mirror is highly reflective with respect to mid wave infrared (MWIR) energy. The pivotable mirror is positioned to direct energy onto the rear surface of the dichroic mirror, so that the MWIR is reflected by the dichroic mirror into the infrared telescope. Thus, the dichroic mirror enables two modes of operation with a single fixed element.

In an alternative embodiment the second mirror is displaceable and band selectivity is not critical. That is, the second mirror may be replaced with a more conventional mirror or other assembly for redirecting the energy from the pivotable mirror to the telescope. In the exoatmospheric mode, the mirror is removed from the direct line-of-sight axis of the telescope. In the endoatmospheric mode, the displaceable mirror is switched into the line of sight of the telescope. As in the fixed dichroic mirror embodiment, the energy from the pivotable mirror is reflected at the internal surface of the displaceable mirror to direct the energy into the telescope.

The collected and focused energy from the telescope is directed to a sensor that converts the optical signal into an electrical signal representative of the collected radiation. The sensor may be a focal plane array (FPA) or a Dewar, but this is not critical. Mode switching can take place downstream of the telescope. For example, a switching filter assembly may be used to select between filtering MWIR and LWIR prior to optical signal sensing.

As previously noted, the optical elements are preferably cryogenically cooled for exoatmospheric operation. In the preferred embodiment, the optical elements of the telescope are made of silicon carbide, which has stiffness approaching that of beryllium and a desirable coefficient of thermal expansion close to that of an optical bench which may be a structure made of aluminum-doped graphite epoxy, for example. This substantially matches the thermal expansion of the silicon carbide optics, so that the optical system remains in focus under dynamic thermal environments. The combination of cryogenic cooling and the selection of the materials provides a substantially athermal design which also reduces the thermal noise contribution of the mirrors to the total noise at the sensors (FPA). In the exoatmospheric mode, the preferred embodiment uses cryogenically cooled optical elements, including the window, second mirror, telescope and other optical elements in the line of sight to the sensor FPA. One method of removing heating effects from the window is to eject the window (by a suitable actuator device) from the seeker forecone after the missile is in the exoatmosphere.

Another advantage is that the multi-mode capability is achieved with a substantially small number of additional components. For the preferred embodiment in which the dichroic mirror is fixed in position, the mirror eliminates the need for moving components in order to switch modes, eliminates the need for a pre-launch command to select either the wide field of regard for endoatmospheric operation or the narrow field of view for exoatmospheric operation, and allows simultaneous split band spectra to be present at the focal plane, so that a single in-flight command to the internal filter can be used to select between LWIR and MWIR wavelength bandpass. Alternatively, a dual FPA/Dewar or a single dual-band FPA, such as a gallium arsenide multiple quantum device, could eliminate the need for filtering, thereby eliminating all mode switching. Also, the use of cryo-cooled optical elements increases the effective range in the exoatmospheric mode by increasing the signal-to-noise ratio of the seekers.