TERCOM, System and Symbol: Part One

On the opening night of the first Gulf War in 1991, seven B-52 bombers flew 14,000 miles from Barksdale Air Force base in Louisiana to launch the first attack in history with GPS-guided weapons. The bombers, whose operation was formally code-named SENIOR SURPRISE but quickly nicknamed “Secret Squirrel,” launched thirty-five Conventional Air-Launched Cruise Missiles (CALCM), each equipped with a single-channel GPS receiver. That made the CALCM was the first and only GPS-guided weapon fired during Operation Desert Storm, though GPS navigation was used by many attack aircraft.

SENIOR SURPRISE was, it turns out, the first step towards a era of cheap and easy GPS-guided weapons. But during Operation Desert Storm those thirty-five missiles fired were vastly outnumbered by their close cousin, the US Navy’s BGM-109 Tomahawk cruise missile. The two missiles shared a common engine, the Williams F107 turbofan, and they had been designed with a common guidance system in mind too. The CALCM’s were a variant of the US Air Force’s nuclear cruise missile: the nuclear version was designated AGM-86A, and it and the BGM-109 shared a guidance system built around Terrain Contour Matching, or TERCOM.

US Navy photo of a Block IV "Tactical Tomahawk" in flight. 021110-N-0000X-003 China Lake, Calif. Courtesy Wikipedia.

US Navy photo of a Block IV “Tactical Tomahawk” in flight. 021110-N-0000X-003 China Lake, Calif. Courtesy Wikipedia.

Today, we think of the first Gulf War as GPS’s coming-out party – which it was – but imagine it as the backbone of the many new weapons the US unleashed during the war – which it wasn’t. That begs the question, what actually was guiding US weapons during the first Gulf War, and why those systems never captured the imagination – for better or worse. After all, in 1992 two activists took axes to unlaunched GPS satellites because GPS was “military in its origins, military in its goals, [and] military in its development.”

TERCOM was the guidance system at the heart of the US cruise missile triad: an air-launched missile that extended the range and capabilities of the US bomber fleet (the ALCM); a submarine and ship-launched missile that gave US Navy ships a strategic nuclear weapon (the Tomahawk); and a ground-launched missile to be based in Europe as a counterpart to the Soviet intermediate-range nuclear arsenal (the Gryphon). Of the three, Tomahawk would have the most significant career. While the ground-launched Gryphon was fairly swiftly traded away in exchange for the Soviets demobilizing their intermediate-range missiles through the INF Treaty, the Tomahawk became the American military’s go-to weapon for punitive strikes during the 1990s. Tomahawks were fired against Iraqi military targets in 1993 and at Afghan and Sudanese targets in response to the East African embassy bombings in 1998, as well as used in conjunction with airstrikes in Bosnia (1995), Iraq (1998), and Serbia (1999). They remain, as journalist Mark Thompson put it in TIME magazine last year, “the curtain-raiser on U.S. military strikes since 1991’s Gulf War.”

(Since the Block III version was introduced in 1993, Tomahawk missiles without nuclear warheads have had GPS guidance in addition to the TERCOM system. The nuclear-armed version remains TERCOM only.)

How TERCOM Worked

Tomahawk flight diagram. From GAO/NSIAD-95-116 (1995), p. 17

Tomahawk flight diagram. From GAO/NSIAD-95-116 (1995), p. 17

Unlike GPS or other radionavigation systems, TERCOM did not provide continuous position information. Instead, it was an adjunct to the cruise missile’s inertial navigation system (INS), which used gyroscopes and accelerometers to measure the weapon’s movement. What TERCOM did was provide updates to the INS by comparing the measurements from the missile’s radar altimeter with stored information on the terrain the missile was flying over. Assuming this was a sufficiently “bumpy” area, the altimeter measurements should match only location – providing a precise position fix. By using TERCOM to update its location three times during flight, the cruise missile had far superior accuracy to a missile using only inertial navigation. (On non-nuclear missiles, a separate system known as Digital Scene Matching Area Correlation [DSMAC] offered even more precise guidance for the final stage of the flight.)

Making Maps for TERCOM

The terrain information which the TERCOM system used (called a “matrix”) was based on a database of Digital Terrain Elevation Data (DTED), which contained the elevation above sea level for all the points on a (roughly) 300 foot grid. Luckily enough, the Defense Mapping Agency (DMA) had already been collecting this information for a far more mundane purpose: to use in flight simulators. TERCOM, however, required much, much more of this information. The DMA had been involved in the early testing of TERCOM but the agency wasn’t involved in the decision to put the cruise missiles using the technology into full-scale development. DMA had only been creating TERCOM matrices by hand (so to speak), and now it was looking at the requirement for more than 8,000 of them. According to Jay L. Larson and George A. Pelletiere, “it took DMA five years of ‘crash’ effort to fulfill these requirements.” Demands for TERCOM matrices had to be triaged, with three-quarters of the effort going to the ALCM and the other quarter to the submarine-launched and ground-launched cruise missile programs. If the US military had gone to war in the interim, they would have been unable to use cruise missiles except in particular areas.

To extend the Digital Terrain Elevation Data database over the entire world, and particularly over areas like the interior of the Soviet Union, the DMA used photography collected by the United States’ reconnaissance satellites. The technical principle involved, stereogrammetry, had been well understood since the First World War. Start with two photographs of the same scene taken from slightly different vantage points, such as two moments along the flight path of a plane or satellite. Look at the two images, one with each eye, and move them until they like a single three-dimension image – just like how the human brain normally combines the images from each eye … if your eyes were several hundred meters apart.

If you know the distance between the points from which the images were taken (that hundred metre bridge of the nose) and the angles between the two viewpoints and an object in the image, you can calculate the size of that object. Add in the altitude from which the images were taken and you can calculate the elevation. Conventionally, stereogrammetry is done using a human brain to process illuminated transparencies or with mirrors and hardcopy prints. In the early 1960s computerized system like the Universal Automated Map Compilation Equipment (UNAMACE) took over, speeding up the process tremendously.

UNAMACE. From Hexagon (KH-9) Mapping Camera Program and Equipment (CSNR, 2012), p. 301.

Pictures of UNAMACE, from Hexagon (KH-9) Mapping Camera Program and Equipment (CSNR, 2012), p. 301.

Even once TERCOM matrices were readily available, the mission planning process remained long and complex. During the first Gulf War, it had to be performed at a specially equipped facility (one of the Navy’s two Cruise Missile Support Activities) and took between 24 and 80 (!) hours.

The US cruise missile program attracted a lot of controversy in the 1970s and 1980s, particularly from critics who argued that a new, flexible, and less detectable nuclear weapons delivery system could be destabilizing or even provocative, but TERCOM itself did not itself draw that much of the attention. The exception was up in Canada, where a guidance system subcontractor found itself the focus of a blast of “direct action” against the cruise missile.

Forward to Part Two


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