A modular avionics system for an Unmanned Aerial Vehicle (UAV) has a control module that executes flight control and vertical and lateral guidance algorithms to generate control commands. A data link module communicates with a remote control station and receives control commands from the remote control station. A data acquisition module communicates with the control module and the data link module. The data acquisition module is configured to receive and process data from one or more onboard sensors and to actuate a plurality of servo motors in response to control commands. A switching module selectively couples the data acquisition module to the control module or to the data link module responsive to an input from the remote control station to respectively switch between a fully autonomous mode of UAV operation and a manual mode of UAV operation. Power may be provided by a power module.