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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. [ABSTRACT FROM AUTHOR]

Sagnac interferometers have established themselves firmly in the Geosciences, so that we can now begin to separate and mitigate error sources that are not related to the rotation sensing process from the Sagnac interferogram. As a result of that we were able to reduce the measurement error of the 16 m2 G ring laser of the Geodetic Observatory Wettzell by a factor of more than two. At the same time the residual sensor drift reduced significantly. Until now most of the investigations on large ring laser gyroscopes were dealing with a single component ring laser, orientated horizontally on the ground. Over the last two years we have constructed the 4 component ring laser structure, which is arranged in the form of an inverted tetrahedron with the tip in the ground. For the first time this arrangement can provide three dimensional rotation measurements with additional redundancy.On this road of a high resolution monitoring of a global measurement quantity (Earth rotation) with a local sensor (ring laser), we have encountered a number of serious challenges. This talk illustrates the recent progress in high resolution Sagnac Interferometry and its application to the Geosciences. [ABSTRACT FROM AUTHOR]

Large-scale laser gyroscopes with sufficiently high sensitivity for measurement of the rotation rate of the Earth Ω E are inertial sensors with the capability to provide Earth orientation parameters, i.e., rotation rate and polar motion in near real time. Larger-scale passive resonant gyroscopes (PRGs) theoretically have a lower shot-noise limit. However, the cavity perimeter fluctuations and laser frequency noise become challenges in a passive gyro. In this paper, we introduce a three-wave differential locking scheme for large-scale PRGs, resulting in an in situ measurement of the cavity perimeter with nanometer resolution. Furthermore, the laser frequency noise is effectively suppressed with an additional gain of 30 dB by a double-stage locking system, based on the three-wave differential locking scheme. Finally, the rotation rate resolution of our 3 m ×3 m gyroscope improves to 1.1×10 -9 r a d / s over 200 s. The simplicity, robustness, and effectiveness of the locking scheme are important to the long-term operation of large-scale PRGs aiming for applications in the geosciences.