Delfi-C³ Command and Data Handling Subsystem
The Command and Data Handling Subsystem (CDHS) provides command handling, data handling, onboard autonomy and configurability. The Delfi-C³ reliability philosophy is fully autonomous OnBoard Computer (OBC) for nominal centrally controlled operations and a decentralized backup mode in which the OBC plays no role. The OBC is a commercial FM-430 board from Pumpkin Inc. It controls the whole satellite, but each subsystem has one or more microcontrollers for local control. Most internal communication takes place via I²C on the System Bus.
Every subsystem has a local microcontroller (PIC) and a local EPS part, under control of the CDHS. A combination board is used as an interface-to-interface between the FM-430 with a PC/104 interface and our custom developed bus interface which is used by all other subsystems. It also houses the receiver side of the sun sensor payload.
The OBC gathers data every second and packs this in an alternating series of housekeeping and payload frames which are sent to the radio transceiver, which on its turn directly sends these frames in AX.25 format. There is no data storage onboard, so reception of the data is dependent on visibility of the satellite by ground stations.
The in-orbit results show some anomalies in the I²C communication. It has been found that the low speed microcontrollers cannot cope with the relatively fast and heavy bus traffic, even though the I²C specification is followed and clock stretching is allowed. Fortunately, this issue did not have a negative impact on the mission success and the lifetime of the satellite. It does however sometimes cause arbitrary periods of time in which the satellite is not transmitting data. This is recovered by either a telecommand or after eclipse.