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AIRPASS was a British
airborne interception radar and
fire-control radar system developed by
Ferranti
Ferranti or Ferranti International plc was a UK electrical engineering and equipment firm that operated for over a century from 1885 until it went bankrupt in 1993. The company was once a constituent of the FTSE 100 Index.
The firm was known ...
. It was the world's first airborne
monopulse radar
Monopulse radar is a radar system that uses additional encoding of the radio signal to provide accurate directional information. The name refers to its ability to extract range and direction from a single signal pulse.
Monopulse radar avoids prob ...
system and fed data to the world's first
head-up display
A head-up display, or heads-up display, also known as a HUD (), is any transparent display that presents data without requiring users to look away from their usual viewpoints. The origin of the name stems from a pilot being able to view informa ...
. The name is an acronym for "Airborne Interception Radar and Pilot's Attack Sight System". In the
Royal Air Force
The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) an ...
(RAF) it was given the official name Radar, Airborne Interception, Mark 23, normally shortened to AI.23. AIRPASS was used on the
English Electric Lightning
The English Electric Lightning is a British fighter aircraft that served as an interceptor during the 1960s, the 1970s and into the late 1980s. It was capable of a top speed of above Mach 2. The Lightning was designed, developed, and manufa ...
throughout its lifetime.
The basic AIRPASS electronics system was later adapted as the basis for a
terrain following radar for navigation and targeting for air-to-ground attacks. This AIRPASS II was originally intended for the
BAC TSR.2, but when that aircraft was cancelled in 1965, it was subsequently used in the
Blackburn Buccaneer. Elements of the AIRPASS design were used on many subsequent radars from Ferranti, while its head-up display was licensed for use in the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
where it was quickly adopted for many aircraft.
History
Development of the monopulse radar underlying AIRPASS began in 1951. The AIRPASS system was announced to the public in late June 1958. It was initially tested on
Douglas DC-3 TS423 (later civilian registered as G-DAKS) and later on an
English Electric Canberra
The English Electric Canberra is a British first-generation, jet-powered medium bomber. It was developed by English Electric during the mid- to late 1940s in response to a 1944 Air Ministry requirement for a successor to the wartime de Havil ...
WJ643 for higher speed trials, replacing the nose sections of these aircraft. After testing use, WJ643 was renamed T.Mk 11 and used as a trainer aircraft for the radar operators of the
Gloster Javelin
The Gloster Javelin is a twin-engined T-tailed delta-wing subsonic night and all-weather interceptor aircraft that served with Britain's Royal Air Force from the mid-1950s until the late 1960s. The last aircraft design to bear the Gloster name ...
. Several further T.Mk 11 were produced, but these mounted the AI.17 from the Javelin. The first flight on the
English Electric Lightning
The English Electric Lightning is a British fighter aircraft that served as an interceptor during the 1960s, the 1970s and into the late 1980s. It was capable of a top speed of above Mach 2. The Lightning was designed, developed, and manufa ...
took place on airframe XG312 on 29 December 1958.
It entered service on
interceptor aircraft
An interceptor aircraft, or simply interceptor, is a type of fighter aircraft designed specifically for the defensive interception role against an attacking enemy aircraft, particularly bombers and reconnaissance aircraft. Aircraft that are c ...
from the early 1960s. It was initially linked to the
de Havilland Firestreak
The de Havilland Firestreak is a British first-generation, passive infrared homing (heat seeking) air-to-air missile. It was developed by de Havilland Propellers (later Hawker Siddeley) in the early 1950s, entering service in 1957. It was the ...
air-to-air missile. AIRPASS was developed by Ferranti Ltd on Ferry Road in
Edinburgh
Edinburgh ( ; gd, Dùn Èideann ) is the capital city of Scotland and one of its 32 council areas. Historically part of the county of Midlothian (interchangeably Edinburghshire before 1921), it is located in Lothian on the southern shore of t ...
. It introduced the
HOTAS
HOTAS, an acronym of hands on throttle-and-stick, is the concept of placing buttons and switches on the throttle lever and flight control stick in an aircraft's cockpit. By adopting such an arrangement, pilots are capable of performing all vit ...
(Hands On Throttle-And-Stick) system whereby the radar and gun sight controls were situated on the
control column
A yoke, alternatively known as a control wheel or a control column, is a device used for piloting some fixed-wing aircraft.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 563. Aviation Supplies & Academics, 1997. ...
and
throttle lever
Thrust levers or throttle levers are found in the cockpit of aircraft, and are used by the pilot, copilot, or autopilot to control the thrust output of the aircraft's engines, by controlling the fuel flow to those engines.
In multi-engine air ...
instead of elsewhere in the cockpit, eliminating the need for the pilot to take his hands off the controls while making an interception.
The radar entered service with the
RAF
The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) and ...
in 1960 in the
English Electric Lightning
The English Electric Lightning is a British fighter aircraft that served as an interceptor during the 1960s, the 1970s and into the late 1980s. It was capable of a top speed of above Mach 2. The Lightning was designed, developed, and manufa ...
interceptor. The next version of the system was called AIRPASS II, or "
Blue Parrot", and was a system optimised for use at low-level and originally developed for the cancelled
BAC TSR.2 and subsequently used in the
Blackburn Buccaneer.
Design
AIRPASS was based on a
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 ...
source which provided pulses of about 100 kW peak. Pulses were about one microsecond in duration and sent 1000 times a second. To make the system as compact as possible, Ferranti invested in a
numerical control system to mill the
waveguide
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
s from single blocks of aluminium. The signal was sent and received from feedhorns that were split vertically to produce two outputs, one on either side of the reflector centerline. The reflector was shaped as two partial paraboloids, so that the two signals re-combined in space in front of the aircraft. The entire assembly was mounted on a servo system that allowed the antenna assembly to be pointed in two dimensions.
On reception of a pulse, the signal was sent into a
klystron
A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,Pond, Norman H. "The Tube Guys". Russ Cochran, 2008 p.31-40 which is used as an amplifier for high radio frequen ...
local oscillator
In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequenc ...
and then into two conventional
superheterodyne
A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carri ...
receivers with an
intermediate frequency of 30 MHz. The monopulse technique requires the signals from the two channels to be compared in strength, so the output of the amplifiers must be precisely matched. This was accomplished with a highly advanced
automatic gain control system with 100 dB range that adjusted the pulse-to-pulse outputs. To this point the system was entirely analogue, using miniaturized
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as ...
s cooled by forced air.
[
Behind the analog section was the ]analog computer
An analog computer or analogue computer is a type of computer that uses the continuous variation aspect of physical phenomena such as electrical, mechanical, or hydraulic quantities (''analog signals'') to model the problem being solved. In ...
portion of the system. This took the outputs from the radar system, calculated the proper intercept course based on the selected weapon, and presented the results in the reflector gunsight mechanism. The system also read data from various aircraft systems like the altimeter
An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...
and air speed indicator and combined this into the same display. The result was the world's first heads-up display
A head-up display, or heads-up display, also known as a HUD (), is any transparent display that presents data without requiring users to look away from their usual viewpoints. The origin of the name stems from a pilot being able to view informa ...
, a concept that was soon licensed by US manufacturers.
AIRPASS had an average detection range against a Tupolev Tu-95 "Bear" bomber of about .[ This was more than enough to allow the Lightning to be directed into the general area of the target through ground controlled interception and then use AIRPASS to hunt it down. Some consideration was given to sending the aircraft into the right area using commands sent from the ground to the aircraft's ]autopilot
An autopilot is a system used to control the path of an aircraft, marine craft or spacecraft without requiring constant manual control by a human operator. Autopilots do not replace human operators. Instead, the autopilot assists the operator' ...
, allowing the pilot to concentrate solely on their radar display, but this project was ultimately cancelled.
Later models, the AI.23B, added additional small antennas for S-band reception of signals from ground based radars. This allowed the aircraft to seek out targets with its own radar transmitter turned off. The signal was displayed in A-scope fashion along the bottom of the radar display, which the pilot could use to seek out targets while under ground direction. When a peak of the required height appeared, the pilot would take over the approach and then turn on their own radar when the conditions appeared correct. This allowed the aircraft to make accurate approaches without signalling their presence or inviting jamming from the target. The same system was used in the E/F-band to provide home-on-jam.
Further development
As part of a fighter purchase programme for the Swiss Air Force, Saab AB modified a single Saab 35 Draken
The Saab 35 Draken (; 'The Kite' or 'The Dragon') is a Swedish fighter-interceptor developed and manufactured by Svenska Aeroplan Aktiebolaget ( SAAB) between 1955 and 1974. Development of the Saab 35 Draken started in 1948 as the Swedish air f ...
was modified by replacing its relatively simple radar system with an AIRPASS. This produced the J35H, for Helvetia, but the contract was ultimately won by the Mirage III
The Dassault Mirage III () is a family of single/dual-seat, single-engine, fighter aircraft developed and manufactured by French aircraft company Dassault Aviation. It was the first Western European combat aircraft to exceed Mach 2 in horizonta ...
.
In the 1960s, Ferranti won the contract to supply radars for the Blackburn Buccaneer aircraft in Royal Navy
The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against ...
service. This version was modified to handle low-level scanning by eliminating the reflections from waves. Since the waves reflected away much of the signal, the resulting "Blue Parrot" system, a rainbow code
The Rainbow Codes were a series of code names used to disguise the nature of various British military research projects. They were mainly used by the Ministry of Supply from the end of the Second World War until 1958, when the ministry was broke ...
name, used a more powerful 250 kW transmitter and a larger Cassegrain antenna
In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in which the feed antenna is mounted at or behind the surface of the concave main parabolic reflector dish and is aimed at a smaller convex secondary reflector suspende ...
.[
During tests of the monopulse systems, Ferranti engineers noticed that the systems produced high quality ranging information of ground reflections.][ Older systems without monopulse processing made the determination of range difficult as the radar returned signals were from the entire beamwidth, meaning that it received signals that were from the ground that was closer and further from the aircraft. Monopulse processing allowed the beam to be discriminated vertically and thus range against a single feature very accurately.
The ability of the radar to produce highly accurate range measurements, combined with a system that displayed the resulting data as a map, opened the possibility of producing a ]terrain-following radar
Terrain-following radar (TFR) is a military aerospace technology that allows a very-low-flying aircraft to automatically maintain a relatively constant altitude above ground level and therefore make detection by enemy radar more difficult. It is ...
guidance system. Ferranti developed this concept extensively through the 1960s, first with their Dakota and Canberra aircraft, and later with the Buccaneer.
The idea was simple; the computer calculated a ski-ramp shaped ideal trajectory, flat directly under the aircraft and then sloping upward in a gentle curve. This pattern is rotated to follow the aircraft's velocity vector. The radar scanned in a U-shaped pattern, taking accurate measurements of the altitude and range to objects in front and slightly to either side of the aircraft. The computer compared the range and altitude of objects in the radar to the pre-calculated path, and then rotated the path so that the terrain feature would be overflown at a pre-set altitude between . This was relayed to the pilot as a dot in the heads-up display, and by following the dot the aircraft would attempt to maintain the selected altitude by continually raising or lowering the dot as the terrain moved. The ski-shaped curve was selected to ensure any required manoeuvres were half-gee, lowering loads on the crew.[
The concept became the basis for the ill-fated ]BAC TSR-2
The British Aircraft Corporation TSR-2 is a cancelled Cold War strike and reconnaissance aircraft developed by the British Aircraft Corporation (BAC), for the Royal Air Force (RAF) in the late 1950s and early 1960s. The TSR-2 was designed ...
project, which used another modified version of the AIRPASS hardware, now extensively transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
ized.
Structure
The radar of AIRPASS I weighed around 90 kg.
See also
* Radar configurations and types This is a list of different types of radar.
Detection and search radars
Search radars scan great volumes of space with pulses of short radio waves. They typically scan the volume two to four times a minute. The waves are usually less than a meter ...
* Airborne interception radar
* Ferranti Blue Fox __NOTOC__
The Ferranti Blue Fox was a British multi-role airborne radar designed and built for the Royal Navy by Ferranti Defence Systems in the late 1970s. It had a mixed record in service, and was replaced by the more capable Blue Vixen.
Desig ...
References
{{reflist
External links
APSS
1960 establishments in the United Kingdom
1960 in aviation
1960 in military history
Ferranti
Military radars of the United Kingdom
Cold War military equipment of the United Kingdom
1960 in Scotland
1960s in Edinburgh
Military equipment introduced in the 1950s