The AN/FPS-20 was a widely used

L band The L band is the Institute of Electrical and Electronics Engineers (IEEE) designation for the range of frequencies in the radio spectrum from 1 to 2 gigahertz (GHz). This is at the top end of the ultra high frequency (UHF) band, at the lower en ...

early warning An early warning system is a warning system that can be implemented as a chain of information communication systems and comprises sensors, event detection and decision subsystems for early identification of hazards. They work together to for ...

and

ground-controlled interception Ground-controlled interception (GCI) is an air defence tactic whereby one or more radar stations or other observational stations are linked to a command communications centre which guides interceptor aircraft to an airborne target. This tactic was ...

radar system employed by the

United States Air Force The United States Air Force (USAF) is the Aerial warfare, air military branch, service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part ...

Air Defense Command, the NORAD

Pinetree Line The Pinetree Line was a series of radar stations located across the northern United States and southern Canada at about the 50th parallel north, along with a number of other stations located on the Atlantic and Pacific coasts. Run by North Ame ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

, the USAF

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in the continental United States, and a variety of other users. The design started life as the Bendix AN/FPS-3 in 1950, was upgraded to the FPS-20, then spawned over a dozen different variants as additional upgrades were applied. The FPS-20 formed the backbone of the US air defense network through the early Cold War with over 200 units deployed. Most FPS-20 sites were replaced by modern equipment in the late 1960s, although a number were turned over to the

FAA The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic m ...

, modified for

air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airs ...

use, and became ARSR-60s. The first AN/FPS-3 arrived in December 1950, slated for installation at

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to control aircraft involved in the atomic bomb tests of early 1951. Over the next few years, 48 FPS-3s were installed to replace older systems in the

Lashup Radar Network The Lashup Radar Network was a United States Cold War radar netting system for air defense surveillance which followed the post-World War II "five-station radar net" and preceded the "high Priority Permanent System". ROTOR was a similar expedient ...

. The FPS-3 and was also produced as the AN/MPS-7, a mobile version. The system used two 5J26

magnetron The cavity magnetron is a high-power vacuum tube used in early radar systems and currently in microwave ovens and linear particle accelerators. It generates microwaves using the interaction of a stream of electrons with a magnetic field while ...

s at 750 kW peak power, operating at 1300 MHz a 400 Hz

pulse repetition frequency The pulse repetition frequency (PRF) is the number of pulses of a repeating signal in a specific time unit. The term is used within a number of technical disciplines, notably radar. In radar, a radio signal of a particular carrier frequency is tu ...

(prf) and 2 microsecond pulse width. The antenna was driven at three fixed speeds of 3.3, 5 or 10 RPM, normally operating at 5. Many of the operating modules were mounted on the rotating platform, with the output signals fed via slip rings to amplifiers and displays at the base of the unit. The FPS-3 was limited to about altitude, which was seen as a limitation in light of new jet-powered bombers known to be in development in the USSR. This led to the development of the AN/GPA-27 add-on unit, increasing the altitude to .The GPA-27 may be the slotted waveguide mounted on the top of the main antenna seen in images of the FPS-20. However, some images show this on radars labeled FPS-3, although this may be before they were renamed. Installations began in 1956.Winkler refers to upgraded units as FPS-3B on page 76, but other sources agree with his own page 33 that these were actually called FPS-3A. New-build units from Bendix with this equipment pre-installed became the AN/FPS-20 in 1957.Winkler does not mention this directly, but notes on page 77 that -3A units were also renamed FPS-20 when receiving updates. Otherwise similar units with an antenna from General Electric were known as the AN/FPS-20B. The slotted-waveguide antenna pictured above the AN/FPS-20 is for the associated IFF system.A high-resolution Slotted-waveguide IFF antenna for the AN/FPS-3 radar by J.Y. Wong 1957 NRC Canada. The FPS-20s were simple pulse-radar systems and subject to jamming using basic techniques. This led to a second series of upgrades to provide anti-jamming capabilities starting in 1959. Among these were the GPA-102 (MK-448) which turned an FPS-20 into an FPS-64 and a FPS-20A to an FPS-66, and the GPA-103 (MK-447) which turned a FPS-20 into an FPS-65 and the FPS-20A into an FPS-67. Similar upgrades using a Canadian OA-4831 system produced the AN/FPS-87 and AN/FPS-87A. The BADGE 412-L upgrade of the FPS-20A created the AN/FPS-82. A more major update was the MK-747, which added a new antenna from Raytheon, the Diplex Gating Unit (DGU), a bandpass filter and other modifications, to produce the AN/FPS-91 and 91A. The similar MK-748 applied to the -60 series resulted in the AN/FPS-64A, -65A, -66A and -67A. Canadian AN/FPS-87s were also converted, becoming AN/FPS-93 and 93A. These units were used with the

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system. The FPS-20 was widely used in Japan. They produced their own series of modifications, including a

tunnel diode A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively " negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suz ...

based amplifier and a separate receiver for the Airborne Instruments Laboratories system, producing the AN/FPS-20/20A JAPAN, or -J. The Indian Air Force used the FPS-20 with a new digital

moving target indicator Moving target indication (MTI) is a mode of operation of a radar to discriminate a target against the clutter. It describes a variety of techniques used for finding moving objects, like an aircraft, and filter out unmoving ones, like hills or trees ...

(MTI) system from Bendix to create the AN/FPS-100 and 100A. These systems were known as the "Blue Pearl", or Bendix Radar Processor BRP-150. These were remanufactured units with a new antenna, and included a new low-noise front-end amplifier. General Dynamics later produced a similar digital MTI system, the AN/FPS-113.

Notes

References

Citations

Bibliography

* * ;Further reading
AN/FPS-3, 3A, 3B; MPS-7
radomes.org

radomes.org

radomes.org

{{DEFAULTSORT:AN FPS-020 Ground radars Military radars of the United States Radars of the United States Air Force Military equipment introduced in the 1950s Bendix Corporation