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Defense Support program

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The Defense Support Program (DSP) is a survivable and reliable satellite-borne system that uses infrared detectors to sense heat from missile plumes against the earth background, to detect and report in real-time missile launches, space launches and nuclear detonations.

DSP satellites have been the spaceborne segment of NORAD's Tactical Warning and Attack Assessment System since 1970. The satellites feed warning data, via communications links, to NORAD and US Space Command early warning centers within Cheyenne Mountain. These centers immediately forward data to various agencies and areas of operations around the world. Members of the Air Force Space Command 50th Space Wing's 1st Space Operations Squadron provide command and control support for the satellite.

Its effectiveness was proven during the Persian Gulf conflict, which took place from August 1990 through February 1991. During Desert Storm, DSP detected the launch of Iraqi Scud missiles and provided timely warning to civilian populations and coalition forces in Israel and Saudi Arabia.

Under contract to Air Force Materiel Command's Space and Missile Systems Center at Los Angeles AFB, CA, in support of the Air Force Program Executive Officer for Space, TRW in Redondo Beach, CA, builds the satellites and integrates the sensor payload built by Aerojet Electronics Systems Division of Azusa, CA.

Over the past 20 years, DSP has repeatedly proven its reliability and potential for growth. DSP satellites have exceeded their specified design life by some 30 percent through five upgrade programs. These upgrades have allowed DSP to provide accurate, reliable data in the face of changing requirements -- greater numbers, smaller targets, advanced countermeasures -- with no interruption in service. Planned evolutionary growth has improved satellite capability, survivability and life expectancy without major redesign.

On-station sensor reliability has provided uninterrupted service well past their design lifetime.

The Satellite Early Warning System (SEWS) consists of five Defense Support Program spacecraft.(1) Three of these provide frontline operational service, with two additional spacecraft available as backups should problems emerge with the primary satellites. The standard operating procedure is that primary reliance is placed on the three most recently launched satellites, with the two older satellites providing backup.(2) Because of the critical importance of this mission, a replacement satellite will normally be launched around the time that the oldest of the five spacecraft on-orbit nears the end of its operational life. This newly launched satellite will assume frontline duty, the eldest of the three frontline spacecraft will assume backup status, and the oldest satellite will be retired.

At the beginning of 1990 five DSP spacecraft were operational. DSP F-13, launched in 1982 respectively, and DSP F-12 launched in 1984, were on backup status. DSP F-6R, launched in 1984, DSP F-5R, launched in 1987, and DSP-I F-14, launched in 1989, were the primary operational spacecraft.(3) The November launch of DSP-I F-15 resulted in the removal of DSP-10 F-13 from operational status.

Developments in the DSP have enabled it to provide accurate, reliable data in the face of tougher requirements such as greater numbers of targets, smaller targets and advanced countermeasures. Through five upgrade programs, DSP satellites have exceeded their design lives by some 30 percent.


The original DSP weighed 2,100 pounds, had 400 watts of power, 2,000 detectors and a design life of three years.


In the 1970's, the satellite was upgraded to meet new mission requirements. As a result, the weight grew to 3,960 pounds, the power to 680 watts, the number of detectors increased by threefold to 6,000, and the design life was three years with a goal of five years.

Adding a reaction wheel removed unwanted orbit momentum from DSP vehicles. The wheel can be made to turn rapidly by command from ground stations. Its spinning motion then acts as a negative force on the satellite's movements. This "zero momentum" approach permits spacecraft orbit control with minimum fuel expenditure. As a result, reaction wheels have been added to other systems including Defense Meteorological Satellite Program, Navstar Global Positioning System and Defense Satellite Communication System satellites.


As their designation indicates, F-5R and F-6R are both refurbished spacecraft that were originally manufactured in the mid-1970's, but placed in storage because of the unexpectedly long operational life of the DSP series. In the early 1980's these two spacecraft were refurbished under the Sensor Evolutionary Development Program (SEDS), which greatly improved the sensitivity of their sensors.(4)

The major elements of the Sensor Evolutionary Development (SED) sensor are: IR Telescope Subsystem (IR); Star Sensor Subsystem (SS); Status Monitor Subsystem (SMS); Signal Electronics Subsystems (SES); Thermal Control Subsystem (TCS); and Advanced Radec I (ARI). Detection of IR sources is accomplished with the telescope and Photo-Electric Cell (PEC) array portions of the IR telescope Subsystem. The PEC detector array, mounted in the telescope center line to coincide with the image surface of the telescope optics, scans the Earth's surface through rotation of the satellite. As a detector passes across an IR source it will develop an electronic signal. the many signals are relayed to processing units where they are grouped and sent to the ground for mission usage.

DSP-I (Improved) dsp_sts_s.jpg - 45.6 K

Today's DSP satellite weighs 5,200 pounds, requires 1250 watts of power, and is approximately 33 feet long, 14 feet in diameter, Recent technological improvements in sensor design includes above-the-horizon capability for full hemispheric coverage and improved resolution. Increased on-board signal-processing capability improves clutter rejection enhancing reliability and survivability.

The DSP-I (Improved) satellites, of which spacecraft 14 through 25 were on order in early 1989 with options for 26 through 28 under consideration,(5) will incorporate the upgraded sensors of the SEDS satellites, as well as improved resistance to laser attack.(6) The DSP-I satellites will also carry a laser communications package that will enable the satellites to relay warning information to each other.(7) This will greatly reduce the vulnerability of this system to attacks on its ground stations, since all the satellites will be able to communicate with any of the system's ground stations. However, the June 1989 DSP-I (F-14) did not incorporate this laser communication systems, due to technical problems.(8) Instead, DSP F-14 carried an experimental sensor package for the Strategic Defense Initiative Organization to assess the utility of ultraviolet sensors for tracking missiles.

The sensor and the spacecraft, which together comprise the satellite, are placed in geosynchronous-equatorial orbit so that the telescope is pointed toward the earth and rotated at six revolutions per minute. To provide a scanning motion for the infrared (IR) sensor, the satellite is spun about its Earth-pointing axis. The axis of the satellite's rotation is normal to the earth's surface. A prime requirement of the spacecraft is to provide attitude control to maintain the pointing direction accurately. Satellite-spin momentum is reduced to a nominal value of zero by introducing an equal and opposite momentum achieved through operation of a Reaction Wheel. The resulting "zero momentum" satellite is attitude controlled by gas thrusters.

The basics functions of the spacecraft are to: provide a spin-controlled, stable, Earth pointing vehicle for the mission data sensing and processing equipment; furnish the on-board functions required to position, control, and maintain the satellite in its proper Earth orbit; furnish, condition, and control the electrical power for all satellite requirements; provide secure downlink capabilities to transmit mission data, State-of-Health (SOH), and other relevant information to the ground for final processing; and provide a secure uplink command receiving, processing, and distribution capability for both spacecraft and sensor ground-generated commands. The spacecraft consists of the following principal systems: structure; Communication and Command and Mission Data Message; Electrical Power and Distribution; Propulsion; Attitude Control; and Thermal.

The sensor's purpose is to detect, locate, and identify targets of interest that are intense sources of IR radiation.


1. Ball, Desmond, A Base for Debate, (Allen & Unwin, London, 1987) is perhaps the most comprehensive discussion of the DSP system.

2. Kenden, A., "Military Maneuvers in Synchronous Orbit," Journal of the British Interplanetary Society, February 1983, V. 36, pp. 88-91.

3. "Advanced Missile Warning Satellite Evolved From Smaller Spacecraft," Aviation Week & Space Technology, 20 January 1989, page 45.

4. Cushman, J., "AF Seeks Invulnerable Warning Satellites," Defense Week, 16 January 1984, pp 1, 10-14.

5. "Air Force to Decide by End of Month on DSP Acquisition Method," Aerospace Daily, 5 October 1989, page 30-31.

6. Covault, Craig, "New Missile Warning Satellite to be Launched on First Titan 4," Aviation Week & Space Technology, 20 January 1989, page 34-40. (This article is an excellent review of the history an status of this program).

7. Cushman, J., "AF Seeks Invulnerable Warning Satellites," Defense Week, 16 January 1984, pp 12.

8. Goodman, Adam, "Problems Plague McDonnell Douglas Laser," St. Louis Post Dispatch, 13 August 1989, page 1 (an extremely thorough treatment of this problem).

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