Development of a Real-Time Capable Ethernet Gateway for the SHEFEX III Navigation System

Abstract

To date, the subsystems of spacecraft are often still interconnected with application-specific pointto- point links, for example using UART protocols. A more versatile interconnect would allow components to be changed more easily without much impacting the overall system, driving down system cost by increasing flexibility during development and creating new opportunities for reusing components across different space missions. Ubiquitous, high-speed Ethernet hardware may offer a cost-effective solution. TTEthernet is one of a handful of recent approaches for adding real-time and fault-tolerance capabilities to the Ethernet protocol. The German Aerospace Center (DLR) is currently evaluating the option of using TTEthernet for its upcoming SHEFEX III re-entry mission. Time-critical navigation solution messages have to be transmitted from the vehicle’s navigation system to the guidance and control subsystem, which controls the spacecraft’s actuators to keep the desired trajectory and orientation. The navigation system’s on-board computers only offer serial interfaces. Therefore, a solution has to be found to transport the messages from the on-board computers via the TTEthernet network to the guidance and control system. This thesis evaluates several possible designs and proposes a solution based on the development of a dedicated RS-422 to TTEthernet gateway. On this basis, different methods for the realtime transmission of the navigation solution messages through the gateway and the TTEthernet network are compared. With the help of simulations, the best solution is selected. An FPGAbased development environment is set up and a gateway application is developed that allows to transport the navigation messages under real-time constraints. Real-time tests are conducted to verify the viability of the solution. The test results show that the gateway fulfills the timing and reliability requirements.