Optical Synchronizing Formation Flying Spacecraft Systems by Optical Integrating Synchronization Module

ESA’s mission Proba-3 consist of two space segments, the Coronagraph spacecraft and the Occulter spaceraft operated at a distance between 25m and 250m, and co-aligned to the sun direction. In addition to in-orbit Formation Flying demonstration, one science objective is to achieve science observation of the Sun’s corona close to the solar rim at a specific inter-satellite distance of 150m. This configuration creates challenging illumination conditions for optical instruments which are needed to control the Formation at various operations scenarios, e.g. when cameras on the Sunwards Occulter are pointed to the Coronagraph, with the fully sunlit Earth in the background. To achieve robust and reliable operations of these cameras, high accuracy and robust relative timing information between the spacecraft are required. The solution developed for Proba-3 is the topic for this work.

The Vision Based Sensor (VBS) extension of the micro Advanced Stellar Compass (μASC) enables general optical rendezvous and docking navigation capabilities in-between formation flying satellites – in both cooperative and non-cooperative modes. The robustness of the cooperative pose and position determination method by the VBS system rely on high accuracy synchronization between the camera system, located on the observing spacecraft, and the pulse-powered LED mires, located on the target spacecraft.

In order to achieve high accuracy synchronization in between satellites, an optically based Integrating Synchronization Module (ISM) and an Acquisition Mire Unit (AMU) have been developed. The AMU transmits a Hadamard based coded light pulse sequence using an array of LEDs. The ISM samples the incoming light by a band-passed photodiode followed by an amplification stage and fed into an FPGA, performing correlation matching towards the known coded pulse sequence of the AMU. This enables the ISM to determine the pulse train alignment with high accuracy, enabling the μASC camera system to synchronize the image capturing timing. The ISM and AMU are fully integrated with the heritage μASC system as plug-on peripherals to the existing camera port connections.
We discuss the design aspects of this system, outlining its performance envelope, and discuss the technology ‘s applicability to cover future space mission synchronization needs, e.g. Lisa, Starshade etc.