Airborne Optical Adjunct (AOA)
Airborne Optical System (AOS)

The US Army operated the Boeing 767-200 prototype with a dorsal cupola. It was converted as the Airborne Optical Adjunct (AOA) also referred to as the Airborne Optical System (AOS), with a unique long-wavelength infra-red sensor. The extra keel area forward is compensated for by a pair of enormous ventral fins.

The first airworthiness flight for the Airborne Optical Adjunct (AOA) was conducted on 21 August 1987. In July 1988 Hughes Aircraft delivered the Airborne Surveillance Testbed Sensor, the most complex long-wavelength infrared sensor built to date. The AST completed system integration and testing at Boeing in January 1990.

The AOS aircraft might also include laser range-finder systems to supply accurate estimates of the distance to each target-and possibly to discriminate decoys from RVs by measuring minute velocity changes caused by drag in the upper atmosphere.

System architecture contractors proposed tens of AOA-like aircraft as part of a sensor system. As of 1987 the Pentagon intended to acquire 40 such aircraft, of which 14 would be placed on alert. Some proposed rocket-borne, pop-up probes with LWIR sensors for rapid response in a surprise attack until the aircraft could reach altitude.

Airborne Optical Adjunct Design Features

The AOS system is a Boeing 767-200 aircraft, modified by the addition of an 86 foot long cupola (inverted canoe), which houses a large aperture, long wavelength infrared telescope. The main cabin contains the signal and data processors to translate infrared target energy into tracking data as the events occur. The system can provide the precise location and apparent temperature of more than 400 targets simultaneously. Operating at altitudes above 43,000 feet, the AOA conducts long-range detection, tracking, discrimination, and infrared signature characterization of ballistic targets in all phases of their flight, from boost through reentry phases.

The purpose of the AOA experiment was to demonstrate that component technologies can be developed and integrated into a system which can acquire, track, discriminate, and hand over the data (deleted). The experiment was expected to:

• demonstrate the resolution of critical technology issues associated with developing airborne optical sensors,
• establish a technology base from which further airborne optical sensor development can proceed, and
• reduce risk of any future full-scale engineering development.

The large cupola fairing on top of the fuselage was designed to house two optical sensor devices. Each optical sensor resides in its own compartment, one forward and one aft. Each compartment had a viewing port on the side of cupola, which will be opened during the observation part of the mission. Therefore, the external cupola air must flow past two large open cavities.

The sensor is, in essence, a telescope mounted on the airplane. It detects tarqets based on the contrast between the heat or infrared energy of an object and that of its surrounding background. The detection process begins when the infrared energy from an object is focused by the telescope on a set of detectors within the sensor. Data from these detectors, or focal plane assembly, flow through a signal processor which converts the information to a form which can be used by the data processor. Because the AOA will have only a few seconds to detect and correctly identify a large number of threatening objects, it must process the data from the sensor virtually as soon as it is received.

During a missile attack, enemy nuclear warheads will likely be accompanied by decoys and other penetration aids. To avoid having the defense system shoot at objects which are not a threat, the AOA must distinguish between the warheads and these other objects. This function is performed by discrimination algorithms in the data processinq software. These algorithms, which are being developed, will compare what the sensor detects with certain preprogrammed characteristics of Soviet warheads, such as speed and the amount of heat radiated, and decide whether it is a threatening object.

One of the major objectives of the AOA experiment was to demonstrate that technology needed to build an airborne optical sensor is available. To be successful, the AOA must demonstrate the capability to acquire, track, correctly identify, and handover information on threatening enemy warheads. The Army had identified the technology issues that must be resolved before the experiment objectives can be achieved. These issues relate to the sensor, data processor, and airborne platform.

The AOA sensor must scan a sector of the sky very rapidly and accurately gather the data needed for detecting and identifying threatening objects. The sensor requires a very accurate pointing and stabilization system to perform this function. One of the major problems is the environment in which the sensor must operate -- the vibrations, pitch, and roll of the airplane.

The telescope optics were also a critical element of the sensor. The optics have to be of such quality that they do not degrade the infrared data received from the threatening objects. The primary, secondary, and tertiary mirror figures are off-axis sections of high-order aspheric surfaces. The three mirrors are used as a wide field-of-view, non-reimaging IR telescope. Such a complex system had never been constructed prior to the start of the AOA project. Production of these mirrors has pushed the state-of-the-art in fabrication technology. The mirrors were figured prior to being cut to size, lightweighted, and having precision mounts machined. Mirror figures were checked both mechanically and optically. Of special importance are problems overcome and lessons learned in this endeavor.

The design of a highly reliable data processor with the required speed and capacity and within the size, weight, and power constraints of the AOA was a challenging technological task. In addition, the development of algorithms to perform the discrimination and handover functions was a technology risk. Once the AOA detects an object, it must very quickly and correctly identify the object and hand the information over to another defense system. The ability of an airborne optical sensor to do this with the required speed and accuracy was a critical issue.

The AOA sensor was mounted in a cupola on top of a Boeing 767 airplane. It gathered data by looking through an open port in the cupola. The ability of the sensor to accurately gather data through the turbulence produced by the open port was a technology issue. In addition, the sensor was looking through the atmosphere. The varying densities of the atmosphere disturb incoming optical signals and will affect the accuracy of the sensor. The intent was to design a cavity configuration that would have acceptable aerodynamic, acoustic, and optical transmission characteristics. The thin, stable shear layer, which reattaches smoothly onto the aft ramp of the viewing port (good aerodynamic properties), was thought to result in good optical transmission properties for the sensor as well as a low acoustical noise level in the cavity. Experiments with a single cavity were performed under the sponsorship of the U.S. Army/Strategic Defense Command (SDC) in the Model Boeing Transonic Wind Tunnel facility for different cavity sizes and aft ramp shapes.

Above most of the atmosphere, this sensor could look up against the cold space background and track RVs as they flew through mid-course. There was some uncertainty regarding the infrared background that an airborne sensor such as AOA would see. Sunlight scattered from either natural or (particularly) man-made "noctilucent clouds" might obscure the real RV targets. These clouds form at altitudes from 60 to 100 kilometers (km). During a battle, the particles ablating from debris reentering the atmosphere would form nucleation centers. Long-lived ice crystals would grow at these centers, possibly creating a noisy infrared background that would obscure the real targets arriving later. Intentional seeding of these clouds is also a possibility.

Airborne Optical Adjunct and the ABM Treaty

A series of experiments by the United States called into question they had exceeded the permissible research allowed by Article V for other than fixed land-based ABM systems. The issue was made more difficult by the language in Article V that pertains to "ABM systems or components." Consequently, much work proceeded on ABM-related projects that is characterized as "adjuncts," "subcomponents," or "subsystems," which are not limited by the Treaty, as opposed to "components," which are limited. As might be expected, "components" were not defined by the Treaty.

The Reagan Administration interpreted "components" as including only devices capable of standing on their own as substitutes for the ABM missiles, launchers, or radars mentioned in Article 11. "Adjuncts," or other terms implying the same, on the other hand, were held to be parts of the independent component. The problem's practical application was illustrated best by the controversy over the Reagan Administration's characterization of elements of ABM sensors, currently being "demonstrated," as adjuncts to larger ABM components. Demonstrations of the Airborne Optical System (AOS) in particular received much attention. Under the Reagan Administration's interpretation, the two functions when tested separately were not forbidden "tests" of "components," but instead were permitted "demonstrations" of "adjuncts" as neither element acting alone has ABM utility.

As planned, the AOA could not launch and guide interceptors to destroy threatening objects. Therefore, it must be able to transfer information that it obtains to the Terminal Imaging Radar. Resolution would be relatively coarse: a follow-up system based on this technology might eventually be able to direct ground-based radars, which in turn would hand target track data over to high speed hit-to-kill projectiles.