This is also known as inertial navigation, widely used in airliners and cruise missiles. With enough money and time, very good accuracy is possible. By using lots of laser gyros and complex computing, the airliners manage very good accuracy. - Robert :O) -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body