The Infrared Background Signature Survey

The IBSS Shuttle Pallet Satellite being deployed from the bay of the orbiter Discovery by the remote grappler. (NASA)

The Infrared Background Signature Survey: A NASA Shuttle Experiment

Frederick Simmons, Lindsay Tilney, and Thomas Hayhurst

The development of remote sensing systems requires an accurate understanding of the phenomena to be observed. Aerospace research helped characterize space phenomena of interest to missile defense planners.

SDIO, the Strategic Defense Initiative Organization (precursor of the Missile Defense Agency), conducted numerous experiments in the late 1980s to study phenomena related to the passage of intercontinental ballistic missiles through the upper atmosphere. Understanding such phenomena was considered a critical step in building systems to detect and track such missiles.

In an effort to involve NATO allies in its research, SDIO invited the West German government to join in an experiment involving deployment of the Shuttle Pallet Satellite (SPAS-II), developed and flown by West Germany in a prior research mission. In its primary mode, deployed from the cargo bay, it would transport sensors for remote observations and be retrieved once the data were collected.

The Germans proposed installing an infrared scanner and spectrometer on the satellite to measure the radiance profiles of the Earth limb, the bright background against which a missile defense system would have to discriminate midcourse targets. Hence, the experiment was termed the Infrared Background Signature Survey, or simply IBSS.

Visible image of the orbital maneuvering system plume recorded by the video camera aboard the Shuttle Pallet Satellite. (NASA)

A panel of scientists from several organizations (including Aerospace) was assembled to review the plan. Their immediate reaction was that the German instrument was ill suited for the job. Moreover, they pointed out that SDIO was already funding development of an instrument at the Air Force Geophysics Laboratory for that very purpose (a cryogenic infrared radiometer, which in fact flew on the same shuttle mission as IBSS). Accordingly, the group began looking for other experiments that could effectively use the German instrument.

Aerospace recommended two experiments that were accepted by SDIO. The first involved using the sensors aboard SPAS-II to observe the plumes from the shuttle's orbital maneuvering system engines (OMS) and the primary reaction control system thrusters (PRCS). These engines would approximate the thrusters that powered the various postboost vehicles of concern to SDIO.

The second experiment involved the deployment of small canisters that would release liquid rocket propellants to simulate the rupture of a missile tank by a boost-phase interceptor. Characterization of such propellant releases could provide a basis for a missile defense system's "kill assessment."

The plume observations were planned and coordinated by the Institute for Defense Analyses, with subsequent analyses performed at Aerospace and other organizations. The responsibility for the propellant releases was given to Aerospace.

Preparations and Deployment

Aerospace played a large role in the program as a whole by overseeing the integration of the IBSS payload into the orbiter and managing its orbital operations. The complex operations of this mission were planned and designed at the Aerospace Conceptual Flight Planning Center using the NASA Flight Design System software. Aerospace also helped develop crew procedures and flight-planning requirements to ensure that the astronauts carried out the experiments properly.

The IBSS experiments were conducted from shuttle flight STS-39, launched April 28, 1991, into a circular orbit of 260-kilometer altitude and 57-degree inclination. Aerospace engineers served as technical advisors for the director and manager for cargo operations. The various onboard activities required two full shifts of astronauts (Guion Bluford, Jr., now an Aerospace trustee, was a mission specialist for the accompanying payload on this flight).

After the shuttle was launched, several deviations from the nominal timeline posed great challenges—most notably, dealing with the effects of a change in launch date, a delayed SPAS-II deployment, increased allocation of data collected while the satellite was attached to the remote manipulator system, and a delay in the timing of the high-priority observations. Aerospace knowledge of the payloads and orbiter capabilities facilitated the successful implementation of contingency plans, mission timeline changes, and operational workarounds.

Aerospace assisted the team that continuously updated 12-hour timelines for the upcoming shifts of personnel on the ground and in the orbiter. Aerospace provided continuous support at NASA Johnson Space Center to ensure that the data collection requirements were adequately met. Aerospace personnel were on 12-hour shifts at consoles, supporting tests and helping in the experiment timeline replanning efforts.

Orbital Burns

The postboost-vehicle simulation burns of the OMS and PRCS engines were conducted with the thrust vectors in a direction normal to the orbiter flight path. This orientation represented cross-range burns of a postboost vehicle deploying its payload of reentry vehicles. Each burn for observations was followed by a "null" burn to maintain orbital position. The orbiter remained behind SPAS-II to prevent exhaust products or natural particles in the upper atmosphere from contaminating the sensors. A total of 22 burns were made in the course of these observations.

Orientation of the orbiter and the Shuttle Pallet Satellite during the observation of the orbital maneuvering system burns. The plume was scanned with the 22-element detector array oriented as indicated. A total of 22 burns of the OMS and PRCS thrusters were made in these experiments.

The design of the experiment was based on the observation that rocket engines discharging into a rarified atmosphere while moving at high velocity create a plume consisting of two components. The "near-field" or "intrinsic-core" component, localized near the nozzle exit (within a few meters or tens of meters), is independent of vehicle altitude and velocity and represents a minimum observable infrared intensity. Further from the nozzle, the plume interacts with the atmosphere to form the "far field" or "enhancement" of the total intensity. The latter component is highly dependent on the vehicle's altitude as well as its attitude and velocity with respect to the atmosphere. For a missile in a rising trajectory, the apparent enhancement peaks at about 100 kilometers in altitude and 3 kilometers/second in velocity and then diminishes rapidly until only the intrinsic core can be observed. For that reason, the intrinsic core is sometimes termed the "vacuum-limit." The near-field observations of the OMS and PRCS plumes were made at a range of about 1 kilometer; those of the far fields required separation of 10 kilometers.

	Medium-wave infrared spectra of the near field of the orbital maneuvering system plume. The bands of carbon dioxide and carbon monoxide are both evident in the plume close to the nozzle exit. The narrowness of the bands is consequent to the very low temperatures resulting from the rapid expansion of the exhaust gases.
	Medium-wave infrared spectra of the far field of the orbital maneuvering system plume. The quasi-periodic structure is due to "band heads" characteristic of changes in the vibrational energy of carbon monoxide consequent to the very energetic interaction with atomic oxygen entering the plume at the orbital velocity of more than 7 kilometers/second.
	Radiances in the orbital maneuvering system plume in the 4–5-micron region. The individual detectors in the 20-element array of the scanner show the variations in the near field, then coalesce into a single value in the far field.

The principal goals of the observations were to measure the spatial distributions of radiances in two spectral bands selected as candidates for postboost-vehicle detection in a defense system and to measure the spectra for both components of both plumes. Of particular significance were the observations in the 4–5-micron region for the emission from the characteristic bands of carbon dioxide and carbon monoxide, observations that can only be made in space because of the blanketing effect of absorption by carbon dioxide in the atmosphere.

These plume observations led to two significant discoveries. First, the constancy of the far-field radiances in the expanding plumes—up to 1600 meters from the nozzle exit in the OMS plume—implied that the rate-controlling process was the influx of highly reactive atomic oxygen, the principal species in the upper atmosphere. Second, the spectra of the far field indicated the principal radiating species in the plume to be carbon monoxide rather than carbon dioxide, the latter being dominant in the near field and previously thought to be in the far field as well. Accordingly, these results provided a much better basis for estimating the infrared emission from postboost vehicles observable to space-based sensors; such studies have recently been performed at Aerospace in support of the development of an advanced surveillance system intended to replace that of the Defense Support Program.

Propellant Releases

A chemical release observation canister being deployed via the launch tube in the orbiter bay. (NASA)

The propellant-release experiments were quite different in nature. A liquid propellant vented into a near vacuum will undergo a flash evaporation. Part of the mass will expand rapidly as a cloud of vapor, which will interact with atomic oxygen in the upper atmosphere and produce chemiluminescent emission in the infrared; the rest will form a cloud of frozen particles embedded within the vapor cloud, which will strongly scatter sunlight. The propellant release observations were designed to evaluate the infrared properties of such clouds, which could impact the functioning of a missile-detection system. The propellants were transported in three canisters or "subsatellites" deployed in sequence from launchers in the orbiter bay; two contained about 25 kilograms of the fuels monomethylhydrazine and unsymmetrical dimethylhydrazine, and one contained about 6 kilograms of the oxidizer nitrogen tetroxide.

Prior to each chemical release, the orbiter would maneuver for a separation of about 100 kilometers for the observations. The subsatellites carried an optical beacon and a radar reflector to facilitate acquisition of the canister by the astronauts using the video camera aboard SPAS-II, thus ensuring the precise pointing of the other sensors. These subsatellites were designed and built by a defense contractor under the close supervision of Aerospace, with particular attention to NASA safety requirements.

These experiments required considerable planning for the orbital arrangements. In particular, the liquid propellants had to be released in sunlight and in view of the ground station, from which commands were sent to turn the optical beacon on and to open the propellant valves.

Video image of the release of monomethylhydrazine. The cloud had grown to about 4 kilometers in diameter; the bright spot in the center is a sun glint from the subsatellite body. The cloud was simultaneously scanned across the center with the radiometer. This provided the infrared radiance profiles in selected spectral bands. (NASA)

All three chemical release operations were successful. In each case, the astronaut in control was able to acquire the optical beacon with the video camera to optimize the pointing of the infrared sensors. The subsatellites discharged their contents upon command from the Western Test Range, which had been providing radar tracking and relaying position information to the orbiter. The video pictures of the releases and growths of the resultant clouds were relayed to the ground; the infrared data were recorded aboard SPAS-II and subsequently transmitted to Aerospace for analysis. The results of these experiments contributed immensely to the knowledge of such phenomena and its impact on missile surveillance. The success of the actual data collections were in great measure due to the early design of the experiment timelines. Many Aerospace people contributed to that planning.

Other Experiments

There were other important and productive experiments, conducted mainly by the Air Force Geophysics Laboratory, which provided valuable data in viewing terrestrial scenes, the Earth limb, and the orbiter environment, and observing effects resultant to the release of various gases from containers in the cargo bay. Particularly important were the observations of the "shuttle glow" seen by astronauts on previous flights (a phenomenon that has been attributed in part to the recombination of atomic oxygen in the atmosphere on the surfaces of the orbiter). It was also observed that the glow was considerably enhanced during and immediately following OMS and PRCS burns. A series of measurements of Earth backgrounds in the midwave infrared bands provided information much needed in the design of advanced space-based sensors for improved missile surveillance. Finally, some of the most spectacular images of auroras viewed from space were obtained during this mission.

Acknowledgements

Individuals from a number of organizations played key roles in the planning and execution of the IBSS experiments. Among the people at Aerospace who made significant contributions are Ron Thompson, Larry Sharp, Kitty Sedam, Jo-Lien Yang, Jim Covington, and Linda Woodward.