A missile warning satellite spots the launch of a ballistic missile and immediately relays that information to the ground, enabling the targeted country to retaliate, take cover, or possibly shoot down the missile. The first missile warning satellites were born in the early days of the space race, when the 1957 Soviet launch of Sputnik atop its converted intercontinental ballistic missile (ICBM) reminded U.S. generals that the country had become increasingly vulnerable to attack. The U.S. military knew that an ICBM launched from the Soviet Union to the United States would take about 30 minutes to reach its target, but in the 1950s, American radars located on overseas bases in Norway, Greenland, and Great Britain could spot an ICBM only after it had travelled half its distance. This meant that, at best, the United States would have only 15 minutes to get their strategic bombers, like the B-52 Stratofortress, off the ground. This was not enough time, and the generals realized that most of their bombers would be blown up before they ever lifted off the runway.

A year before Sputnik, a few scientists and engineers proposed using infrared sensors aboard aircraft to spot the hot exhaust of Russian ICBMs soon after they blasted off. They proposed placing these sensors on high-flying U-2 spyplanes flying around the edge of the Soviet Union. But this plan required a lot of planes flying all the time and would have been expensive, and the plan was not approved by the Air Force.

In 1956, Joseph Knopow, an engineer at Lockheed Aircraft Corporation, proposed putting an infrared sensor on a telescope aboard a spacecraft flying in low-Earth-orbit. This spacecraft could spot the hot exhaust of a rising ICBM once it lifted clear of the clouds and increase warning time of an ICBM attack from only 15 minutes to more than 25 minutes. But because satellites are constantly moving around the Earth, as many as 24 satellites would be needed so that several would be positioned over Russia at any one time to spot the missiles. The Air Force adopted this proposal and named it MiDAS, for Missile Defence Alarm System.

The Air Force launched its first MiDAS test satellite in February 1960, but it did not reach orbit. The next several test launches suffered various problems, many of them pertaining to the spacecraft and not its telescope. But committees of scientists asked to evaluate the MiDAS system were sceptical that the telescope's sensors would ever work. In particular, they were concerned that sunlight reflecting off of clouds would confuse the infrared detectors and register as false alarms.

A second group of test satellites, known as Program 461, was launched in 1966. These satellites had more powerful telescopes capable of spotting cooler solid-propellant rockets like the American submarine-launched Polaris missile. These satellites were more successful than their predecessors and finally proved that the concept could work. But the requirement for a large number of satellites meant that the program would be expensive, and Department of Defence officials cancelled it.

In 1966, a high-level scientific committee determined that a satellite with a big infrared telescope and located in geosynchronous orbit could spot an ICBM launch. The chief advantage of operating in such a high orbit was that only a few satellites, rather than the dozen or more needed for a low-altitude system, were needed to observe all the primary launching sites because each satellite could now see virtually half the Earth. The Air Force abandoned the low-altitude approach and selected satellite maker TRW and sensor manufacturer Aerojet to build the new satellites. This project was soon named the Defence Support Program, or DSP. The first DSP satellite was launched in 1971 into an improper orbit, but the spacecraft itself worked and proved that the technology worked. DSP proved to be a very successful military satellite program.

Ever since 1971, the U.S. Air Force has been launching bigger and more powerful DSP satellites into orbit. The 14th satellite launched entered service in 1989 and was the first of a new class of satellites named DSP-1. In 1992, a DSP was launched from the Space Shuttle. The satellites are barrel-shaped, with four solar panels on extendable panels at their rear. They spin in orbit for stabilization. They have a large telescope that is mounted at an angle at their front end and which sweeps across the face of the Earth six times every minute, meaning that they can detect, or “image,” an infrared heat source, such as a missile, every ten seconds, tracking its trajectory.

Initially two ground stations were built to receive their data, one near Denver, Colorado, and another in Alice Springs, Australia. The Australian site was classified because the Australian government feared public opposition. It was closed in 2001, and now all missile warning data is relayed to the Colorado station.

DSP controllers soon discovered that the satellites could spot other heat sources besides land and submarine-launched missiles. These included forest fires, surface-to-air missiles, and even military aircraft using their afterburners. The U.S. Navy began to use DSP satellites to warn of Soviet bomber attacks on its aircraft carriers. Other intelligence agencies used this information as well.

By the 1980s, the United States began operating infrared sensors on top-secret satellites in highly elliptical orbits that carry them high over the northern part of Russia and low and fast over the southern hemisphere. These sensors augmented the existing early warning system and also focused on specialized targets, such as the anti-ballistic missile system around Moscow.

Like with most military space technologies, the Soviet Union lagged behind the United States by several years. The Soviet Union launched its first missile warning test satellite in 1972, and its first operational satellite in 1977. The Soviet satellites, named Prognoz, operated in highly elliptical orbits that were highly inclined to the equator. These orbits, also known as “Molniya” orbits after the first Soviet communications satellite to use them, enabled the satellite sensors to view a missile above the horizon of the Earth against the cold background of space, which is easier than viewing it against the warm Earth background. But because the satellites are always moving, more of them are needed and the Soviets required a “constellation” of nine satellites to provide full coverage of American ICBM launch sites. This still left other areas of the globe uncovered, like the oceans that hide U.S. missile submarines. The large constellation size, combined with the short lifetimes of the satellites, meant that the Soviets had to launch up to seven satellites in a single year to keep the system running. After the Cold War ended, the Russian government was no longer able to maintain the system and during the 1990s only about half of the constellation was operating at any one time. During the mid-1980s the Soviets began launching missile warning satellites to geosynchronous orbit, like the DSP, but they launched far fewer of these satellites and Western space experts believe that these satellites, called Oko (or “Eye”) have not been very successful.

 
SBIRS architecture.