TERCOM

TERCOM
Name	TERCOM
Type	Terrain contour matching
Used by	United States Air Force, United States Navy
Manufacturer	McDonnell Douglas
Designer	Charles Stark Draper Laboratory
Inception	1960s
Service	1970s–present

TERCOM. Terrain Contour Matching is a passive navigation system used primarily by cruise missiles to achieve a high degree of accuracy without relying on external signals. It functions by comparing measured terrain elevations against a pre-stored digital map to determine the weapon's position and guide it to its target. This technology was a critical enabling component for long-range, low-altitude strategic weapons during the Cold War.

Overview

The core function of this guidance method is to provide autonomous, all-weather terminal guidance for air-launched cruise missiles and ground-launched cruise missiles. It represented a significant technological leap from earlier systems dependent on celestial navigation or vulnerable radio navigation aids. By enabling missiles to fly complex, terrain-hugging routes at very low altitudes, it dramatically improved their ability to penetrate sophisticated air defense networks operated by nations like the Soviet Union. The integration of this technology was pivotal for weapons like the AGM-86 ALCM and the BGM-109 Tomahawk.

Technical principles

The system operates by utilizing a radar altimeter to measure the height of the terrain directly below the missile during flight. These real-time elevation profiles are then cross-referenced with a series of digitized maps stored in the weapon's onboard computer. The matching algorithm, often involving correlation analysis, identifies the missile's precise location by finding the best fit between the measured contour and the reference data. This process occurs periodically along the flight path, with updates correcting the inertial navigation system that controls the missile between fixes. Key components enabling this include advanced microprocessors and high-density data storage media developed by contractors such as Honeywell and Texas Instruments.

Development and history

Initial research into terrain correlation concepts began in the 1950s, with significant impetus provided by the Advanced Research Projects Agency. The need for a survivable guidance system for the proposed Supersonic Low Altitude Missile (SLAM) drove early experimentation. Following the cancellation of that program, development continued through the 1960s, with major contributions from the Charles Stark Draper Laboratory and MIT Lincoln Laboratory. The system saw its first successful full-scale tests in the early 1970s, leading to its deployment on the U.S. Navy's Subroc weapon and later as the cornerstone of the United States Department of Defense's cruise missile initiatives. Its success directly influenced parallel developments in the Soviet Union, leading to similar systems on missiles like the RK-55 Relief.

Operational use

This navigation method entered widespread service with the fielding of the BGM-109 Tomahawk in the 1980s. It allowed these weapons to execute precise, long-range strikes from platforms such as Los Angeles-class submarines and Ticonderoga-class cruisers. During the Gulf War, missiles using this guidance famously demonstrated their capability by striking high-value targets in Baghdad. The system remains in use on later variants of the Tomahawk, though often integrated with Global Positioning System updates. Other platforms that have employed it include the AGM-129 ACM and the now-retired AGM-86 ALCM, launched from aircraft like the B-52 Stratofortress.

Advantages and limitations

A primary advantage is its complete independence from external radio frequency emissions, making it immune to jamming and unaffected by the degradation or denial of satellite networks. It provides exceptional accuracy for striking fixed targets, even in electronic warfare environments. However, its effectiveness is constrained by the requirement for detailed, pre-mission terrain intelligence over the entire flight corridor. Significant changes to the landscape, such as from seismic activity or seasonal snow cover, can degrade performance. Furthermore, the system is generally unsuitable for use over very flat or uniformly featureless terrain like open ocean or deserts, and it provides no capability against moving targets.

Related navigation systems

To overcome its limitations, it is often fused with other technologies in a hybrid guidance suite. Digital scene matching area correlator (DSMAC) provides terminal guidance by matching optical images of the target area. The widespread adoption of the Global Positioning System has largely supplanted it for mid-course updates due to GPS's global coverage and consistency. Modern developments include Terrain Profile Matching (TERPROM), used in aircraft like the Panavia Tornado, and more advanced image-based navigation systems. Research into next-generation autonomous navigation continues at institutions like the Air Force Research Laboratory focusing on alternatives like scene matching and LiDAR-based correlation.

Category:Guided missiles Category:Avionics Category:Military robotics Category:Navigation