Delta-T: All Optical Time Tagging in Satellite Laser Ranging And Optical Delay Compensation For Very Long Baseline Interferometry Based On Ultra-Short Mode-Locked Laser

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Satellite Laser Ranging (SLR) is an advanced technology, which is used in the field of space geodesy for precise orbit determination. More recently SLR demonstrated its suitability for accurate time transfer between a ground station and the Jason 2 satellite in a low Earth orbit. While a single shot precision of less than 15 ps is typically achieved by several SLR systems, the respective obtained accuracy for the reconstruction of the satellite orbit is not better than 20 – 45 ps. This is caused by some fundamental short-comings of the current realization of the SLR technique, since the measurement is done in the optical domain and the time tagging in the microwave regime. In order to improve the accuracy of optical time transfer by reducing systematic errors to less than 10 ps, we propose to use a delay compensated fs-pulse laser (available on the observatory) to move the complete time tagging process of the laser ranging technique entirely into the optical domain and exploit the fact that mode-locked fs-pulse lasers have ultra-low noise in the optical and microwave regime simultaneously. This will significantly improve the time transfer precision and accuracy between the Wettzell Laser Ranging System on the ground and the Atomic Clock Ensemble in Space (ACES), which is prepared for the operation on the International Space Station (ISS). Similar considerations apply for the Very Long Baseline Interferometry (VLBI). Long cables, subject to squeezing and stretching when the antenna is in motion and temperature sensitive amplifier and mixer delays are causing systematic errors. These can only partially be absorbed by clock adjust-ments and modeling tropospheric delay corrections. Therefore we propose to use the same two-way optical delay compensated time and frequency distribution concept also for the VLBI in order to obtain a stable unambiguous system reference as well as an ultra-wideband fs pulse laser based phase calibration for the new VGOS systems, covering also the Ka band. Last but not least we shall demonstrate the reduction of systematic measurement errors in a closure measurement concept based on a common actively stabilized timebase.