Development of the European Laser Timing instrumentation for the ACES time transfer using laser pulses

We are presenting the work progress and recent results in the field of the European Laser Timing instrumentation for the ACES time transfer using laser pulses. European Laser Timing (ELT) is an optical link presently under preparation in the frame of the ESA mission “Atomic Clock Ensemble in Space” (ACES). The on-board hardware consists of a comer cube retro-reflector (CCR), an optical receiver based on a single-photon avalanche diode (SPAD) and an event timer board connected to the ACES time scale. Short laser pulses fired towards ACES by a laser ranging station will be time tagged with respect to the ground time scale. They will also be detected in space by the SPAD diode and time tagged in the ACES time scale. At the same time, the CCR will re-direct the laser pulse towards the ground station providing precise ranging information. This procedure provides ground-to-space and ground-to-ground time transfer with a precision and accuracy outperforming the radiofrequency techniques. Extensive work has been done on the photon counting detector. Three different versions of the photon counter for space application have been developed and tested. The main areas of interest were: minimal power consumption requirements of the optical detector package, long term stability of detection delay and a broad operational temperature range. The detector shall be capable to operate within a temperature range of - 50° to + 50° C while keeping the detection delay stability on the picosecond level for temperature variations of 6.5 K peak-to-peak. The detection delay shall be characterized and controlled - e.g. delay between the event of photon absorption and the appearance of the electrical pulse on the detector output. A configuration of the optical receiver able to maintain uniform sensitivity over a wide field of view of 120 degrees was developed. The parameters of the optical receiver obtained in the ground tests meet safely all the requirements for the ELT mission.