Time distribution on the Athena WFI

The Wide Field Imager (WFI) is one of two instruments of the X-ray Advanced Telescope for High-ENergy Astrophysics (Athena) mission selected by ESA. The WFI instrument uses a camera with a DEPFET sensor, Detector Electronics (DE) to control the camera, and additional electronics units to communicate with the spacecraft on-boardcomputer (OBC). The Spacecraft Event Time (SCET) is generated on the OBC and synchronized with ground. The SCET timing synchronization between the OBC and the sensor photon detection presents particular challenges. The science user requirement of the absolute knowledge error of the WFI time stamp relative to the OBC clock is 5 mu s with a confidence level of 99.73%. In this paper, we present the WFI timing distribution implementation. The three main contributors of the timing distribution are: (1) Time delays and jitter between OBC and DE, (2) Internal delays of the DE, and (3) Delay between a photon capture and the time stamping in the DE. The first contributor is the most critical and two solving methods are identified. The first method uses only the timecode of the SpaceWire (SpW) communication network, and the second method uses a combination of pulse-per-second (PPS) signal and SpW network. SpW network standard was published in 2003 and few missions such as ESA Solar Orbiter use it exclusively for time distribution. In our analysis, we found that using the second method with a PPS signal, delays contribution is in order of nanoseconds.