1. A combined autonomous device with both true north finding and navigation functionalities, comprising: a) an INS unit which comprises: i. an IMU which in turn comprises a set of at least three gyros and at least three accelerometers, all mounted on a rotatable stage; and ii. an INS algorithm for measuring the behavior of said gyros and said accelerometers during a mission, and calculating a navigation solution based on said measurements; b) an inertial-type autonomous true north finding unit, which comprises: i. one or more from said IMU gyros and one or more from said IMU accelerometers, said one or more IMU gyros and accelerometers are common to said INS unit and to said true north finding unit; and ii. a true north finding algorithm which utilizes measurements from said common gyros and accelerometers during an initial conditions stationary state in which said stage is positioned in at least two separate stationary orientations, said north finding algorithm determines a north finding solution which is provided to the INS unit for initializing its said INS algorithm: wherein during navigation, said stage is positioned at a fixed non-rotated orientation or managed between various angular positions.