Precise Time Transfer for High Throughput Satellite Communications Links

Kurzfassung

Future satellite constellations for high throughput communications with optical links are seen to set the coverage of broadband network access and as a backup of the ground fibre network. With dense wavelength division multiplexing techniques optical links are able to transmit Tbps. Their physical characteristics make them hard to jam or to spoof. A large bandwidth offers excellent conditions for precise time transfer. To increase the exploitation of these expensive communications constellation we propose to simultaneously perform time transfer within the network. Current time transfer concepts like frequency comb pulse transmission offer advantages like high accuracy and sensitivity but require a large bandwidth and specific hardware which makes it not straight forward to implement. Other techniques like high rate PRN transmission with optical correlation utilize a single DWDM channel but still employ a receiver structure which is not yet compatible with the communications setup. Finally, the space development agency defined a time transfer standard which enables time transfer within a data channel but the stability remains in the ns-range. Aiming towards picoseconds time transfer This study shall elaborate the requirements on a time transfer system being integrated into an optical communications system. Compared to pure time transfer systems the constraints are mainly defined by the data exchange. The time transfer shall be performed with minimum effort and only with the hardware which is anyways available, only adapting the processing of the received data. Further, the overhead of the time transfer shall be minimized to maintain the data throughput. As the link budget is defined by the data throughput there is more power being received compared to a pure time transfer system which is used to optimize the overhead. Given the requirements, a communications channel including precise time transfer is designed and presented. The processing units of the transceiver a referenced to an ultra-stable oscillator (USO). The transmitter signal is generated by expanding the commonly used frame structure consisting of a frame header and user data with a short data field for time transfer. The optical transmission is independent of the modulation scheme and supports on-off-keying, higher order modulation formats like phase shift keying or quadrature amplitude modulation and polarization multiplexing. At the receiver the signal is detected and demodulated depending on the respective modulation. The time transfer is gained at the time and frame recovery where the time of arrival is estimated with the timing recovery observables and the time transfer data field is read out to calculate the two-way time transfer. To verify the feasibility of precise time transfer via an optical high throughput channel the system is set up in the laboratory. A binary phase shift keying transmission system with flexible data rates are tested in a common mode and the time transfer precision is evaluated. Lower data rates up to 5 Gbps resembling the current state-of-art of optical inter-satellite communications are compared to high rates for future developments. To optimize the overhead different header lengths are tested.