Terrain contour matching, or TERCOM, is a navigation system used primarily by
cruise missiles. It uses a pre-recorded
contour map
A contour line (also isoline, isopleth, or isarithm) of a function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value. It is a plane section of the three-dimensional grap ...
of the terrain that is compared with measurements made during flight by an on-board
radar altimeter
A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it t ...
. A TERCOM system considerably increases the accuracy of a missile compared with
inertial navigation system
An inertial navigation system (INS) is a navigation device that uses motion sensors (accelerometers), rotation sensors ( gyroscopes) and a computer to continuously calculate by dead reckoning the position, the orientation, and the velocity (dir ...
s (INS). The increased accuracy allows a TERCOM-equipped missile to fly closer to obstacles and generally lower altitudes, making it harder to detect by ground radar.
Description
Optical contour matching
The
Goodyear Aircraft Corporation Goodyear may relate to:
Companies
* Goodyear Tire and Rubber Company
* Goodyear Redwood Company, foresting company operated from 1916 to 1932
* Edward Goodyear, a florist company in London, England
* Goodyear Lumber Company, one of many lumber ...
ATRAN (''Automatic Terrain Recognition And Navigation'') system for the
MGM-13 Mace
The Martin Mace was a ground-launched cruise missile developed from the earlier Martin TM-61 Matador. It used a new self-contained navigation system that eliminated the need to get updates from ground-based radio stations, and thereby allowed i ...
was the earliest known TERCOM system. In August 1952,
Air Materiel Command
Air Materiel Command (AMC) was a United States Army Air Forces and United States Air Force command. Its headquarters was located at Wright-Patterson Air Force Base, Ohio. In 1961, the command was redesignated the Air Force Logistics Command wi ...
initiated the mating of the Goodyear ATRAN with the
MGM-1 Matador
The Martin MGM-1 Matador was the first operational surface-to-surface cruise missile designed and built by the United States. It was developed after World War II, drawing upon their wartime experience with creating the Republic-Ford JB-2, a ...
. This mating resulted in a production contract in June 1954. ATRAN was difficult to jam and was not range-limited by line-of sight, but its range was restricted by the availability of radar maps. In time, it became possible to construct radar maps from
topographic map
In modern mapping, a topographic map or topographic sheet is a type of map characterized by large- scale detail and quantitative representation of relief features, usually using contour lines (connecting points of equal elevation), but histori ...
s.
Preparation of the maps required the route to be flown by an aircraft. A
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, we ...
on the aircraft was set to a fixed angle and made horizontal scans of the land in front. The timing of the return signal indicated the range to the landform and produced an
amplitude modulated (AM) signal. This was sent to a light source and recorded on
35 mm film, advancing the film and taking a picture at indicated times. The film could then be processed and copied for use in multiple missiles.
In the missile, a similar radar produced the same signal. A second system scanned the frames of film against a
photocell
Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or b ...
and produced a similar AM signal. By comparing the points along the scan where the brightness changed rapidly, which could be picked out easily by simple electronics, the system could compare the left-right path of the missile compared with that of the pathfinding aircraft. Errors between the two signals drove corrections in the autopilot needed to bring the missile back onto its programmed flight path.
Altitude matching
Modern TERCOM systems use a different concept, based on the altitude of the ground over which missile flies and measure by radar altimeter of the missile and comparing that to measurements of prerecorded terrain altitude maps stored in missile avionics memory. TERCOM "maps" consist of a series of squares of a selected size. Using a smaller number of larger squares saves memory, at the cost of decreasing accuracy. A series of such maps are produced, typically from data from radar mapping satellites. When flying over water, contour maps are replaced by magnetic field maps.
As a radar altimeter measures the distance between the missile and the terrain, not the absolute altitude compared to sea level, the important measure in the data is the change in altitude from square to square. The missile's radar altimeter feeds measurements into a small buffer that periodically "gates" the measurements over a period of time and averages them out to produce a single measurement. The series of such numbers held in the buffer produce a strip of measurements similar to those held in the maps. The series of changes in the buffer is then compared with the values in the map, looking for areas where the changes in altitude are identical. This produces a location and direction. The guidance system can then use this information to correct the flight path of the missile.
During the cruise portion of the flight to the target, the accuracy of the system has to be enough only to avoid terrain features. This allows the maps to be a relatively low resolution in these areas. Only the portion of the map for the terminal approach has to be higher resolution, and would normally be encoded at the highest resolutions available to the satellite mapping system.
TAINS
Due to the limited amount of memory available in mass storage devices of the 1960s and 70s, and their slow access times, the amount of terrain data that could be stored in a missile-sized package was far too small to encompass the entire flight. Instead, small patches of terrain information were stored and periodically used to update a conventional
inertial platform. These systems, combining TERCOM and inertial navigation, are sometimes known as TAINS, for TERCOM-Aided Inertial Navigation System.
Advantages
TERCOM systems have the advantage of offering accuracy that is not based on the length of the flight; an inertial system slowly drifts after a "fix", and its accuracy is lower for longer distances. TERCOM systems receive constant fixes during the flight, and thus do not have any drift. Their absolute accuracy, however, is based on the accuracy of the radar mapping information, which is typically in the range of meters, and the ability of the processor to compare the altimeter data to the map quickly enough as the resolution increases. This generally limits first generation TERCOM systems to targets on the order of hundreds of meters, limiting them to the use of
nuclear warhead
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...
s. Use of conventional warheads requires further accuracy, which in turn demands additional terminal guidance systems.
Disadvantages
The limited data storage and computing systems of the time meant that the entire route had to be pre-planned, including its launch point. If the missile was launched from an unexpected location or flew too far off-course, it would never fly over the features included in the maps, and would become lost. The INS system can help, allowing it to fly to the general area of the first patch, but gross errors simply cannot be corrected. This made early TERCOM-based systems much less flexible than more modern systems like
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
, which can be set to attack any location from any location, and do not require pre-recorded information which means they can be given their targets immediately before launch.
Improvements in computing and memory, combined with the availability of global
digital elevation map
A digital elevation model (DEM) or digital surface model (DSM) is a 3D computer graphics representation of elevation data to represent terrain or overlaying objects, commonly of a planet, moon, or asteroid. A "global DEM" refers to a discrete gl ...
s, have reduced this problem, as TERCOM data is no longer limited to small patches, and the availability of
side-looking radar allows much larger areas of landscape contour data to be acquired for comparison with the stored contour data.
Comparison with other guidance systems
DSMAC, Digital Scene Matching Area Correlator
DSMAC was an early form of
AI which could guide missiles in real time by using camera inputs to determine location. DSMAC was used in Tomahawk Block II onward, and proved itself successfully during the first Gulf War. The system worked by comparing camera inputs during flight to maps computed from spy satellite images. The DSMAC AI system computed contrast maps of images, which it then combined in a buffer and then averaged. It then compared the averages to stored maps computed beforehand by a large mainframe computer, which converted spy satellite pictures to simulate what routes and targets would look like from low level. Since the data were not identical and would change by season and from other unexpected changes and visual effects, the DSMAC system within the missiles had to be able to compare and determine if maps were the same, regardless of changes. It could successfully filter out differences in maps and use the remaining map data to determine its location. Due to its ability to visually identify targets instead of simply attacking estimated coordinates, its accuracy exceeded GPS guided weapons during the first Gulf War.
["Image Processing For Tomahawk Scene Matching". Johns Hopkins APL Technical Digest, Volume 15, Number 3. Geoffrey B. Irani and James P. Christ.]
The massive improvements in memory and processing power from the 1950s, when these scene comparison systems were first invented, to the 1980s, when TERCOM was widely deployed, changed the nature of the problem considerably. Modern systems can store numerous images of a target as seen from different directions, and often the imagery can be calculated using image synthesis techniques. Likewise, the complexity of the live imaging systems has been greatly reduced through the introduction of solid-state technologies like
CCDs. The combination of these technologies produced the digitized scene-mapping area correlator (DSMAC). DSMAC systems are often combined with TERCOM as a terminal guidance system, allowing point attack with conventional warheads.
MGM-31
Pershing II
The Pershing II Weapon System was a solid-fueled two-stage medium-range ballistic missile designed and built by Martin Marietta to replace the Pershing 1a Field Artillery Missile System as the United States Army's primary nuclear-capable thea ...
,
SS-12 Scaleboard Temp-SM and
OTR-23 Oka
The OTR-23 Oka (russian: OTP-23 «Ока»; named after Oka River) was a mobile theatre ballistic missile (russian: оперативно-тактический ракетный комплекс) deployed by the Soviet Union near the end of the Co ...
used an
active radar homing version of DSMAC (digitized correlator unit DCU), which compared radar
topographic map
In modern mapping, a topographic map or topographic sheet is a type of map characterized by large- scale detail and quantitative representation of relief features, usually using contour lines (connecting points of equal elevation), but histori ...
s taken by satellites or aircraft with information received from the onboard active radar regarding target topography, for terminal guidance.
Satellite navigation
Yet another way to navigate a cruise missile is by using a
satellite positioning system
A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
as they are precise and cheap. Unfortunately, they rely on satellites. If the satellites are interfered with (e.g. destroyed) or if the satellite signal is interfered with (e.g. jammed), the satellite navigation system becomes inoperable. Therefore, the GPS-based (or GLONASS-based) navigation is useful in a conflict with a technologically unsophisticated adversary. On the other hand, to be ready for a conflict with a technologically advanced adversary, one needs missiles equipped with TAINS and DSMAC.
Missiles that employ TERCOM navigation
The cruise missiles that employ a TERCOM system include:
*
''Supersonic Low Altitude Missile'' project (early version of TERCOM was slated to be used in this never-built missile)
*
AGM-86B
The AGM-86 ALCM is an American subsonic air-launched cruise missile (ALCM) built by Boeing and operated by the United States Air Force. This missile was developed to increase the effectiveness and survivability of the Boeing B-52H Stratofortre ...
(United States)
*
AGM-129 ACM
The AGM-129 ACM (Advanced Cruise Missile) was a low-observable, subsonic, turbofan-powered, air-launched cruise missile originally designed and built by General Dynamics and eventually acquired by Raytheon Missile Systems. Prior to its withdraw ...
(United States)
*
BGM-109 Tomahawk (some versions, United States)
*
C-602
The YJ-62 () is a Chinese subsonic anti-ship cruise missile. It is manufactured by the China Aerospace Science and Industry Corporation Third Academy.
Description
In a September 2014 article published in Joint Forces Quarterly, the YJ-62 is c ...
anti-ship & land attack cruise missile (China)
*
Kh-55 Granat
NATO reporting name AS-15 Kent (Soviet Union)
*Newer Russian cruise missiles, such as Kh-101 and Kh-555 are likely to have TERCOM navigation, but little information is available about these missiles
*
C-802 or YJ-82
NATO reporting name CSS-N-8 Saccade (China) – it is unclear if this missile employs TERCOM navigation
*
Hyunmoo III (South Korea)
*
DH-10 (China)
*
Babur (Pakistan) land attack cruise missile
*
Ra'ad
The Ra'ad ( ur, رعد, "Thunder") is a Pakistani air-launched cruise missile (ALCM) . The missile was first tested in August 2007.
Development and design
The Ra'ad is a joint project developed by Pakistan Air Force's Air Weapons Complex and ...
(Pakistan) air-launched cruise missile
*
Naval Strike Missile
The Naval Strike Missile (NSM) is an anti-ship and land-attack missile developed by the Norwegian company Kongsberg Defence & Aerospace (KDA).
The original Norwegian name was Nytt sjømålsmissil (literally ''New sea target missile'', indicatin ...
(anti-ship and land attack missile, Norway)
*
SOM (missile)
The SOM ( tr, ) is a next-generation autonomous, stealth, high precision cruise missile developed by TÜBİTAK SAGE, Defence Research and Development Institute of Turkey. It was first revealed during the 100th anniversary celebrations of the Tu ...
(air-launched cruise missile, Turkey)
*
HongNiao
HN (reported short for Hong Niao, or Hongniao, 红鸟 meaning Red Bird) missiles are a series of turbofan powered Chinese land attack cruise missiles, based on the X-600.
Development
Although China was satisfied with the performance of its own ...
1/2/3 cruise missiles
*
9K720 Iskander
The 9K720 Iskander (russian: «Искандер»; NATO reporting name SS-26 Stone) is a mobile short-range ballistic missile system produced and deployed by the Russian military. The missile systems () are to replace the obsolete OTR-21 ''Toc ...
(
short-range ballistic missile
A short-range ballistic missile (SRBM) is a ballistic missile with a range of about or less. In past and potential regional conflicts, these missiles have been and would be used because of the short distances between some countries and their rela ...
and cruise missile variants, Russia)
See also
*
Missile guidance
Missile guidance refers to a variety of methods of guiding a missile or a guided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness. Guidance systems improve missile accuracy by improving its P ...
*
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 ...
References
{{Reflist
External links
"Terrestrial Guidance Methods" Section 16.5.3 of Fundamentals of Naval Weapons Systems
Missile guidance
Aircraft instruments
Aerospace engineering