Within the TeSeR project, the work objectives of Bundeswehr University Munich were the development of concepts and the definition of a functional architecture of the post-mission-disposal (PMD) module.
It is critical to discern when a spacecraft is not operating nominally. Therefore, as a first step of the analysis, it was investigated how this can be detected. A number of symptoms indicating a troubled spacecraft are identified (see figure 1). The majority of the symptoms can only be checked through checks of the telemetry of the host spacecraft. This is done through the ground station. In addition, certain physical parameters can indicate a non-nominal state of the host spacecraft: excessive rotational rates, temperatures exceeding acceptable limits, and lack of power. In order to detect these symptoms from the PMD module, a suitable set of sensors is identified. As an added bonus, this allows the PMD module to establish the position and attitude of the host spacecraft if needed.
Figure 1: Spacecraft status detection
The operational and autonomy concept is aimed at “triggering” the removal from orbit and the passivation of the host spacecraft (i.e. safely disabling) at the end of its operational lifetime. It was found that future spacecraft using the PMD module technology will be required to be designed to be self-passivated. The PMD module itself will also have to be passivated having performed the removal operations unless in case of a direct, controlled atmospheric re-entry. The operational autonomy concept foresees – for the foreseeable future – a human operator “in the loop”. However, in the case of satellite mega-constellations, autonomous removal presents a viable business case as it can significantly reduce unnecessary cost. The autonomy concept features three levels of increasing on-board autonomy capabilities (see figure 2).
Figure 2: Illustration of the three progressively more advanced autonomy levels defined by Bundeswehr University Munich for removal from orbit
The removal subsystems are placed on the PMD module platform. The PMD module platform is placed on the host spacecraft. The system design comprises of the concept of operations for the PMD module (see figure 3), the functional design (“What the system should do.”), and the performance design (“How well the system should do it”). The established requirements for the design will form the basis for future, most likely modular, more detailed designs. Regarding the level of self-sufficiency of the PMD module, the analysis showed that the optimal route is to not be reliant on the host spacecraft.
Figure 3: The top-level operational concept for removal from orbit as defined by Bundeswehr University Munich
Figure 4: A functional block diagram of a version of the PMD module, its subsystems, the connections between them, and the two interfaces to the host spacecraft and the removal subsystem (RS) respectively
For the interfaces between the host spacecraft and the PMD platform as well as the PMD platform and the removal subsystem, the approach follows the idea to standardize as much as possible (see figure 5). Regarding the physical/mechanical, thermal, electrical, and data interface to the spacecraft, the aim was to achieve minimal reliance on the resources of the spacecraft. As the PMD module platform will “trigger” the removal subsystems via Standard Interface #2, this “fuller” interface encompasses a larger set of options.
Figure 5: The concept of the PMD module: the PMD module platform is attached to the host spacecraft via standardized interface #1. Each removal subsystem can be attached to the PMD platform via standardized interface #2