During the last weeks, the engineering model of the Power Control and Distribution Unit (PCDU) was connected to the satellite simulation environment in the MDVE-Lab (Model Based Development and Verfification Environment) in order to test it using the complete commanding path.

The setup of the commanding path in the MDVE-Lab is shown in the figure above. The path starts with the mission control software SCOS-2000, from where commands are sent to a TM/TC-Frontend, which in turn processes the commands and sends them to the satellite. In the simulation environment, a simple cable connection is used instead of the radio link which would follow in the real space link system. In the simulated satellite, the signals from the frontend, which are encoded applying the CCSDS (Consultative Committee on Space Data Systems) standard, first reach the so-called CCSDS-Board, where they are being decoded again. After that, there are two possibilities: If the signal is a so-called High Priority Command (HPC), it is routed directly to the PCDU, where it can command devices to be turned on or off in case of a contingency. A so-called Common Command (CC) on the other hand is routed to the On-Board Computer (OBC), from where it is routed to the devices it is addressed to.

The aim of the test was to send both HPCs and CCs from the SCOS-2000 Software to the PCDU and to verify if they were executed successfully. We started with the HPCs, because we did not need any further software for these tests. During the first tests we noticed that the TM/TC-Frontend software and the PCDU software calculate error correction bytes, which are attached at the end of every command, in a different way, which is why they were no valid commands for the PCDU. After we received an update of the PCDU software, we were able to send HPCs from the SCOS-2000 software to the PCDU without any problems. We verfified the execution of the commands by measuring the voltage at the PCDU outputs that were activated by the command sent. After we received an update of the TM/TC-Frontend software, we were also able to successfully send sequences of HPCs, i.e. up to four HPCs in one command, to the PCDU. The following figure shows a screenshot of a HPC recorded with an oscilloscope at the input of the PCDU.

For the tests of the CCs a simplified piece of software was uploaded to the OBC, with which the received commands are routed to the PCDU. We verfified the successful reception and execution of the commands in the same way as the HPCs. Furthermore, a “reply” from the PCDU, the so-called echo, is recorded in the memory of the OBC. We were able to read this memory and verify that the command had not just been correctly executed, but the echo from the PCDU had also been correctly transmitted to the OBC.

Thus for the first time we were able to successfully send commands from the mission control software to a device of the satellite and save the echo in the OBC memory. At the same time, we were able to detect and correct several errors in the commanding path. In the future, the on-board software is being developed further, so we will soon be able to perform similar tests with software, which is closer to the final flight software.