page_title: Nike Zeus
section_title: Early detection
hierarchical_section_title: Nike Zeus / Description / Early detection
caption_attribution_description: The Zeus Acquisition Radar (ZAR) for the Nike Zeus anti-ballistic missile program on Kwajalein, Marshall Islands. The transmitter is the triangle in the foreground, the receiver is in the background. The transmitter was built in a triangle form to allow the rapid scanning of the sky. In a typical single-reflector radar, the reflector would have to rotate through a complete circle before it re-visited a particular target. If you need rapid re-scanning, the rotation rate has to be increased. The ZAR solved this by using three reflectors and switching the transmitter to the one closest to the desired target. By continually switching between the faces, the scanning rate was tripled without requiring the antenna to rotate more rapidly. Designed to track warheads at hundreds of miles, the ZAR was extremely powerful, and its broadcasts were dangerous to anyone near the antenna. The large metal fence surrounding the transmitter is opaque to microwaves, and protects anyone walking around the site outside the fence. To access the radar while it was running, staff entered through the metal tunnel. The receiver antenna is located under the radome and is thus not visible in this image. It consisted of a stack of foam blocks with metal strips inside forming a Luneberg lens, nearly filling the entire radome. The entire assembly rotated in synchronicity with the transmitter. The hashed area surrounding the receiver is a large field of metal wires suspended off the ground on poles. This formed a reflector that make a perfect ground plane, improving the accuracy of vertical angle measurements. This technique was first used by British anti-aircraft radars early in WWII. The rectangular building between the two parts of the ZAR is a power plant that powers the system. The circular objects on the right are part of the island's water processing system.
context_page_description: Nike Zeus was an anti-ballistic missile system developed by the US Army during the late 1950s and early 1960s that was designed to destroy incoming Soviet intercontinental ballistic missile warheads before they could hit their targets. It was designed by Bell Labs' Nike team, and was initially based on the earlier Nike Hercules anti-aircraft missile. The original, Zeus A, was designed to intercept warheads in the upper atmosphere, mounting a 25 kiloton W31 nuclear warhead. During development, the concept changed to protect a much larger area and intercept the warheads at higher altitudes. This required the missile to be greatly enlarged into the totally new design, Zeus B, given the tri-service identifier XLIM-49, mounting a 400 kiloton W50 warhead. In several successful tests, the B model proved itself able to intercept warheads, and even satellites.
  The nature of the strategic threat changed dramatically during the period that Zeus was being developed. Originally expected to face only a few dozen ICBMs, a nationwide defense was feasible, although expensive.
context_section_description: When Zeus was still in the early stages of design, Bell Labs suggested using two similar radars to provide extended range tracking and improve reaction times. Located at the Zeus bases would be the Local Acquisition Radar (LAR), a UHF monopulse radar able to track between 50 and 100 targets. The Forward Acquisition Radar (FAR) would be positioned 300 to 700 miles (480–1,130 km) ahead of the Zeus bases to provide early warning of up to 200 to 300 seconds of tracking data on up to 200 targets. The FAR would broadcast 10 MW pulses at UHF between 405–495 MHz, allowing it to detect a 1 square metre radar reflection at 1,020 nautical miles (1,890 km) or a more typical 0.1 m² target at 600 nautical miles (1,100 km). Each track would be stored as a 200 bit record including location, velocity, time of measure and a measure of the quality of the data. Clouds of objects would be tracked as a single object with additional data indicating the width and length of the cloud. Tracks could be updated every five seconds while the target was in view, but the antenna rotated at a relatively slow 4 RPM so targets moved significantly between rotations. Each FAR could feed data to up to three Zeus sites.
  By the time the Zeus plans were being finalized in 1957, plans for FAR were deemphasized, and LAR had been upgraded to become the Zeus Acquisition Radar (ZAR) which provided wide area early warning and initial tracking information. This enormously powerful radar was driven by multiple 1.8 MW klystrons and broadcast through three 80-foot (24 m) wide antennas arranged as the outside edges of a rotating equilateral triangle. The ZAR spun at 10 RPM, but with three antennas it simulated a single antenna rotating three times as fast. Each target was scanned every two seconds, providing much more data than the earlier FAR/LAR concept.
  The signal was received on a separate set of three antennas, situated at the centre of an 80 foot (24 m) diameter Luneburg lens, which rotated synchronously with the broadcaster under a 120-foot (37 m) diameter dome. Multiple feed horns were used in the receiver to allow reception from many vertical angles at once. Around the receiver dome was a large field of wire mesh, forming a flat ground plane reflector. The ZAR operated in the UHF on various frequencies between 495–605 MHz, giving it frequency agility. ZAR had detection range on the order of 460 nautical miles (850 km) on a 0.1 m² target.
  The entire transmitter was surrounded by a 65-foot (20 m) high clutter fence located 350 feet (110 m) away from the antenna, which reflected the signal away from local objects on the ground that would otherwise create false returns. The ZAR was so powerful that the microwave energy at close range was far beyond the mandated safety limits and potentially lethal within 100 yards (91 m). In order to allow for maintenance while the radar was operating, the equipment areas were shielded in a partial Faraday cage of metal foil, and a metal tunnel was run from the outside of the clutter fence, which blocked the signal outside the fence line. The other radars completing the system featured similar protection.
caption: The Zeus Acquisition Radar's triangular transmitter is in the foreground, with the dome covered receiver in the background.
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