A method for operating an autonomous vehicle at an airport comprises the steps of: receiving a set of general purpose navigation signals; receiving a supplementary set of navigation signals; receiving constraint data representing a permitted area of operation; calculating a present position of the vehicle; comparing the calculated present position of the vehicle with the constraint data, thereby to determine whether the vehicle's present position lies within the permitted area; and producing a signal indicative of the result of said comparing step. The autonomous vehicle for use at an airport, comprising: a navigation processor for calculating a present position of the vehicle; a constraint store for storing constraint data indicating a permitted area of operation; navigation receivers for receiving general purpose navigation signals and supplementary navigation signals, for guiding aircraft to land at the airport; and a comparator for comparing the constraint data with the present position.