Nonmilitary GPS specialists say the United States probably has been selectively jamming signals from the Global Positioning System (GPS) in Afghanistan since the start of the air campaign.
These experts emphasize that the jamming in Afghanistan will have no effect on civilian users, including airlines, which increasingly rely on GPS for transoceanic navigation. Signals from the GPS satellite system available to civilian users provide an accuracy of 36 meters or better, while separate, encrypted military signals used to guide so-called smart bombs in Afghanistan provide accuracy within 6 meters, according to Richard Langley, a professor of geodesy and precision navigation at the University of New Brunswick. Langley's Web site plots the GPS military signal over Kabul as of October 11.

Langley said the Pentagon has developed the capability to jam civilian GPS signals within a specific targeted area and could easily deny the 36-meter-accuracy civilian signal to the Taliban forces without interfering with users in other areas of the world. Depending on whether the Pentagon -- which developed and operates the 28-satellite GPS constellation -- uses airborne or ground jammers, this could deny the signal to the Taliban over a wide area, with some of the jamming potentially spilling over into Pakistan.

GPS receivers, which sell for as little as $100 for a simple version used by hikers, plot position through sophisticated triangulation operations with at least three GPS satellites. Area jamming would prevent GPS receivers used by the Taliban from locking on to the satellites and deriving a highly accurate location. The Pentagon has developed this jamming capability to ensure that enemies don't use the GPS signal to guide their own smart missiles or bombs in an attack on U.S. forces.

Sam Wormley, a researcher at Iowa State University in Ames and manager of an authoritative GPS resources and accuracy Web site, agreed that the Pentagon has the capability to jam civilian GPS signals in a given area without interfering with more precise military signals. Wormley said that's because the military signals occupy a different and smaller slice of the GPS frequency band than that used by the civilian signals.

The Pentagon doesn't discuss its GPS jamming capabilities, especially in time of war.(from CNN.com)