PAVE PAWS (PAVE Phased Array Warning System) is a complex Cold War early warning radar and computer system developed in 1980 to "detect and characterize a sea-launched ballistic missile attack against the United States". The first solid-state phased array deployed used a pair of Raytheon AN/FPS-115 phased array radar sets at each site to cover an azimuth angle of 240 degrees. In accordance with the Joint Electronics Type Designation System, the "AN/FPS-115" designation represents the 115th design of an Army-Navy fixed radar(pulsed) electronic device for searching. Two sites were deployed in 1980 at the periphery of the contiguous United States, then two more in 1987–95 as part of the United States Space Surveillance Network. One system was sold to Taiwan and is still in service.
Mission
The radar was built in the Cold War to give early warning of a nuclear attack, to allow time for US bombers to get off the ground and land-based US missiles to be launched, to decrease the chance that a preemptive strike could destroy US strategic nuclear forces. The deployment of submarine launched ballistic missiles (SLBMs) by the Soviet Union by the 1970s, significantly decreased the warning time available between the detection of an incoming enemy missile and its reaching its target, because SLBMs can be launched closer to the US than the previous ICBMs, which have a long flight path from the Soviet Union to the continental US. Thus there was a need for a radar system with faster reaction time than existing radars. PAVE PAWS later acquired a second mission of tracking satellites and other objects in Earth orbit as part of the United States Space Surveillance Network.
A notable feature of the system is its phased array antenna technology, it was one of the first large phased array radars. A phased array was used because a conventional mechanically-rotated radar antenna cannot turn fast enough to track multiple ballistic missiles. A nuclear strike on the US would consist of hundreds of ICBMs and SLBMs incoming simultaneously. The beam of the phased array radar is steered electronically without moving the fixed antenna, so it can be pointed in a different direction in milliseconds, allowing it to track many incoming missiles at the same time.
Description The AN/FPS-115 radar consists of two phased arrays of antenna elements mounted on two sloping sides of the 105 ft high transmitter building, which are oriented 120° apart in azimuth. The beam from each array can be deflected up to 60° from the array's central boresight axis, allowing each array to cover an azimuth angle of 120°, thus the entire radar can cover an azimuth of 240°. The building sides are sloped at an angle of 20°, and the beam can be directed at any elevation angle between 3° and 85°. The beam is kept at least 100 ft above the ground over public-accessible land to avoid the possibility of exposing the public to significant electromagnetic fields.
Each array is a circle 72.5 ft (22.1 m) in diameter consisting of 2,677 crossed dipole antenna elements, of which 1,792 are powered and serve as both transmitting and receiving antennas, with the rest functioning as receiving antennas. Due to the phenomenon of interference the radio waves from the separate elements combine in front of the antenna to form a beam. The array has a gain of 38.6 dB, and the width of the beam is only 2.2°. The drive current for each antenna element passes through a device called a phase shifter, controlled by the central computer. By changing the relative timing (phase) of the current pulses supplied to each antenna element the computer can instantly steer the beam to a different direction.
The radar operates in the UHF band between 420 - 450 MHz, which is shared with the 70 centimeter amateur band (just below the UHF television broadcast band), that is a wavelength of 71–67 cm, with circular polarization. It is an active array (AESA); each of the 1,792 transmitting elements has its own solid-state transmitter/receiver module, and radiates a peak power of 320 W, so the peak power of each array is 580 kW. It operates in a repeating 54 millisecond cycle in which it transmits a series of pulses, then listens for echoes. Its duty cycle (fraction of time spent transmitting) is never greater than 25% (so the average power of the beam never exceeds 25% of 580 kW, or 145 kW) and is usually around 18%. It is reported to have a range of about 3,000 nautical miles (3,500 statute miles, 5,600 km); at that range it can detect an object the size of a small car, and smaller objects at closer ranges.
The functioning of the radar is completely automatic, controlled by four computers. The software divides the beam time between "surveillance" and "tracking" functions, switching the beam back and forth rapidly between different tasks. In the surveillance mode, which normally consumes about 11% of the duty cycle, the radar repeatedly scans the horizon across its full 240° azimuth in a pattern between 3° and 10° elevation, creating a "surveillance fence" to immediately detect missiles as they rise above the horizon into the radar's field of view. In the tracking mode, which normally consumes the other 7% of the 18% duty cycle, the radar beam follows already-detected objects to determine their trajectory, calculating their launch and target points.
Background
Fixed-reflector radars with mechanically-scanned beams such as the 1955 GE AN/FPS-17 Fixed Ground Radar and 1961 RCA AN/FPS-50 Radar Set were deployed for missile tracking, and the USAF tests of modified AN/FPS-35 mechanical radars at Virginia and Pennsylvania SAGE radar stations had "marginal ability" to detect Cape Canaveral missiles in summer 1962. A Falling Leaves mechanical radar in New Jersey built for BMEWS successfully tracked a missile during the October 1962 Cuban Missile Crisis, and "an AN/FPS-85 long-range phased array (Passive electronically scanned array) radar was constructed at Eglin AFB" Site C-6, Florida beginning on 29 October 1962 (the Bendix Radio Division FPS-85 contract had been signed 2 April 1962). Early military phased array radars were also deployed for testing: Bendix AN/FPS-46 Electronically Steerable Array Radar (ESAR) at Towson, MD (powered up in November 1960), White Sands' Multi-function Array Radar (1963), and the Kwajalein Missile Site Radar (1967).
Submarine Launched Ballistic Missile Detection and Warning System The Avco 474N Submarine Launched Ballistic Missile (SLBM) Detection and Warning System (SLBMD&W System) was deployed as "an austere…interim line-of-sight system" after approval in July 1965 to modify some Air Defense Command (ADC) Avco AN/FPS-26 Frequency Diversity Radars into Avco AN/FSS-7 SLBM Detection Radars. The 474N sites planned for 1968 also were to include AN/GSQ-89 data processing equipment (for synthesizing tracks from radar returns), as well as site communications equipment that NORAD requested on 10 May 1965 to allow "dual full period dedicated data circuits" to the Cheyenne Mountain 425L System, which was "fully operational" on 20 April 1966. (Cheyenne Mountain Complex relayed 474N data to "SAC, the National Military Command Center, and the Alternate NMCC over BMEWS circuits", for presentation by Display Information Processors—impact ellipses and "threat summary display" with a count of incoming missiles and "Minutes Until First Impact" countdown).
By December 1965 NORAD decided to use the Project Space Track "phased-array radar at Eglin AFB…for SLBM surveillance on an "on-call" basis" "at the appropriate DEFCON". By June 1966 the refined FPS-85 plan was for it "to have the capability to operate in the SLBM mode [interlaced transmissions] with the Spacetrack surveillance and tracking modes" Rebuilding of the "separate faces for transmitting and receiving" began in 1967 after the under-construction Eglin FPS-85 was "almost totally destroyed by fire on 5 January 1965". FPS-85 IOC was in 1969, 474N interim operations began in July 1970 (474N IOC was 5 May 1972), and in 1972 20% of Eglin FPS-85 "surveillance capability…became dedicated to search for SLBMs,"Jane's Radar and Electronic Systems, 6th edition, Bernard Blake, ed. (1994), p. 31 and new SLBM software was installed in 1975. (the FPS-85 was expanded in 1974).
The Stanley R. Mickelsen Safeguard Complex with North Dakota phased arrays (four-face Missile Site Radar and single-face GE Perimeter Acquisition Radar, PAR) became operational in 1975 as part of the Safeguard Program for defending against enemy ballistic missiles.
Development
The SLBM Phased Array Radar System (SPARS) was the USAF program initiated in November 1972 by the Joint Chiefs of Staff (JCS) while the Army's PAR was under construction. A 1974 SPARS proposal for "two new SLBM Phased Array Warning Radars" was submitted to replace the east/west coast 474N detection radars, which had "limitations against Soviet SLBMs, particularly the longer range SS-N-8" on 1973 "Delta" class submarines. Development began in August 1973, SPARS was renamed PAVE PAWS on 18 February 1975, and system production was requested by a 13 June 1975 Request for Proposals (RFP). Rome Air Development Center (RADC) "was responsible for the design, fabrication installation, integration test, and evaluation of" PAVE PAWS through 1980. (PAVE PAWS concept drawing on p. 141)
The differing USAF AN/FPS-109 Cobra Dane phased array radar in Alaska achieved IOC on 13 July 1977 for "providing intelligence on Soviet test missiles fired at the Kamchatka peninsula from locations in southwestern Russia". The Safeguard PAR station that closed in 1976, had its radar "modified for the ADCOM mission during 1977 [and] ADCOM accepted [the Concrete Missile Early Warning Station] from the Army on 3 October 1977" for "SLBM surveillance of Arctic Ocean areas". By December 1977 RADC had developed the 322 watt PAVE PAWS "solid state transmitter and receiver module", and the System Program Office (ESD/OCL) issued the AN/FPS-115 "System Performance Specification …SS-OCLU-75-1A" on 15 December 1977. (cited by ADA088323) IBM's PAVE PAWS "beam-steering and pulse schedules from the CYBER-174" duplexed computers to the MODCOMP IV duplexed radar control computers were "based upon" PARCS program(s) installed for attack characterization in 1977 when the USAF received the Army's PAR. Bell Labs enhanced the PARCS beginning December 1978, e.g., "extending the range" by 1989 for the Enhanced PARCS configuration (EPARCS).
Environmental and health concerns