Wolfgang Schafer

Ground-based demonstration of the European Laser Timing (ELT) experiment

The development of techniques for the comparison of distant clocks and for the distribution of stable and accurate time scales has important applications in metrology and fundamental physics research. Additionally, the rapid progress of frequency standards in the optical domain is presently demanding additional efforts for improving the performances of existing time and frequency transfer links. Present clock comparison systems in the microwave domain are based on GPS and two-way satellite time and frequency transfer (TWSTFT). European Laser Timing (ELT) is an optical link presently under study in the frame of the ESA mission Atomic Clock Ensemble in Space (ACES). The on-board hardware for ELT consists of a corner cube retro-reflector (CCR), a single-photon avalanche diode (SPAD), and an event timer board connected to the ACES time scale. Light pulses fired toward ACES by a laser ranging station will be detected by the SPAD diode and time tagged in the ACES time scale. At the same time, the CCR will re-direct the laser pulse toward the ground station providing precise ranging information. We have carried out a ground-based feasibility study at the Geodetic Observatory Wettzell. By using ordinary satellites with laser reflectors and providing a second independent detection port and laser pulse timing unit with an independent time scale, it is possible to evaluate many aspects of the proposed time transfer link before the ACES launch.

The ACES Microwave Link: Instrument Design and Test Results

The microwave link (MWL) is a key piece of equipment of the ACES payload. Consisting of a flight segment and a ground terminal, the MWL is used to transmit the ACES clock signal to the ground. It performs space-to-ground clock comparison and ground-to-ground comparison of time & frequency reference systems.

Unprecedented long-term frequency stability with a microwave resonator oscillator

This article reports on the long-term frequency stability characterization of a new type of cryogenic sapphire oscillator using an autonomous pulse-tube cryocooler as its cold source. This new design enables a relative frequency stability of better than 4.5 × 10-15 over one day of integration. To the best of our knowledge, this represents the best long-term frequency stability ever obtained with a signal source based on a macroscopic resonator.

The european laser timing (ELT) experiment on-board ACES

The development of techniques for the comparison of distant clocks and for the distribution of stable and accurate time scales has important applications in metrology and fundamental physics studies. Additionally, the rapid progress of frequency standards in the optical domain is presently demanding additional efforts for improving the performances of existing time and frequency transfer links. Present clock comparison systems in the microwave domain are based on GPS and TWSTFT (Two-Way Satellite Time and Frequency Transfer). ELT (European Laser Timing) is an optical link presently under study in the frame of the ESA mission “Atomic Clock Ensemble in Space”. The on-board hardware consists of a corner cube retro-reflector (CCR), a single-photon avalanche diode (SPAD), and an event timer board connected to the ACES time scale. Light pulses fired towards ACES by a laser ranging station will be detected 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 paper will present the ELT scientific objectives, the recent studies performed on the ELT hardware, and the dedicated test campaign carried out at the Wettzell laser ranging station to demonstrate the experiment feasibility. Recent test results will be also discussed.

Future Gravity Field Satellite Missions

The project “Future Gravity Field Satellite Missions” (FGM) was a logical consequence of two previous phases in Theme 2 “Observation of the System Earth from Space” in the BMBF/DFG (Federal Ministry of Education and Research/German Research Foundation) Research and Development Programme GEOTECHNOLOGIEN.

Data analysis and phase ambiguity removal in the ACES microwave link

The ACES (atomic clock ensemble in space) mission is an ESA - CNES project with the aim of setting up onboard the international space station (ISS) several highly stable atomic clocks with a microwave communication link (MWL). The specifications of the MWL are to perform ground to space time and frequency comparisons with a stability of 0.3 ps at one ISS pass and 7 ps at one day. The ACES mission has applications in several domains such as fundamental physics, metrology or geodesy.

Design of the frequency and timing subsystem for ESA's Deep Space Antenna 3

In December 2012, ESA inaugurated their third Deep Space Antenna tracking station near Malargüe, Argentina. Due to the nature of the deep space operations, exigent requirements for stability of reference signals and low phase noise characteristics were necessary on the ground station equipment. In order to fulfill the requirements, new concepts and hardware development were carried out, resulting in an improvement of the performance of the frequency and timing reference system.

State-of-the-art time link calibration with a mobile TWSTFT station

Two-Way Satellite Time and Frequency Transfer (TWSTFT) using geostationary telecommunication satellites has become an important technique for comparing remote timescales. There is a need for periodic calibration of the signal delays along the links which are part of the network for the realization of the International Atomic Time (TAI). In order to minimize the efforts of such TWSTFT calibration campaigns, TimeTech GmbH has assembled a mobile station on a trailer. The trailer contains the usual TWSTFT equipment, including the two-way modem, the up/down converters, filters, and amplifiers, as well as monitoring systems, air conditioner, and weather sensors. The steerable antenna is mounted on top of the trailer. The 1 PPS and 10 MHz signals required as input to the TWSTFT equipment are transferred from inside the timing laboratory by an optical link and two reference generators in a master-slave configuration.

High stability Cryocooled 10 GHz oscillator for the European Space Agency

This paper reports on a 10 GHz ultra-stable Cryocooled Sapphire Oscillator (CSO) developed for the European Space Agency. This CSO presents a frequency stability better than 3 × 10⁻¹⁵ between 1 s and 1,000 s and a phase noise lower than −100 dBc/Hz at 1 Hz.

New technologies for laser time transfer and their possible application in the Galileo programme

We are presenting the new instrument, new technology available and new measurement technique proposal for the Galileo programme - optical detector for the laser time transfer. Combining the laser pulse emission times, propagation delays and satellite arrival times the ground to space clock comparison may be accomplished. The timing precision of the order of 1 × 10-12 seconds and a time transfer accuracy of 50 picoseconds is achievable. This precision and accuracy is at least one order of magnitude better in the optical region than in the radio frequency wavelength region. All the components of the proposed instruments are available in Europe, the ground segment of the proposed project is existing, the measurement techniques and data flow and processing procedures are well established. The implementation of new picosecond timing technologies and the laser time transfer into the Galileo programme will improve the precision and accuracy of the satellite on-board time scale and position prediction with unprecedent precision and accuracy. Both these facts will contribute to the Galileo system overall accuracy and performance and simultaneously will enable new experiments in fundamental physics.