 Launch of the Program 461 satellite by an Atlas Agena from Vandenberg Air Force Base on August 19, 1966, into a 3700-kilometer circular orbit. (USAF) |
IR Eyes High in the Sky: The Defense Support Program
Fred Simmons and Jim Creswell
Early in the morning on a day in August 1972, all satellites in the constellation that would alert the United States of a missile attack suddenly lost their warning capability. The detectors and circuitry, according to the status data, had been hit by a strong source of ionizing radiation. This appeared to be an ominous event to operators at the ground stations, where the initial interpretation was that the Russians had detonated a nuclear warhead in space, possibly as a precursor to a ballistic missile attack.
Prompt analysis of the sensor outputs by an Aerospace expert in nuclear and space physics on duty at one of the sites provided the actual cause: The satellites had been hit with a massive proton flux from an extraordinarily intense solar flare. An unwise reaction by the government was averted. The Aerospace Corporation subsequently worked with the U.S. Air Force and the system contractor to provide fixes to assure uninterrupted operation through such events. Aerospace has often provided invaluable assistance to the Air Force, playing a key role in the development, operation, and success of this national asset—the Defense Support Program (DSP).
Deployed 40,000 kilometers above Earth in the equatorial plane, a constellation of satellites equipped with infrared sensors ("IR eyes") looks for ballistic missiles aimed at the United States or its allies. The period of their orbits is 24 hours, so the satellites remain at constant longitudes, that is, in geostationary orbits, guarding against an attack on the United States or its allies from anywhere in the world. The system has been in operation continuously since it went on line in 1971. Fortunately, the United States has never experienced a missile attack; of course, the extent to which DSP has served as a deterrent to such an attack cannot be known.
In addition to performing their primary mission, DSP sensors have produced a wealth of information on a variety of sources, military and otherwise, that has served many other purposes. Certain civilian uses of these surveillance satellites are described in the premier issue (January 2000) of Crosslink. Those particular applications of course are peripheral to the principal mission of DSP.
Early Development
The U.S. national early warning program had its beginnings in the early 1960s, when it became evident that the United States was vulnerable to attack by the intercontinental ballistic missiles (ICBM) then under development in the Soviet Union. By the mid-sixties, ICBMs appeared in test flights, and the United States adopted the MAD (mutual assured destruction) strategy as its national defense posture.
 Sources of confusion for a space-based surveillance system. From low altitudes (0 to 10 kilometers), cultural sources include industrial sites (such as heat exchangers, flare gas burners, smelters, and coke ovens), petroleum and pipeline fires, explosions and dumps, and slash-and-burn regions; geophysical sources include forest fires, volcanoes, and solar scatter from cloud edges, coastlines, water surfaces, high deserts, and snow-covered heights. From high altitudes (above 30 kilometers), cultural sources include target-related clouds and tracks, objects in low Earth orbits, and vehicle reentries; natural sources include meteors and bolides, volcanic clouds, air glow, and aurora, as well as the sun, moon, planets, and stars. |
Early warning became critical to the survival of U.S. retaliatory forces, and launch detection by space-based sensors was essential. Aerospace was called upon at the start to perform trade studies and prepare technical specifications for an operational system. It provided the general system engineering and technical direction for the development of the program.
 The blue and green curves show the spectra of typical terrain and a solar-illuminated cloud at 10 kilometers altitude; the red curve shows a hypothetical missile at 20 kilometers altitude. |
The ballistic missile defense studies as a whole had been initiated earlier. As part of its Project Defender, the Advanced Research Projects Agency (ARPA) of the Department of Defense in the late 1950s explored concepts for early warning based on the detection of the infrared emission from rocket exhaust plumes by sensors stationed in space. The ARPA program consisted mainly of system studies and various measurement programs to characterize the infrared properties of ballistic missiles and the backgrounds against which they would have to be observed.
A space experiment designated as Program 461, the final element of the MIDAS program, provided the proof of principle to support the development of a system with far greater capabilities. Although the exhaust plume from a rocket emits a great deal of infrared radiation, so do many other sources that might appear in the background. To discriminate the rocket from the background sources, the sensors must operate in specific regions of the spectrum. In their characteristic molecular bands between two and three microns, water vapor and carbon dioxide in the atmosphere greatly suppress emissions from fires and other hot terrestrial sources and solar reflections from the ground and low clouds.
 | The RTS-1 payload for Program 461 was built by Lockheed Missiles and Space Company (now Lockheed Martin) for the Air Force Space Systems Division. The sensor, built by the Aerojet-General Corporation, included an 8-inch aperture concentric telescope and a focal plane containing a linear array of 442 lead-sulfide detectors. A spectral filter defined a narrow bandpass within the band of water-vapor absorption in the atmosphere. The sensor was mounted on a spin table to rotate at 6 rpm, providing a scan of Earth every 10 seconds. A pair of star sensors provided information for attitude determination. Three such payloads were launched in 1966; two were successful. Each sensor collected data for about a year. (USAF) |
Concurrent with much of that work, the Air Force, aided by Aerospace, began development of its own Missile Defense Alarm System (MIDAS). That system, had it been implemented, would have employed a constellation of many satellites in low Earth orbits.
Because water vapor and carbon dioxide are the principal products of rocket-propellant combustion, the hot exhaust plume from the missile appears as a very bright source moving against the background in those same spectral bands. Consequently, as a missile rises through the atmosphere and absorption diminishes, the apparent intensity of the plume rapidly increases. Accordingly, the sensors are designed with detectors filtered to accept radiation only in those molecular bands. Furthermore, this spectral region favors the use of lead-sulfide detectors, which offer the advantage of high sensitivity with passive cooling.
The Program 461 satellite was designed to operate in a spinning mode to scan Earth below from a low circular polar orbit (~3226 kilometers) with a period of 10 seconds. |
Sensor Design
In the design of a sensor for missile detection, a basic engineering decision involves a choice between two approaches. A sensor can be designed to have relatively few detectors that scan the field of view, or it can be designed to have a very large number of detectors staring at the scene to detect targets by their motion through the field of view. The technology of the 1960s enabled only the former approach, which was incorporated in Program 461 and subsequently, DSP. In either case, a basic design tradeoff is the optimization of the spectral filter: The wider the bandpass, the more target intensity is collected, but also the greater the amount of highly variable background. The early target and background measurement programs provided sufficient information, mostly with airborne instruments, for the design of the Program 461 sensors; the results of that program provided the basis for further optimization of the sensors in the DSP system that followed.
Program 461 satellites, built by Lockheed Missiles and Space Company, observed many launches of missiles and space launch vehicles from Cape Canaveral and Vandenberg Air Force Base, as well as from different sites in the Soviet Union. After processing the signals received at the ground stations, the target intensities were reported as radiant intensities in the system bandpass as functions of time for the particular viewing aspect and other parameters of the observation. Extraction of such signatures from the raw data was a formidable task in view of the relatively coarse pointing system of these satellites by today's standards; Aerospace provided much of the analysis for that purpose.
Such data were obtained from observations of three SS-9 missiles in test flights from the Tyuratam facility near the Aral Sea to the Kamchatka peninsula in the Sea of Japan. At the time, this liquid-propellant ICBM was the largest missile in the USSR inventory and the principal threat to the United States. Among other observations in the Eastern hemisphere, a particular sighting of significance was that of a single-stage missile launched in a test flight from Kapustin Yar on February 3, 1967. The relatively short burn of that missile afforded observations in only three scans between cloud break and thrust termination. That missile was later identified as an SSN-6 medium-range submarine-launched ballistic missile (SLBM), the smallest missile of a direct threat to the continental United States.
 | Program 461 observation. This plot, illustrative of the product, shows the characteristic intensity profile of a two-stage missile. The initial increase in intensity occurs as the missile rises and the atmospheric absorption decreases. After the maximum, the "afterburning" of hydrogen and carbon monoxide in the exhaust diminishes as the vehicle and exhaust velocities become comparable and the density of oxygen in the air decreases. After a minimum, the reported intensity again increases as the missile velocity exceeds the exhaust velocity, until the image of the plume exceeds the detector field of view. |
During 1966 and 1967, Program 461 collected data on many of the ballistic missiles and space launch vehicles in the Soviet and U.S. arsenals, totaling dozens of test launches. In the course of those observations, Program 461 sensors produced a substantial database on the clutter created by the scanning of the Earth-cloud backgrounds, information also needed for the optimization of the DSP sensors, the development of which was commencing at that time. Thus were provided the proof of principle for space-based surveillance and a valuable database for the design of the sensors in the national early warning system to follow.
 DSP Flight 1 satellite prior to shipment to Cape Canaveral (1970). Note the offset of the telescope from the vehicle axis. The two small telescopes pointing normal to the axis of rotation are the star sensors that provide the data necessary to determine the precise pointing of the primary telescope. The main body of the satellite contains a reaction wheel to control the spinning rate, propulsion units for station keeping, and electronic components for data processing and transmittal. Solar cells covering the body and four paddles provide power. The radiator, used for passive cooling of the detectors, is located near the base of the telescope. (USAF) |
DSP Sensors
In the late-1960s, the design of the sensors for the DSP system to some extent followed the concept for MIDAS. A linear array of passively cooled infrared detectors, with spectral filters providing a bandpass in the center of an atmospheric absorption band, was positioned in the focal plane of a telescope mounted in a satellite rotating at six revolutions per minute. The idea of a constellation of many satellites in a low Earth orbit was abandoned in favor of a few satellites in geostationary orbits positioned at longitudes affording views of the launch sites of concern. Accordingly, a sensor with a much more powerful telescope and many more detectors was designed by Aerojet for installation in a satellite built by TRW Inc. The DSP development effort was originally known as Program 266, then 949, and later 647. The system became DSP when it achieved full functional capability.
DSP sensors incorporated many features representative of an advanced technology for that time. The design included a larger array of detectors (2000 initially, 6000 eventually), spectral filters, electrical circuitry for optimizing discrimination of targets from a cluttered background, and improvements in data processing onboard and at the ground station. A key feature of the DSP design (insisted upon by the Aerospace advisors to the Air Force), was the absence of moving elements in the sensor optics, elements that enormously simplified the bore sighting and precise attitude determination. The optical axis of the telescope was offset from the axis of rotation of the satellite; the field of view of the detector array extended from near the nadir to slightly above the horizon. Thus, the rotation of the satellite provided a scan of most of a hemisphere every 10 seconds. Two ground stations were initially built, one located near Denver, Colorado, and the other deep in the outback of Australia.
 DSP mode of scanning. The detector arrays cover the range from near nadir to slightly above the horizon. The downlinked data include the responding detector identification and the universal time, which specify the target position in satellite coordinates of azimuth and elevation. |
The initial satellite, Flight 1, was launched in November 1970 by a Titan IIIC launch vehicle with a Transtage third stage. Unfortunately, it didn't quite reach a geosynchronous orbit, and the subpoint circled Earth every five days. The orbit was high enough, however, to allow checkout of the ground data-processing sites and the mission software and to provide nominal Earth-pointing and sensor operation. Aerospace advisors at the ground sites and the Air Force Satellite Control Facility provided the leadership for debugging and modifying the software with "field-fixes" and configuring the satellite for collecting data. The anomalous orbit was fortuitous because it provided the opportunity to observe launches from both the United States and the Soviet Union. The functioning of the system was proved, and data from a considerable number of observations were collected.
In May 1971, Flight 2 was successfully launched, and after on-orbit testing (accomplished in a very short time), it was turned over to the Air Force Systems Command. The satellite was stationed over the Indian Ocean to view the major launch sites of the Eurasian continent. For Flight 2 to effectively perform the warning mission, it had to recognize and report ICBM launches and reject all infrared phenomena from other sources. This required templates (intensities vs. time) for threat missiles to compare with the data as it was transmitted to the ground. Analysis to that end by other organizations would have taken months.
 Launch of DSP Flight 1 from Cape Canaveral in November 1970. (USAF) |
Aerospace and the sensor contractor, working closely with personnel at the site, produced templates in a few weeks and continually upgraded them as the data from missile sightings were accumulated. This analysis also facilitated some necessary modifications of the system software by Aerospace and contractor personnel assigned to the ground site. Transition of Flight 2 to a fully operational status, consequently, was greatly accelerated.
The sensor met the requirements and rapidly created a database of all the ballistic missiles and space launches of the time. In later years, additional satellites were deployed to maintain a constellation with satellites in the East to report ICBM launches and in the West to cover the ocean areas from which SLBMs could be launched. Later, a larger constellation of satellites, stationed over a range of longitudes, provided multisensor viewing of areas of particular concern.
The sensors have been upgraded and improved in several respects throughout the life of the program. One significant improvement was the addition to Flight 6 and subsequent sensors of an array of sensors to view targets above the horizon. For such fields of view, it was not necessary to restrict the spectral bandpass to suppress terrestrial sources. That addition and other changes in the detectors and electrical circuitry provided a substantial improvement in sensitivity, which was needed to detect and track upper stages. The average orbit lifetime of a sensor has been five years, with considerable variation. To date, 19 satellites have been built. All but one were launched with Titan vehicles, those since 1989 with Inertial Upper Stages. The exception was Flight 16, deployed from the shuttle into a low Earth orbit for subsequent boost by an Inertial Upper Stage motor to a geostationary altitude.
 Ground station in Australia. (USAF) |
The research test series that preceded the development of DSP provided no effective means for establishing the precise pointing of the sensors. Aerospace realized that a foolproof backup needed to be added to DSP for determining the precise attitude of the infrared sensors. (In retrospect, the use of an Earth sensor for pointing and star sensors for instantaneous, precise attitude determination proved to be effective.) Aerospace concluded that a ground-based laser operating in the infrared band of the sensor—analogous to a beacon or transponder in the field of view of a radar—could serve that purpose. The idea was vigorously pursued, and a DSP satellite was successfully illuminated within a year of the first satellite launch. Aerospace developed and provided hydrogen-fluoride chemical lasers for that use.
 Deployment of DSP Flight 16 from the shuttle in November 1991. (NASA) |
Although the lasers were rarely needed for precise attitude determination of DSP satellites, they were used as a beacon for functional checkout of the overall system. Also, they were used for determining sensor resolution and bore sighting, new software validation and evaluation of stray light properties, and assessing system sensitivity to uncooperative laser illumination and developing means for its mitigation. Incidentally, after two star sensors on Flight 8 failed, lasers were used in their originally intended applications—beacons in the sensor field of view as the primary means of determining the precise attitude.
Target data downlinked from the satellites to the ground include the intensities of the detected source above a prescribed threshold, the identification of the responding detectors, and the universal time, the latter two providing the instantaneous target position in satellite coordinates: elevation and azimuth. By appropriate processing, including a comparison with stored templates of intensity versus time based on prior sightings, the target can be identified and its heading established. The principal product of the DSP system in near real time is a warning message from the ground station relaying that information to the national command authority. DSP satellites have fulfilled their primary mission by reporting thousands of missile launches over three decades.
Data for Off-Line Analysis
The data from those DSP sightings have also been provided to various centers for off-line analysis, when appropriate, and for archiving in a comprehensive database maintained by Aerospace. The database contains the reports on all ballistic missile sightings, as well as on a vast number of other events, military in nature and otherwise (for example, space launch vehicles). The data listings include not only the maximum intensities in a given scan, but also the lower intensities of the target distributed in the vicinity. For such analysis, data from the satellites can be displayed in a variety of forms.
 |  | Apparent motion of target in the satellite field of view over the Atlantic Ocean (left), with time increasing upward to the left, and over the western Pacific Ocean (right), where the launch site was beyond the horizon and the target appeared after rising above the Earth limb. Green indicates target positions of maximum intensity per scan in the main array; blue indicates positions reported by the more sensitive cells in the above-the-horizon array. Note the increasing spread as the vehicle accelerates. |
 |  |  |
Time variation of maximum single-detector intensities over a range of two orders of magnitude. (GMT: Greenwich mean time). | (Left) Spatial distribution of signals reported in a single scan. The signals from the extended plume merge into those of a persistent trail. (Right) Distributions intensities reported in a single scan with altitude. The solid symbol on both graphs indicates the position of the vehicle. | |
DSP Support of Theater Operations
DSP took on a more direct and proactive role in its missile-warning mission during Desert Storm operations in 1991. In that conflict, Iraq launched a large number of Scud missiles at targets located in Saudi Arabia and Israel. Specifically, the DSP satellites stationed in the Eastern Hemisphere detected and tracked the missiles during the boost period and reported their headings to the appropriate Patriot missile batteries fielded by the U.S. Army in those areas. The information was provided by telephone communication links, some of which were staffed by Aerospace personnel, allowing the Patriots to intercept the incoming Scuds. Although the effectiveness of the Patriots in destroying the warheads can be questioned, the interceptions did take place, establishing the feasibility of defense against theater missiles.
 The ALERT architecture. (CONUS: Continental United States) |
Largely because of their success in Desert Storm, DSP satellites currently play the key role in the Air Force's Attack and Early Reporting to Theater (ALERT) system, an operational function of the 11th Space Warning Squadron of the 21st Space Wing. Aerospace provided invaluable assistance to the Air Force in the procurement of that system by generating specifications and providing support with the contractor selection process. For the ALERT system, data from the entire DSP constellation and other sources are integrated and processed at one facility located at Schriever Air Force Base in Colorado. The detection reports, considerably improved in accuracy, are transmitted rapidly to commanders in the theaters through space-based communication links. DSP satellites provide worldwide coverage so that the ALERT system can monitor all major regional conflicts and areas of concern simultaneously, and provide threat-missile descriptors, such as launch point, heading, position, velocity, and predicted impact location.
Observations of Other Sources
The database contains hundreds of sightings of other sources that appear in the fields of view of the sensors; many are assigned descriptors that characterize the nature and time variation of their movement across the monitor screen. In no instance has analysis failed to identify the sources of those sightings. (Contrary to some assertions in the popular press, there have been no sightings of alien spacecraft.) Among the objects of current interest are the occasional meteors of significant size. Earth is constantly bombarded by small meteors, most the size of a grain of sand. Their numbers, and intensities due to atmospheric drag, appear to vary inversely with mass. Large meteors of potentially catastrophic size are rare. Nevertheless, during the last 30 years, DSP has observed some very sizable meteors. For example, in 1972 an exceptionally large meteor was observed in a grazing trajectory that came within an astronomical whisker of hitting Salt Lake City. Analysis by Aerospace led to the conclusion that the object was of sufficient mass that a slightly deeper penetration of the atmosphere would have resulted in an impact equal to the explosive force of the atomic bombs that destroyed Hiroshima and Nagasaki in World War II.
 Path of a meteor moving north at 18 kilometers per second over several western states, at closest approach a mere 94 kilometers above Earth. |
In addition to such moving objects, very intense stationary thermal sources on the ground can be seen in spite of the background suppression afforded by the spectral filters and electronic circuitry. Such sources include fires, gas flare-offs from oil refineries, volcanic eruptions, nuclear explosions, and solar scatter and reflections. The observation of such events is of course facilitated by very low humidity, which minimizes absorption in the path to space.
Some particularly mysterious sightings occurred in the early 1970s. Extremely bright stationary sources suddenly appeared in the area adjacent to the Caspian Sea, with apparent intensities of nearly a megawatt per steradian lasting for several minutes. Certain analysts elsewhere attached a sinister interpretation to those events. However, analysis at Aerospace solved the mystery simply by noting that these sources all appeared precisely along a pipeline to Moscow from the natural gas fields in the area. Clearly, the sources were burning gas, presumably flared off for maintenance of the pipeline, a conclusion later confirmed by other information. Gas flares from oil refineries are also routinely observed, particularly in dry regions such as Southern California and the Near East. Likewise, volcanoes are frequently observed at various locations throughout the world, sometimes by the emission from the lava flow, but more often by reflected sunlight from the ash plume rising high in the atmosphere.
Observations of many other infrared sources, both stationary and moving, have been routinely observed, reported, and processed at Aerospace for inclusion in the database, archived at the Ballistic Missile Defense Organization Advanced Missile Signature Center, Air Force Arnold Engineering and Development Center, Tullahoma, Tennessee, which is accessible to qualified users. Over the years, there have been innumerable reports analyzing the data to fulfill the needs of various Space and Missile Systems Center program offices and other government agencies. The results of most of those analyses are in the classified literature.
 Large meteor seen in broad daylight in August 1972. The fireball is the bright spot just to the right of the cloud, in the left center, followed by a faint trail to the right. Later, at a closer approach, the fiery source was much brighter, with a teardrop shape and a more visible trail. (Photo extracted from video conversion of a film recording submitted to the government some years ago by an amateur photographer.) |
The database contains an extensive collection of events, the observations of which were not even thought of when the system was originally conceived. Among other uses, that collection provides the fundamental basis for evaluating the effect of such events on the performance of the Space Based Infrared Systems (SBIRS) that will replace DSP in a few years. It is axiomatic in the field of infrared phenomenology that when more sensitive sensors are deployed in space, unexpected observations and other surprises are invariably produced. This is true of the DSP sensors, not only in their original configuration, but especially in the improved versions. The new system will feature many improvements in the sensors and advances in overall capability, and will be assigned additional missions. SBIRS will quite likely bring many surprises when it is deployed.
References
- W. Kellogg and S. Passman, "Infrared Techniques Applied to the Detection and Interception of Intercontinental Ballistic Missiles," Rand Corporation Report No. RM-1572 (October 1955).
- R. Zirkind, "Review of Project Tabstone," Journal of Missile Defense Research, Vol. 4, No. 1 (Summer 1966).
- R. G. Hall, "Missile Defense Alarm: the Genesis of Space-Based Infrared Early Warning," Space and Missile Systems Center Conference Honoring IR Pioneers (The Aerospace Corporation, June 3, 1999).
- E. E. Lapin, "Surveillance by Satellite," Journal of Defense Research, Vol. 8, No. 2 (Summer 1976).
- Col. J. Kidd, and 1st Lt. H. Caldwell, USAF, "Defense Support Program: Support to a Changing World," AIAA Space Programs and Technologies Conference (Huntsville, AL, March 24, 1992).
- Maj. J. Rosolanka, USAF, "Defense Support Program—A Pictorial Chronology 1970–1998," Space and Missile Systems Center Conference Honoring IR Pioneers (The Aerospace Corporation, 3 June 1999).
- Missile Defense Data Center, Bits-n-Bytes, Vol. 5, No. 2 (Spring 1997).
- R. D. Rawcliffe, et al., "Meteor of August 10, 1972," Nature, Vol. 247, 449 (1974).
- D. W. Pack, et al., "Civilian Uses of Surveillance Satellites," Crosslink, Vol. 1, No. 1 (January 2000).
- R. S. J. Sparks, et al., "The Giant Umbrella Cloud of the May 18th Explosive Eruption of Mount St. Helens," Journal of Volcanology and Geothermal Research, Vol. 28, 257–274 (1986).
- E. Tagliaferri, et al., "Detection of Meteoroid Impacts by Optical Sensors in Earth Orbit," Hazards Due to Comets and Asteroids, edited by T. Gehrels (University of Arizona Press, 1994).
- Col. D. L. Burkett II, USAF, "Space Based Infrared Systems SBIRS," Space and Missile Systems Center Conference Honoring IR Pioneers (The Aerospace Corporation, June 3, 1999).
- F. S. Simmons, Rocket Exhaust Plume Phenomenology (The Aerospace Press and American Institute of Aeronautics and Astronautics, El Segundo, CA, 2000).
