On Saturday the U.S. and its allies fired more than 110 Tomahawk cruise missiles at targets in Libya. Tomahawks are equipped with GPS satellite navigation systems, but GPS signals, transmitted at low power from distant satellites, are uniquely susceptible to jamming.

It turns out that everyone from North Korea to truck drivers does it all the time, and militaries around the world are likely to have the capability:

A 1-kilowatt jammer can block a military GPS receiver from as far away as 80 kilometers (50 miles). A Russian company recently marketed a 4-[kilo]watt jammer that can deny a standard GPS signal within up to 200 kilometers (125 miles)

Fortunately, cruise missiles have been around even longer than GPS satellite navigation, so they have a second system to allow them to be as accurate as possible: Terrain Contour Matching.

Terrain Contour Matching, or TERCOM “uses a pre-recorded contour map of the terrain that is compared to measurements made during flight by an on-board radar altimeter.” In other words, elaborate, Google Earth-style satellite maps of the terrain of interest can be compared to data gathered by an onboard radar system that is constantly bouncing signals off the ground below in order to measure changes in elevation.

Such a system is much more difficult to jam; even if the radar on which it depends received interference, TERCOM is combined with another, even older method of navigation known as an intertial navigation system (INS).

Inertial navigation works on dead reckoning – if you know where you used to be, and all of the changes in direction and velocity since that point in time and space, you can calculate where you are now.

What’s interesting about these systems is that when satellite navigation fails, cruise missiles revert to computerized versions of navigation methods that would have been recognizable to navigators centuries ago. Dead reckoning has been around at least since the 16th century, and anyone who has ever tried to triangulate their location on a hike using points on a topographic map has exploited the same underlying principles as TERCOM.

Modern cruise missiles have one other navigation system that allows them maximum accuracy during the final moments of their approach: Digital Scene Matching Area Correlation (DSMAC), “which compares a stored image of target with the actual target image.” In other words, there’s no substitute for simply confirming that the appearance of a building on a particular block matches your memory of it – especially when it’s your intent to blow it up.

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