A spacecraft control electronics system provides multi-functionality to a spacecraft in a single electronics box. The system is subdivided into a plurality of modular electronic subunits. Each of the modular subunits plugs into a backplane in the electronics box and is positioned side-by-side. A serial data bus in the backplane of the box interconnects the modular subunits to each other. The data bus provides fully redundant, standard interfacing for the modular subunits. The plurality of electronic subunits provides spacecraft attitude determination, control, telemetry and command and data processing functions to the spacecraft as one unit. The electronics box connects to the spacecraft harness via external connectors. The spacecraft has a minimum number of harness connections as a result of the integrated functions in the spacecraft control electronics system. The control system applies selective internal redundancy in its subunits. The CPU subunit has triple mode redundancy through three microprocessors that are voted together to detect and correct errors due to single event upsets. The T&C/GPS subunit has an embedded GPS receiver that performs attitude determination as well as orbital positioning. The modular subunits have a built in self-test that verifies minute circuitry interconnections and detects faults automatically. The subunits are tested as stand-alone subunits, as a part of the spacecraft control electronics system, and/or as part of the spacecraft level integration using this automated built-in self test capability. Faults in the subunits or the system are detected in seconds. The built in self-test feature also provides an end-to-end spacecraft harness verification automatically in minutes. Defective modular subunits are removed and replaced with substitute modular subunits during spacecraft level integration.