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

卫星激光测距（SLR）是一项用于空间大地测量领域精确定轨的先进技术。最近，单反相机展示了它在地面站和近地轨道上的杰森2号卫星之间精确时间传递的适用性。虽然几个单反系统通常可以实现小于15ps的单次射击精度，但各自获得的卫星轨道重建精度不超过20 - 45ps。这是由于目前实现单反技术的一些根本缺陷造成的，因为测量是在光域进行的，而时间标记是在微波区进行的。为了提高光学时间传递的精度，将系统误差降低到10 ps以下，我们建议使用延迟补偿的fs脉冲激光器（天文台有）将激光测距技术的整个时间标记过程完全转移到光域，并利用锁模fs脉冲激光器在光学和微波波段同时具有超低噪声的事实。这将显著提高地面Wettzell激光测距系统与空间原子钟（ACES）之间的时间传递精度和准确度，后者是为在国际空间站（ISS）上运行而准备的。类似的考虑也适用于甚长基线干涉测量（VLBI）。长电缆，在天线运动时受到挤压和拉伸，对温度敏感的放大器和混频器延迟会导致系统误差。这些只能部分地被时钟调整和对流层延迟校正模型所吸收。因此，我们建议对VLBI使用相同的双向光延迟补偿时间和频率分布概念，以便为新的VGOS系统获得稳定的无二义性系统参考以及基于超宽带fs脉冲激光的相位校准，也覆盖Ka波段。最后但并非最不重要的是，我们将展示基于共同主动稳定时基的封闭测量概念中系统测量误差的减少。

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.