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Concept Proposal for Strategic Defense Initiative Space Defense System (SDS)

by Ron Kurtus (updated 31 May 2023)

A concept for a Space Defense System (SDS) to be implemented as part of the Strategic Defense Initiative was proposed by the U.S. Air Force Space Systems Division program offices in February 1988.

SDS was visualized to consist of Surveillance and Tracking Systems, Interception Satellites and Battle Management/Communications, Command, and Control (BM/C3). This document provides an unclassified summary of the proposal.

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Surveillance and tracking systems

The very first part of SDS would be to place a series of space satellite-based sensors to watch for enemy missile launches. Missiles may be launched from ground bases and/or submarines. There are specific ground areas for which surveillance would be emphasized—namely in Russia and other parts of the Soviet Union.

The Surveillance and tracking systems consist of the Boost Surveillance and Tracking System (BSTS), Ground-Based Surveillance and Tracking System (GBTS), and Space-Based Surveillance and Tracking System (SSTS).

Boost Surveillance and Tracking System (BSTS)

BSTS would continuously monitor the earth's surface for signs of a missile launch, relying mainly on infrared sensors to detect the intense burst of heat from a rocket engine.

The brief boost phase of a missile's flight offers a defense system's best opportunity, because the missile still contains all its warheads, and its flaring rockets are easy to see.

Ground-Based Surveillance and Tracking System (GBTS)

After the boost-phase sensors (BSTS) detect a missile launch, the battle-management (BM/C3) computers would order a ground-based second layer of surveillance gear into action. A satellite laden with IR sensors, GBTS, would be launched upon attack to track incoming warheads, as well as decoys, during post-boost phase of missile flight.

By the time the satellite reaches its vantage point high above the earth, the missiles would have already released multiple warheads. Although the warheads would be destined for different targets, they would initially float through space in a cluster.

A critical task of GBTS will be to differentiate these nuclear bombs from the large number of harmless decoys that will accompany them during this mi-course phase. The decoys would probably be much lighter than the warheads, for example, and would thus move differently.

Space-Based Surveillance and Tracking System (SSTS)

Sharing the mid-course watch with GBTS would be a series of satellites stationed in space to monitor the swarm of warheads unleashed from a missile. SSTS would track missiles and warheads during post-boost and mid-course phases of their flight.

Another important function would be "kill assessment" - ascertaining whether a warhead had been disabled after it has been hit by a laser or high-speed projectile. IR sensors would look for evidence of a "kill", such as a sudden change in warhead's trajectory, showing that its guidance electronics have been knocked out. This is a crucial piece of information, because it keeps the weapon from wasting its time firing at targets that have already been destroyed.

Interception satellites

Information from the surveillance and tracking systems would be sent to the Interception satellites, which would seek out and kill the enemy missiles. These satellites consist of the Space Based Interceptor (SBI), Exo-Atmospheric Re-Entry Intercept System (ERIS), and High Endo-Atmospheric Defense Interceptor (HEDI).

Space Based Interceptor (SBI)

A fleet of satellites, SBI, would serve as "garages" for rocket-propelled projectiles.

Equipped with IR sensors and a computerized guidance system, the projectiles would home in on the heat of a missile during its launch or on a warhead during its drift through space. The projectiles would contain no explosives, but they would inflict damage simply by slamming into the target at extremely high speed. These "smart rocks" are shot from low-flying satellites to missiles shortly after launch, during their boost phase.

Exo-Atmospheric Re-Entry Intercept System (ERIS)

Heat seeking projectiles ("smart rocks") are shot from ground at attacking warhead during its midcourse phase, before it enters the atmosphere.

Like SBI, ERIS would knock off warheads by whacking them with non-exploding projectiles. But ERIS would launch rockets by aiming at warheads late in the mid-course phase of their trajectory, before they re-enter the atmosphere.

For guidance, ERIS rockets would rely on tracking data obtained GBTS and SSTS.

High Endo-Atmospheric Defense Interceptor (HEDI)

HEDI is designed to intercept low-flying, submarine-launched missiles, as well as warheads that have already re-entered Earth's atmosphere.

HEDI homes in on its target's infrared signature. The weapon's nose is designed to bathe its IR sensor in nitrogen coolant, thus protecting it from heat caused by the interceptor's speed of up to 10,000 mph.

Battle Management/Communications, Command, and Control (BM/C3)

Monitoring and controlling all the pieces of an SDI system would require large computers and extremely complex software.

The system would collect information from hundreds of sensors, decide what to do (for example, to fire a laser beam or a barrage of smart rocks), relay the signal to the designated weapons, judge from new sensor readings whether the target has been destroyed, and then revise its tactics as needed.

Much work is going into developing electronic circuitry made from gallium arsenide, which is less susceptible to nuclear radiation than silicon, the traditional circuit material. Fiber-optic links might provide resistance to enemy jamming.


The concept for a Space Defense System (SDS) to be implemented as part of the Strategic Defense Initiative was proposed by the U.S. Air Force in February 1988. SDS was to consist of Surveillance and Tracking Systems, Interception Satellites and Battle Management/Communications, Command, and Control (BM/C3). Research and development was implemented, but most of the systems were never put into place.

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Resources and references

Ron Kurtus' Credentials


Federation of American Scientists articles on Strategic Defense Initiative

History of the Missile Defense Organization - U.S. Department of Defense

SDI History Resources


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Secret Weapons of the Cold War: From the H-Bomb to SDI by William Yenne; Berkley (2005) $7.99

US Strategic And Defensive Missile Systems 1950-2004 by Mark A. Berhow, Chris Taylor; Osprey Books (2005) $16.95

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