A global positioning system (gps) receiver, a laptop computer and some software can be used to warn pilots if they are flying too close to the ground. California-based Dubbs and Severino has developed the software from technology originally researched by the US National Aeronautics and Space Administration. Military pilots are testing the system in older aircraft, and the company hopes to develop a civilian version this year. A conventional system costs about us $100,000. The software and a cd-rom map package will cost about us $5,000. The laptop and the gps receiver would be about US $1,200.