ELT: Time transfer by laser pulses

The Atomic Clock Ensemble in Space (ACES) was launched to the International Space Station (ISS) on 21^st of April of this year. Following successful installation on the external payload facility of the Columbus module, the commissioning phase began, which will approximately last until the end of 2025. ACES brought two time transfer methods into orbit: the Microwave Link (MWL) and the European Laser Timing (ELT) link. These two links differ not only in frequency, – one operates in the microwave domain, the other in the optical domain – but also in their detection principle. In this contribution, we introduce the optical pulsed time transfer experiment ELT and compare its measurement principle with that of MWL.

Just like T2L2, ELT is a combination of Satellite Laser Ranging (SLR) and a one-way ranging measurement, in which the laser pulses are time tagged in the ACES timescale. Contrary to T2L2, the complexity of the measurement is not in the space segment but on ground. Although the entire SLR ground segment is available in principle, restrictions exist for ranging to the ISS and the availability of a stable clock signal at these geodetic stations.

The Wettzell Laser Ranging System (WLRS) located at the Geodetic Observatory Wettzell in Germany is the main ground station for the ELT experiment. We describe the steps taken at WLRS to participate in the ELT experiment and the available hardware. We then present the ACES payload and the ELT Data Center, which is responsible for the data processing chain. We highlight the challenges of the data processing based on the first synchronisation measurements between Wettzell and the ACES time scale. Finally, we discuss the objectives of ELT and the benefit optical time transfer will bring for space geodesy and how it fits to the objectives of GGOS.