The Economist explains

Why isn’t Russia blocking GPS in Ukraine?

It is capable of drowning out Ukrainian signals—or even striking satellites

HE9PG5 A High Mobility Artillery Rocket System crew member assembles an antenna prior to radio check operations in support of Emerald Warrior April 29, 2015, at Melrose Air Force Range N.M. Emerald Warrior is the Department of Defense's only irregular warfare exercise, allowing joint and combined partners to train together and prepare for real world contingency operations. Staff Sgt. Matthew Plew)

THE PATTERN of craters across Antonovsky bridge in Kherson, a city in southern Ukraine, illustrates the power of a precision strike. The bridge, one of only two main routes into or out of the city across the Dnipro river, was rendered impassable on July 27th by Ukrainian soldiers, who are fighting to retake Kherson from Russian forces. They used a handful of missiles fired from an American-supplied HIMARS launcher, which relies on global positioning system (GPS) signals to find their target. GPS is a powerful tool of war, but it can often be rendered useless by the enemy. What could Russia do to prevent more strikes of this kind?

The NavStar Global Positioning System was developed by America’s air force in the 1970s. It was the first system to provide portable, accurate navigation anywhere on earth. The receiver picks up precise time signals from several satellites and calculates its position by triangulation. GPS-guided munitions have almost completely replaced unguided bombs in America’s arsenal. Many other weapons, from missiles to artillery shells, are also GPS-enabled. Much of Ukraine’s older Russian-made weaponry is not so advanced. But America and other backers are providing GPS-guided rockets and artillery shells. The technology is expensive but almost guarantees a hit, assuming the target is stationary and its location is known.

But GPS can be vulnerable. The satellites that power it are around 20,000 kilometres from Earth, but their transmitters are no more powerful than a car’s headlight. Their weak signals can be drowned out by radio transmitters operating on the same wavelengths. Some are harder to jam than others. Military GPS receivers may use “M-code”, a military-only signal. Some receivers have electronic filters to separate signals from noise, and directional antennas to pick up only signals from satellites. American weapons, such as HIMARS rockets, also have back-up inertial guidance, which measures acceleration and uses it to extrapolate distance and direction of travel. This kicks in if GPS fails to guide the missile.

Still, Dana Goward, president of the Resilient Navigation and Timing Foundation, an advocacy group, and a member of America’s government advisory board on GPS, reckons that Russia could do more to prevent precision strikes in Ukraine. It may be holding back on jamming to keep some capability in reserve for a possible conflict with NATO, he says. A high-powered jammer is the radio equivalent of a lighthouse, making it a very visible target. Some Russian forces may even be using GPS instead of GLONASS, their own equivalent (but less reliable) technology. Britain’s defence minister, Ben Wallace, has said that some Russian pilots are taping commercial GPS receivers to their instrument panels. The Kremlin may have decided to limit jamming to avoid disrupting its own forces.

Russia is also capable of rendering GPS satellites completely useless, either using a cyber or physical attack. But it is highly unlikely to take such a dramatic step simply to impede Ukrainian strikes. An attack on GPS would be an attack on America, and thus risk drawing NATO into the conflict. Nevertheless, to counter such a threat, America has long been developing alternatives to satellite navigation. In June it tested a system called RATS on a B-2 bomber. Most systems of this kind use the plane’s radar to precisely find its location by comparing the ground below it with a map, a form of the “terrain contour matching”, or TERCOM, used by cruise missiles for decades. Meanwhile, as Ukraine receives more GPS-guided rocket launchers from America and Britain, Russia may increase its jamming efforts. But for the time being, satellite-guided munitions are still hitting targets with impressive precision.

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This article appeared in the The Economist explains section of the print edition under the headline "Why isn’t Russia blocking GPS in Ukraine?"

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