Although both types of satellites are based on the same physical principles, the technological and engineering details of the two differ. The United States deployed its first early-warning satellite in 1970, as part of the Defense Support Program (DSP). This program was so sensitive that even its existence was an official secret for many years. It became known to the general public only in 1991, when it was used to warn populations, as well as Patriot missile units, of Iraqi Scud launches during the Persian Gulf War.
From the DSP satellites' geostationary orbits, the angle between the center and edge of the earth's disk is roughly 10 degrees. The satellite has a cylindrical body with a slightly off-axis infrared telescope, and it spins around the cylinder's axis of symmetry once every 10 seconds. As it nutates around the earth's disk below, the telescope points in a direction roughly two-thirds of the way between the disk's center and its edge.
The telescope consists of a sunshade, an aperture, and a large reflecting mirror. A line array of roughly 6000 infrared-sensitive pixels is positioned at the telescope's focus, which is itself between the aperture and mirror. Each pixel measures the infrared intensity within narrow bands in the infrared background.
The wavelengths that the pixels see are chosen to minimize the brightness of the earth background. This is done by picking bands that correspond to the atmospheric absorption lines of water and carbon dioxide at 2.7 µm, and of carbon dioxide at 4.3 µm. Sunlight reflected from clouds at low altitudes, or the earth-water surface, passes through the absorbing lower atmosphere twice--once on the way down and once on the way up. This setup greatly reduces the infrared sun-reflected background and in turn cuts down the false signal rate.
However, this lower false signal rate also means a slight delay in the detection of a launched missile. Missiles rising out of the atmosphere will be detected by a DSP satellite only after they reach an altitude between 10 and 15 km. For most slow-moving intercontinental ballistic missiles, this corresponds to about 1 minute out of the 25- or 30-minute flight time.
The entire visible face of the earth, a little less than half its surface, is scanned by the telescope as it spins. Individual pixels record the light gathered from roughly 1-km squares as the spinning satellite moves the line array around the image of the earth below. As each sensor sweeps along that image, the slowly varying part of the natural background is removed by digital filtering and the rapidly varying part is retained. The retained component of the signal is checked to see if it exceeds a pre-determined threshold. All signals that exceed this threshold, both real and false, are then transmitted to the ground through a 1Mb/s-data link. Comparing the light intensity in a square with the light gathered 10 seconds before and later indicates the false signals, which are then removed. In most cases, a real missile will have moved out of the original square and into another square during those 10 seconds, while most naturally occurring false signals will have moved little.
Soviet space scientists seem in addition to have exploited the atmospheric absorption bands. There are reports of Russian satellites observing the flashes of explosions from Tomahawk missiles during the December 1998 Desert Fox attacks on Baghdad, but reliable observations of such events would require sensor systems that are much different from those in early-warning satellites.
Drawings of Russian early-warning satellites show vehicles with long unfolding sunshades that appear to be designed to operate as stable non-spinning platforms. Such a satellite would be pointed toward the area of interest, and the field of view would be scanned using a rotating mirror to reflect the image of the earth below across an infrared-sensitive linear array. The scan time of such systems would probably be between 3 to 4 seconds, the exact time being chosen with attention to the pixel resolution and rate of change in the background created by the earth.
The actual scan time and field of view of the system could be changed by adjusting the mirror's rotation. Based on known data transmission rates, a typical field of view, used for a wide-area search early-warning mission, is estimated at roughly 15 to 20 square degrees. Backgrounds could, in principle, be reduced using a digital-filtering scheme similar to that used on the DSP satellite. However, the Russians have apparently chosen to do essentially all data processing on the ground, as opposed to the on-board approach of the DSP.
A big advantage of processing data on the ground rather than in space is that the Russians can obtain an enormously detailed record of the earth background over time. With processing in space, this detailed information is essentially thrown away. The Russian technique yields valuable information on the spatial and temporal characteristics of the earth background. When this data is combined with advanced data processing, it can lead to satellites that have improved sensitivity and lower false alarm rates.