Editing PAVE PAWS (section)

== Description ==
The AN/FPS-115 radar consists of two [[phased array]]s of antenna elements mounted on two sloping sides of the 105&nbsp;ft high transmitter building, which are oriented 120° apart in azimuth.<ref name="EIS">{{cite book |title=National Missile Defense Deployment - Final Environmental Impact Statement, Vol. 4 |publisher=United States Army Space and Missile Defense Command |date=July 2000 |pages=H.1.4–H.1.9}}</ref><ref name="GlobalSecurity"/> 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&nbsp;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&nbsp;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 (wave propagation)|interference]] the radio waves from the separate elements combine in front of the antenna to form a beam.  The array has a [[antenna gain|gain]] of 38.6&nbsp;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 (waves)|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 [[ultrahigh frequency|UHF]] band between 420 - 450&nbsp;MHz, which is shared with the [[70-centimeter band|70 centimeter]] [[Amateur radio|amateur]] band (just below the UHF television broadcast band), that is a wavelength of 71–67&nbsp;cm, with [[circular polarization]]. It is an active array ([[active electronically scanned array|AESA]]); each of the 1,792 transmitting elements has its own solid-state transmitter/receiver module, and radiates a peak power of 320&nbsp;W, so the peak power of each array is 580&nbsp;kW.  It operates in a repeating 54&nbsp;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&nbsp;kW, or 145&nbsp;kW) and is usually around 18%.  It is reported to have a range of about 3,000 [[nautical mile]]s (3,452 statute miles, 5,555&nbsp;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.