The Command & Data Handling Subsystem (CDH for short) is the "brain" of our satellite, so to speak. It collects and manages sensor and housekeeping data and commands the other subsystems.
For our balloon mission, the CDH subsystem will have no "C", i.e. no command handling part as our balloon will only be able to send data but not receive messages and in particular not receive commands from our mission control.
Instead, the data handling part was the main focus of development for the balloon mission. This part listens in on all communication on the satellite, filters out the relevant data, stores this data in so-called beacons, and forwards the beacons to their destination.
This destination can either be the COM subsystem, which sends the beacons directly to the ground station, or the beacon storage. For our balloon mission, the beacon storage consists of two redundant and independent storage systems to guarantee that we can access our valuable experimental data afterward.
The primary storage system consists of a large SD card and a custom SD card driver that was developed as part of a bachelor’s Thesis and implemented as an extension to our distributed operating system DOSIS. The SD card will also become the sole storage mechanism for subsequent balloon missions and satellite missions.
The secondary storage system consists of external UART logger devices - simple low-level communication interfaces - that store all incoming traffic, the beacons in our case, to an internal microSD card.
The CDH subsystem has its own UART logger, but the external logger devices can also be attached to other hardware parts. For example, they can be connected to all Standard Subsystem Boards (SSBs for short, i.e. the boards that hold all necessary sensors and other devices for a single subsystem). In addition, these loggers can even be used with the backplane (that connects all SSBs and coordinates the power distribution) and, thus, allow inter-board communication independent data logging.
In conclusion, the CDH subsystem is already responsible for several important tasks on the balloon mission although its primary features, (semi-autonomous) commanding and housekeeping for a great number of data sources, including our payload, are not yet fully developed.