involving use of the magnetic field of the earth

Full Title

Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 (measuring distance traversed on the ground by a vehicle G01C22/00; control of position, course, altitude or attitude of vehicles G05D1/00; traffic control systems for road vehicles involving transmission of navigation instructions to the vehicle G08G1/0968) > by terrestrial means (G01C21/24, G01C21/26 take precedence) > involving use of the magnetic field of the earth