Terrain-following Radar
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Terrain-following radar (TFR) is a military
aerospace Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial and military applications. Aerospace engineering consists of aeronautics and astrona ...
technology that allows a very-low-flying
aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...
to automatically maintain a relatively constant altitude above ground level and therefore make detection by enemy radar more difficult. It is sometimes referred to as ''ground hugging'' or ''terrain hugging'' flight. The term ''
nap-of-the-earth Nap-of-the-earth (NOE) is a type of very low-altitude flight course used by military aircraft to avoid enemy detection and attack in a high-threat environment. Other, mostly older terms include "ground-hugging", "terrain masking", "flying under t ...
'' flight may also apply but is more commonly used in relation to low-flying military
helicopters A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...
, which typically do not use terrain-following radar. TFR systems work by scanning a radar beam vertically in front of the aircraft and comparing the range and angle of the radar reflections to a pre-computed ideal manoeuvring curve. By comparing the distance between the terrain and the ideal curve, the system calculates a manoeuvre that will make the aircraft clear the terrain by a pre-selected distance, often on the order of . Using TFR allows an aircraft to automatically follow terrain at very low levels and high speeds. Terrain-following radars differ from the similar-sounding terrain avoidance radars; terrain avoidance systems scan horizontally to produce a map-like display that the navigator then uses to plot a route that avoids higher terrain features. The two techniques are often combined in a single radar system, the navigator uses the terrain avoidance mode to choose an ideal route through lower-altitude terrain features like valleys, and then switches to TFR mode which then flies over that route at a minimum altitude. The concept was initially developed at the
Cornell Aeronautical Laboratory Calspan Corporation is a science and technology company founded in 1943 as part of the Research Laboratory of the Curtiss-Wright Airplane Division at Buffalo, New York. Calspan consists of four primary operating units: Flight Research, Transportati ...
in the 1950s. It was first built in production form starting in 1959 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 ...
for use with the
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 ...
aircraft, flying for the first time in 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 ...
testbed in 1962. While the
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 was ultimately abandoned, the concept was widely deployed in 1960s and 70s
strike aircraft An attack aircraft, strike aircraft, or attack bomber is a tactical military aircraft that has a primary role of carrying out airstrikes with greater precision than bombers, and is prepared to encounter strong low-level air defenses while pres ...
and
interdictor An interdictor is a type of attack aircraft that operates far behind enemy lines, with the express intent of air interdiction of the enemy's military targets, most notably those involved in logistics. Interdiction prevents or delays enemy f ...
s, including the
General Dynamics F-111 The General Dynamics F-111 Aardvark is a retired supersonic, medium-range, multirole combat aircraft. Production variants of the F-111 had roles that included ground attack (e.g. interdiction), strategic bombing (including nuclear weapons c ...
,
Panavia Tornado The Panavia Tornado is a family of twin-engine, variable-sweep wing multirole combat aircraft, jointly developed and manufactured by Italy, the United Kingdom and West Germany. There are three primary Tornado variants: the Tornado IDS (inter ...
and
Sukhoi Su-24 The Sukhoi Su-24 ( NATO reporting name: Fencer) is a supersonic, all-weather attack aircraft developed in the Soviet Union. The aircraft has a variable-sweep wing, twin-engines and a side-by-side seating arrangement for its crew of two. It wa ...
"Fencer". The wider introduction of
stealth aircraft Stealth aircraft are designed to avoid detection using a variety of technologies that reduce reflection/emission of radar, infrared, visible light, radio frequency (RF) spectrum, and audio, collectively known as stealth technology. The F-117 N ...
technologies through the 1990s has led to a reduction in low-altitude flight as a solution to the problem of avoiding
anti-aircraft weapon Anti-aircraft warfare, counter-air or air defence forces is the battlespace response to aerial warfare, defined by NATO as "all measures designed to nullify or reduce the effectiveness of hostile air action".AAP-6 It includes surface based, ...
s and the technique is no longer common. Most aircraft of this class have since retired although the Su-24 and Tornado remain in use in some numbers.


Technology

The system works by transmitting a
pencil beam In optics, a pencil or pencil of rays is a geometric construct used to describe a beam or portion of a beam of electromagnetic radiation or charged particles, typically in the form of a narrow beam (conical or cylindrical). Antennas which stron ...
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
signal towards the ground area in front of the aircraft while the radar scans up and down. The reflections off the ground produce very powerful returns. The time the signal takes to travel to and from the terrain produces a range measurement to the terrain in front of the aircraft. The angle relative to the aircraft is returned by a sensor on the vertical gimbal that returns a calibrated voltage. At the same time that the radar is sending out pulses, a
function generator In electrical engineering, a function generator is usually a piece of electronic test equipment or software used to generate different types of electrical waveforms over a wide range of frequencies. Some of the most common waveforms produced ...
is producing a varying voltage representing a preferred manoeuvring curve. This is similar in shape to a
ski jump Ski jumping is a winter sport in which competitors aim to achieve the farthest jump after sliding down on their skis from a specially designed curved ramp. Along with jump length, competitor's aerial style and other factors also affect the final ...
ramp, flat under the aircraft and then curving upward in front of it. The curve represents the path the aircraft would take if it was manoeuvring at a constant
g-force The gravitational force equivalent, or, more commonly, g-force, is a measurement of the type of force per unit mass – typically acceleration – that causes a perception of weight, with a g-force of 1 g (not gram in mass measure ...
, while the flat area under the aircraft extends forward a short distance to represent the distance the aircraft moves in a straight line before starting that manoeuvre due to control lag. The resulting compound curve is displaced by a pilot-selected desired clearance distance. The system makes two measurements at the instant the return is seen. The first is from the vertical encoder on the radar measuring the actual angle to the terrain, and the second is from the function generator indicating the desired angle at the measured range. The difference between these two voltages is a representation of the angle between the radar's image and the preferred location. If the resulting voltage is positive, that means the terrain lies above the curve, negative means it is below. This difference is known as the ''angle error''. To guide the aircraft, a series of these measurements are taken over the period of one complete vertical scan out to some maximum distance on the order of . The maximum positive or minimum negative value of the angle error during the scan is recorded. That voltage is a representation of the change in pitch angle the aircraft needs to fly at to keep itself at the desired clearance altitude above the terrain while manoeuvring at the selected load factor. This can be fed into an
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' ...
or displayed on the pilot's
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 ...
. This process produces a continually computed path that rises and falls over the terrain with a constant manoeuvring load. One problem with this simple algorithm is that the calculated path will keep the aircraft in positive pitch as it approaches the crest of a hill. This results in the aircraft flying over the peak while still climbing and taking some time before it begins to descend again into the valley beyond. This effect was known as "ballooning". To address this, real-world units had an additional term that was applied that caused the aircraft to climb more rapidly against larger displacements. This resulted in the aircraft reaching the desired clearance altitude earlier than normal and thus levelling off before reaching the peak. Because the radar only sees objects in the line-of-sight, it cannot see hills behind other hills. To prevent the aircraft from diving into a valley only to require a hard pull-up, the negative G limit was generally low, on the order of one-half G. The systems also had problems over water, where the radar beam tended to scatter forward and returned little signal to the aircraft except in high
sea state In oceanography, sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, ...
s. In such conditions, the system would fail back to a constant clearance using a
radio altimeter Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
. Terrain avoidance normally works in a relative fashion, the absolute altitudes of objects are not important. In some cases, it is desirable to provide an absolute number to indicate the amount of clearance or lack of it. The height of the top of any particular feature relative to the aircraft can then be calculated through , where H is the altitude over the ground measured by the radio altimeter, φ is the angle and R the range measured by the radar, with h being the resulting height of the object over the current flight path. The clearance between the aircraft and terrain is then .


History


Initial work at Cornell

The TFR concept traces its history to studies carried out at the
Cornell Aeronautical Laboratory Calspan Corporation is a science and technology company founded in 1943 as part of the Research Laboratory of the Curtiss-Wright Airplane Division at Buffalo, New York. Calspan consists of four primary operating units: Flight Research, Transportati ...
for the USAF Aeronautical Systems Division. This led to the development of a system known as "Autoflite." Early Airborne Interception radars used
conical scanning Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target. Conical scanning is similar in concept to the earlier lobe switching concept used ...
systems with beamwidths on the order of four degrees. When the beam hits the ground, some of the signal scatters back toward the aircraft, allowing it to measure the distance to the ground in front of it. When looking downwards at an angle, the near and far side of the radar's circular beam was spread out into an ellipse on the ground. The return from this pattern produced a "blip" that was similarly spread out on the
radar display A radar display is an electronic device to present radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets (intended or otherwise) and retu ...
and not accurate enough for terrain avoidance. It was, however, accurate enough to produce a low-resolution map-like display of the ground below the aircraft, leading to the wartime development of the
H2S radar H2S was the first airborne, ground scanning radar system. It was developed for the Royal Air Force's Bomber Command during World War II to identify targets on the ground for night and all-weather bombing. This allowed attacks outside the ran ...
. To provide the accuracy required for terrain following, TFR systems have to be based on the
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 ...
concept. The monopulse technique produces a beam of the same width as a traditional design, but adds additional information in the radio signal, often using polarization, which results in two separate signals being sent in slightly different directions while overlapping in the center. When the signals are received, the receiver uses this extra information to separate the signals back out again. When these signals are oriented vertically, the signal from the lower beam hits the ground closer to the aircraft, producing a spread-out blip as in the case of earlier radars, while the upper beam produces a similar blip but located at a slightly further distance. The two blips overlap to produce an extended ellipse. The key feature of the monopulse technique is that the signals overlap in a very specific way; if you invert one of the signals and then sum them, the result is a voltage output that looks something like a
sine wave A sine wave, sinusoidal wave, or just sinusoid is a curve, mathematical curve defined in terms of the ''sine'' trigonometric function, of which it is the graph of a function, graph. It is a type of continuous wave and also a Smoothness, smooth p ...
. The exact midpoint of the beam is where the voltage crosses zero. This results in a measurement that is both precisely aligned with the midline of the signal and is easily identified using simple electronics. The range can then be accurately determined by timing the precise moment when the zero-crossing occurs. Accuracies on the order of a meter for measurements of objects kilometers away are commonly achieved.


Development in the UK

The Cornell reports were picked up in the UK where they formed the basis of an emerging concept for a new
strike aircraft An attack aircraft, strike aircraft, or attack bomber is a tactical military aircraft that has a primary role of carrying out airstrikes with greater precision than bombers, and is prepared to encounter strong low-level air defenses while pres ...
, which would eventually emerge as the
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 ...
. The TSR-2 project was officially started with the release of GOR.339 in 1955, and quickly settled on the use of TFR to provide the required low-level performance. The
Royal Aircraft Establishment The Royal Aircraft Establishment (RAE) was a British research establishment, known by several different names during its history, that eventually came under the aegis of the Ministry of Defence (United Kingdom), UK Ministry of Defence (MoD), bef ...
built a simulator of the system using discrete electronics that filled a room. During this same period, 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) and ...
was introducing its newest
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 cap ...
, 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 ...
. The Lightning was equipped with the world's first airborne monopulse radar, the
AIRPASS AIRPASS was a British airborne interception radar and fire-control radar system developed by Ferranti. It was the world's first airborne monopulse radar system and fed data to the world's first head-up display. The name is an acronym for "Air ...
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 ...
in
Edinburgh Edinburgh ( ; gd, Dùn Èideann ) is the capital city of Scotland and one of its 32 Council areas of Scotland, council areas. Historically part of the county of Midlothian (interchangeably Edinburghshire before 1921), it is located in Lothian ...
. In the case of the Lightning, the monopulse signal was used to accurately measure the horizontal angle, in order to allow the AIRPASS computer to plot an efficient intercept course at long range. For TFR use, all that had to change was that the antenna would be rotated so it measured the vertical angle instead of horizontal. Unsurprisingly, Ferranti won the contract for the radar component sometime in 1957 or 58. Shortly after the project started, in 1959 the project lead, Gus Scott, left for Hughes Microcircuits in nearby
Glenrothes Glenrothes (; , ; sco, Glenrothes; gd, Gleann Rathais) is a town situated in the heart of Fife, in east-central Scotland. It is about north of Edinburgh and south of Dundee. The town had a population of 39,277 in the 2011 census, making i ...
, and the team was taken over by Greg Stewart and Dick Starling. The initial system was built from a surplus AI.23B AIRPASS, and could be mounted to a trailer and towed by a
Land Rover Land Rover is a British brand of predominantly four-wheel drive, off-road capable vehicles, owned by multinational car manufacturer Jaguar Land Rover (JLR), since 2008 a subsidiary of India's Tata Motors. JLR currently builds Land Rovers ...
for testing. A significant issue is that the amount of signal returned varies greatly with the terrain; a building's vertical walls produces a partial
corner cube A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves directly towards the source, but translated. The three intersecting surfaces often have square shapes. Radar ...
that returns a signal that is about 10 million times stronger than the signal from sand or dry ground. To deal with the rapidly changing signals, an automatic gain control with 100 dB of range was developed. The radar measures only relative angles in reference to the stabilized boresight line using aircraft instruments, so the aircraft's
radio altimeter Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
is used to produce a reference to calculate actual altitudes. The beamwidth of the radar was small enough that objects to either side of the aircraft's flight path might be a potential hazard if the aircraft was blown sideways or started a turn close to the object. To avoid this, the radar scanned in an O-shaped pattern, scanning vertically from 8 degrees over the flight path to 12 degrees below it, while moving a few degrees left and right of the flight path. Additionally, the system read turn rates from the instruments and moved the scanning pattern further left or right to measure the terrain where the aircraft would be in the future. Tests of the system were carried out using Ferranti Test Flight's existing
DC-3 Dakota The Douglas DC-3 is a propeller-driven airliner manufactured by Douglas Aircraft Company, which had a lasting effect on the airline industry in the 1930s to 1940s and World War II. It was developed as a larger, improved 14-bed sleeper version ...
and, starting over the winter of 1961/62, 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 ...
. The test aircraft carried cameras looking in various directions, including some looking at the aircraft instruments and radar displays. This allowed the system to be extensively examined on the ground after the flight. Each flight returned data for flights over about 100 miles, and over 250 such flights were carried out. Early tests showed random noise in the measurements which rendered the measurements useless. This was eventually traced to the automatic gain control using very high gain while at the top of the scanning pattern where the terrain was normally at long distances and required the most amplification. This had the side-effect of making spurious reflections in the antenna's
side lobe In antenna engineering, sidelobes are the lobes (local maxima) of the far field radiation pattern of an antenna or other radiation source, that are not the ''main lobe''. The radiation pattern of most antennas shows a pattern of "''lobes'' ...
s being amplified to the point of causing interference. This was addressed by moving from an O-shaped pattern to a U-shaped one, and only allowing the gain to increase when scanning upward to prevent it from re-adjusting to high gain when moving downward and thereby avoiding low-lying terrain appearing in the sidelobes with high gain. Advances in electronics during development allowed the original
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 voltage, potential difference has been applied. The type kn ...
electronics to be increasingly
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 e ...
ized, producing a much smaller system overall. As the system was further developed it was moved to a Blackburn Buccaneer for higher-speed testing. The tests were carried out from
RAF Turnhouse Royal Air Force Turnhouse or more simply RAF Turnhouse is a former Royal Air Force Sector Station located in Edinburgh, Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of G ...
at the
Edinburgh Airport Edinburgh Airport is an airport located in the Ingliston area of Edinburgh, Scotland. It was the busiest airport in Scotland in 2019, handling over 14.7 million passengers. It was also the sixth-busiest airport in the United Kingdom by t ...
, close to Ferranti's radar development site in the city. During testing, the radar was not connected to the aircraft's autopilot system and all control was manual. The curve was chosen to produce a one-half G maximum load. The path to fly was indicated by a dot in an AIRPASS
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 ...
. The pilot followed the computed path by pitching until the aircraft's velocity vector indicator, a small ring, was centred around the dot. In tests, the pilots very quickly became confident in the system and were happy to fly it at the minimum clearance setting even in bad weather. As the pilots became familiar with the system, the engineers continually reduced the selected clearance downward until it demonstrated its ability to safely and smoothly operate at an average of only clearance. This was tested against rough terrain, including mountain ridges, blind valleys and even cliff faces. It was also found to property guide over artificial objects like the
television antenna A television antenna (TV aerial) is an antenna specifically designed for use with a television receiver (TV) to receive over-the-air broadcast television signals from a television station. Television reception is dependent upon the antenna as ...
s at
Cairn O' Mounth Cairn O' Mounth/Cairn O' Mount ( gd, Càrn Mhon) is a high mountain pass in Aberdeenshire, Scotland. The place name is a survival of the ancient name for what are now the Grampian Mountains, earlier called "the Mounth" (in Gaelic: "monadh", mea ...
and the
Kirk o' Shotts transmitting station The Kirk o' Shotts transmitting station is a broadcasting and telecommunications site at The Hirst which lies just outside the village of Salsburgh which is near the town of Shotts in North Lanarkshire central Scotland. (Kirk o' Shotts means 'Chur ...
, bridges over the
River Forth The River Forth is a major river in central Scotland, long, which drains into the North Sea on the east coast of the country. Its drainage basin covers much of Stirlingshire in Scotland's Central Belt. The Gaelic name for the upper reach of th ...
, and
overhead power line An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy across large distances. It consists of one or more uninsulated electrical cables (commonly multiples of three for three-p ...
s.


Development in the US

In spite of the early start of Cornell's work, for reasons that are not well recorded, further development in the US ended for a time with the concept in a semi-complete form. This changed dramatically after the
1960 U-2 incident On 1 May 1960, a United States U-2 spy plane was shot down by the Soviet Air Defence Forces while conducting photographic aerial reconnaissance deep inside Soviet territory. The single-seat aircraft, flown by American pilot Francis Gary Power ...
, which led to the rapid switch from high-altitude flying over the
USSR The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...
to the low-altitude "penetrator" approach. In the short term, a number of terrain avoidance radars were introduced for a variety of aircraft. The first true TFR in the US was the
Texas Instruments Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globall ...
AN/APQ-101, which launched the company as the market leader in TFR for many years. In the early 1960s, they developed TFR systems for the RF-4C version of the Phantom II, the Army's Grumman OV-1 Mohawk, and the advanced AN/APQ-110 system for the
General Dynamics F-111 The General Dynamics F-111 Aardvark is a retired supersonic, medium-range, multirole combat aircraft. Production variants of the F-111 had roles that included ground attack (e.g. interdiction), strategic bombing (including nuclear weapons c ...
. For a variety of reasons, the TSR-2 project was cancelled in 1965 in favor of purchasing the F-111, a platform of similar concept based around a similar radar. In contrast to Ferranti's design, the APQ-110 offered several additional controls, including a ride quality setting for "hard", "soft" and "medium" that changed the G force of the calculated curve's descent profile from 0.25 to 1 G, while always allowing a maximum 3 G pullup. It also included a second set of electronics to provide hot-backup in case the primary unit failed, and fail-safe modes that executed the 3 G pullup in the case of various system failures.


Spread

Ultimately the F-111 ran into delays and cost overruns not unlike the TSR-2. After examining several concepts, the RAF eventually decided to use the Buccaneer. Although this platform had been extensively tested with the Ferranti radar, this potential upgrade was not selected for service. Unhappiness with this state of affairs led the RAF to begin discussions with their French counterparts and the emergence of the BAC/Dassault AFVG, an aircraft very similar to the F-111. After successful initial negotiations, the UK dropped its options on the F-111K. Shortly thereafter, Marcel Dassault began to actively undermine the project, which the French eventually abandoned in 1967. The next year, the UK government began negotiations with a wider selection of countries, leading eventually to the
Panavia Tornado The Panavia Tornado is a family of twin-engine, variable-sweep wing multirole combat aircraft, jointly developed and manufactured by Italy, the United Kingdom and West Germany. There are three primary Tornado variants: the Tornado IDS (inter ...
. Texas Instruments used their experience with the F1-11 TFR to win the radar contract for the Tornado IDS.


Use in strike aircraft


Advantages and disadvantages

Terrain following radar is primarily used by military strike aircraft, to enable flight at very low altitudes (sometimes below 100 feet/30 metres) and high speeds. Since radar detection by enemy radars and interception by
anti-aircraft Anti-aircraft warfare, counter-air or air defence forces is the battlespace response to aerial warfare, defined by NATO as "all measures designed to nullify or reduce the effectiveness of hostile air action".AAP-6 It includes surface based, ...
systems require a line of sight to the target, flying low to the ground and at high speed reduces the time that an aircraft is vulnerable to detection to a minimum by hiding the aircraft behind terrain as far as possible. This is known as
terrain mask Nap-of-the-earth (NOE) is a type of very low-altitude flight course used by military aircraft to avoid enemy detection and attack in a high-threat environment. Other, mostly older terms include "ground-hugging", "terrain masking", "flying under t ...
ing. However, radar emissions can be detected by enemy anti-aircraft systems with relative ease once there is no covering terrain, allowing the aircraft to be targeted. The use of terrain-following radar is therefore a compromise between the increased survivability due to terrain masking and the ease with which the aircraft can be targeted if it is seen. Even an automated system has limitations, and all aircraft with terrain-following radars have limits on how low and fast they can fly. Factors such as system response-time, aircraft g-limits and the weather can all limit an aircraft. Since the radar cannot tell what is beyond any immediate terrain, the flight path may also suffer from "ballooning" over sharp terrain ridges, where the altitude becomes unnecessarily high. Furthermore, obstacles such as radio antennas and electricity pylons may be detected late by the radar and present collision hazards.


Integration and use

On aircraft with more than one crew, the radar is normally used by the navigator and this allows the pilot to focus on other aspects of the flight besides the extremely intensive task of low flying itself. Most aircraft allow the pilot to also select the ride "hardness" with a cockpit switch, to choose between how closely the aircraft tries to keep itself close to the ground and the forces exerted on the pilot. Some aircraft such as the
Tornado IDS The Panavia Tornado is a family of twin-engine, variable-sweep wing multirole combat aircraft, jointly developed and manufactured by Italy, the United Kingdom and West Germany. There are three primary Tornado variants: the Tornado IDS (interd ...
have two separate radars, with the smaller one used for terrain-following. However more modern aircraft such as the
Rafale The Dassault Rafale (, literally meaning "gust of wind", and "burst of fire" in a more military sense) is a French twin-engine, canard delta wing, multirole fighter aircraft designed and built by Dassault Aviation. Equipped with a wide rang ...
with
phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
radars have a single antenna that can be used to look forward and at the ground, by electronically steering the beams.


Other uses

Terrain-following radar is sometimes used by civilian aircraft that map the ground and wish to maintain a constant height over it. Military helicopters may also have terrain-following radar. Due to their lower speed and high maneuverability, helicopters are normally able to fly lower than fixed-wing aircraft.


Alternatives

There are very few alternatives to using terrain-following radar for high-speed, low altitude flight.
TERPROM TERPROM (terrain profile matching) is a military navigation Ground Proximity Warning System (GPWS) employed on aircraft and missiles, which uses stored digital elevation data combined with navigation system and radar altimeter inputs to compute th ...
, a terrain-referenced navigation system provides a limited but passive terrain-following functionality.


See also

*
Phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
*
Active electronically scanned array An active electronically scanned array (AESA) is a type of phased array antenna, which is a computer-controlled array antenna in which the beam of radio waves can be electronically steered to point in different directions without moving the an ...


Notes


References


Citations


Bibliography

* * * * *


External links


Ferranti Strike and Terrain Following Radar
lengthy film with complete details of the AIRPASS II development and operational concept * {{cite journal , first1=F.M. , last1=Krachmalnick , first2=G.J. , last2=Vetsch , first3=Michael , last3= Wendl , author-link3 = Michael J. Wendl , title= Automatic flight control system for automatic terrain-following , journal= Journal of Aircraft , volume=5 , issue=2 , pages= 168–175 , date=1968 , doi=10.2514/3.43925 , url= http://pdf.aiaa.org/jaPreview/JA/1968/PVJAPRE43925.pdf , archive-url= https://web.archive.org/web/20090226214651/http://pdf.aiaa.org/jaPreview/JA/1968/PVJAPRE43925.pdf , archive-date=26 February 2009 Aircraft radars Ferranti Low flying