An on-board data system is required to collect, administrate and store the generated data. The type of mission, orbit, payload and the selected ground stations have a signifficant effect on this on-board data system. The collected data can be transmitted from a geostationary satellite constantly, because the spacecraft is visible permanently from one ground segment. A circling satellite in the low-Earth-orbit requires a reliable data storing during the non-contact periods between the spacecraft and the ground station(s). Due to the short LEO/ground station link time the internal generated data of a satellite should not exceed the
sendable data volume during one day. The CDHS is the data- and telecommand administration system of ESTCube-1. The CDHS memories store the generated internal data of the satellite like housekeeping data, experiment data and the transmitted telecommands from the ground station. In a regular time interval the system is requesting data from each system and storing this data in the memory devices. The actual design schedules one memory device for the housekeeping data, one for the experiment data and one for the telecommands. The incoming telecommands will be forwarded to the according systems of the spacecraft.

Requirements
Internal requirements for the onboard data handling system contain the following:

• Enabling the flow of the housekeeping and mission data. Housekeeping and mission data is gathered from subsystems and stored in the memory. CDHS also compiles some of the beacon data (the most vital information is gathered by COM).
• Receiving and distributing commands. CDHS acts as a router between subsystems. For example, it distributes commands received from the COM to other subsystems.
• Performing telemetry and telecommands protocols. Data handling system receives telecommands from the COM subsystem, executes them and sends response telemetry data back to COM.
• Time distribution around the spacecraft. In case every subsystem has its own microcontroller, a common real-time clock is needed for scheduled commands or time-based events for other subsystems.
• Providing data storage. CDHS provides memory for the camera and other subsystems.
• Executing commands and scheduled events. Received telecommands are scheduled and executed when needed.
• Controlling payloads and subsystems. Commands that result from autonomous decisions, are distributed to subsystems. If a central processor unit will be used for all subsystems, then the onboard data handling system is needed to directly execute their code.
• Monitoring spacecraft health. In addition to recording housekeeping and mission data, the system executes schedules based on the changes in the data.
• Making autonomous decisions. Preprogrammed schedules are executed and commands distributed when the status of a subsystem or the payload changes.
• Update program code. There is a possibility that the code of the microcontroller needs to be uptadetd during the mission. In order to do that, the bootloader of the microcontroller has to include code that allows fetching the updates via the communication bus, verify the data and write it to the controller. If the verification process fails, then the old code is executed. Flashing operation will be initiated by the COM subsystem.