http://www.nytimes.com/2010/02/24/world/asia/24satellites.html

Afghanistan's mountains can weaken or block satellite signals that help guide American troops.

OFFUTT AIR FORCE BASE, Neb. — Across the unforgiving terrain of Afghanistan, American combat forces have come to rely on satellites as well as their rifles and body armor to carry out missions effectively, and to stay alive.

But American units have found that satellite signals are weakened and even blocked outright by the breathtaking peaks and backbreaking valleys of Afghanistan — making it hard to pinpoint the troops’ location, navigate on patrol, identify friend from foe in battle or call in bombs and artillery when under attack.

So the top officer of the military’s Strategic Command, which is better known for control of the nation’s nuclear arsenal, has ordered up what might be called a “satellite surge” to increase the coverage and accuracy for GPS devices in the war zone.

The constellation of operational satellites that allows GPS devices to work is being expanded over the next year or two to 27 from 24.

The increase will benefit civilians as well as soldiers. Drivers, sailors, hikers and golfers around the globe will share in the improved performance of their GPS devices.

“We’ve got more than 24 satellites up there,” said Gen. Kevin P. Chilton, the top officer at Strategic Command, during an interview at his headquarters here. “Can we better optimize support to the regional war fighter, particularly as you look at the terrain in Afghanistan?”

His order to increase the number of operational GPS satellites was intended to assist coalition forces carrying out missions in Afghanistan’s deep valleys, where troops say they lose signals because of the narrow window to the sky between mountain peaks and within canyon walls.

Four separate satellite signals are required to pinpoint a location on the earth, including elevation. The more satellites overhead, the more opportunities to get the minimum number of signals. Additional readings beyond four provide even greater accuracy.

While it might seem odd that the commander of America’s nuclear arsenal would spend time focusing on the conventional mission in Afghanistan, General Chilton visited the country in late January for meetings with Lt. Gen. David M. Rodriguez, the No. 2 American commander, who is in charge of day-to-day operations there.

In an e-mail message from his headquarters in Afghanistan, General Rodriguez said he welcomed the increased satellite coverage and noted that “the additional capability will help support the operations here.”

The military always keeps extra GPS satellites in space, hibernating in orbit close to active satellites, but ready to be called from standby status should an operating satellite fail or fall out of position.

Moving the three new satellites into position involves a delicate, calculated effort to expend as little fuel as possible, and then let inertia gently guide the satellite into place without wasting energy. Satellites are expensive to build and expensive to lift into space; once a satellite’s fuel is gone, it is lost.

So while the need for improved GPS signaling to Afghanistan is a priority, the long-term obligation of preserving fuel aboard the satellites prompted Strategic Command, and its subordinate unit overseeing satellites, Air Force Space Command, to allow up to two years to get the new satellites in place.

The first of the new satellites has already begun sailing toward its operating orbit. A Strategic Command spokesman said the second and third additional satellites could be in place as early as next January, a year ahead of the official schedule.