The US Navy plans to upgrade its Tactical Tomahawk cruise missiles with PTAN (Precision Terrain-Aided Navigation) systems to defeat the expected threat of GPS jamming. Tactical Tomahawks are equipped with GPS satellite navigation, an inertial navigation system, and a Digital Scene Camera. The ability to jam GPS signals is easily understood and many potential enemies are already working to blanket their airspace with signals that jam, degrade, or corrupt the GPS signals. The PTAN technology (expected to reach the front lines by 2003, probably after GPS jamming) is based on a recent Space Shuttle mission which mapped 80% of the Earth's surface between 60-degrees North and 56-degrees South.--Stephen V Cole