A control system (100) for an unmanned vehicle (10) includes a control processing unit (102) which receives input signals from one or more sensors (101) and supplies output signals to one or more actuators (103). Processing modules (110) are divided into a plurality of successively linked stages (120). In a first stage (120A), multiple processing modules (110) produce substantially equivalent payload data when operating correctly. Message cryptography units (140) generate cryptographically signed messages (800) containing the payload data. In a second stage, a voting cryptography unit (130) receives and examines the cryptographically signed messages, and applies voting logic to derive a validated payload data for use by the respective processing module (120) of the second stage. A control method is also described.