It was known that hot, high-speed objects caused ionization of the atmosphere that reflects waves, and it appeared that a warhead's signature would be different enough from decoys that a detector could pick out the warhead directly, or alternately, provide added information that would allow operators to focus a conventional tracking radar on the single return from the warhead.Although the concept appeared to offer a solution to the tracking problem, there was almost no information on either the physics of re-entry or a strong understanding of the normal composition of the upper layers of the. Even at this early stage it was clear that the use of would be a serious problem at the long ranges needed to successfully attack a warhead, ranges on the order of 1,000 miles (1,600 km).Among the many Defender projects were several studies based on the concept that a re-entering would cause unique physical signatures while still in the upper atmosphere. Also to the right is the catwalk and part of the rectangular that brings the 2.5 MW 430 MHz radar transmitter's signal up to the focal region.The origins of the observatory trace to late 1950s efforts to develop (ABM) defences as part of the newly formed ABM umbrella-effort, Project Defender.
To the right is the Gregorian sub-reflector, and to the left is the remains of the 96-foot-long (29 m) line feed tuned to 430 MHz (destroyed by Hurricane Maria). The triangular platform at the top is fixed, and the arm rotates beneath it. A detailed view of the beam-steering mechanism. The round trip light time to objects beyond is longer than the 2.6 hour time that the telescope can track a celestial position, preventing observations of more distant objects. 's location near the Northern allows Arecibo to view the planets in the Solar System over the Northern half of their orbit.
This allows the telescope to observe any region of the sky in a forty-degree cone of visibility about the local (between −1 and 38 degrees of ).
The platform has a rotating, bow-shaped track 93 m (305 ft) long, called the arm, carrying the receiving antennas and secondary and tertiary reflectors. A parabolic mirror would have varying when the receiver is off the focal point, but the is uniform in every direction.The receiver is on a 900-ton platform suspended 150 m (492 ft) above the dish by 18 cables running from three towers, one 111 m (365 ft) high and the other two 81 m (265 ft) high, placing their tops at the same elevation. To aim the device, the receiver is moved to intercept signals reflected from different directions by the spherical dish surface of 270 m (870 ft) radius. The ground beneath is accessible and supports shade-tolerant vegetation.The observatory has four transmitters, with of 20 (continuous) at 2380 MHz, 2.5 (pulse peak) at 430 MHz, 300 at 47 MHz, and 6 at 8 MHz.The reflector is a, not a. The dish surface is made of 38,778 perforated aluminum panels, each about 3 by 6 feet (1 by 2 m), supported by a mesh of steel cables.
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