Determination of Physical Heights via Time Transfer

作为中心观测量，我们利用爱因斯坦理论预测的不同位置的时间尺度上的引力效应（即引力红移）。利用高精度光学时钟之间的卫星光学时间传输，我们将在几天的观测期内测量韦特泽尔和布伦瑞克之间的运行时间差异。根据这一观测结果，我们将确定两个地点之间的物理高度差。该项目的创新研究要素是：i)使用时间而不是频率作为基本数量来确定远距离站点之间的物理高度或重力位势差；ii)在相当长的时间跨度内利用与卫星的光学自由空间链接而不是光纤，即集成数天以达到所需的高精度。这不仅对所涉及的时钟和链路提出了挑战，而且对数据分析提出了挑战。我们将研究主要的误差来源（如不同的时钟/链路行为，仪器效应）以及所有相关的校正和数据减少，如潮汐效应或由于更多的局部质量变化而引起的变化，最终以20厘米的精度确定高度差。

As central observation quantity, we exploit the gravitational effect on the time scales at different locations as predicted by Einstein’s theory (i.e. the gravitational redshift). Using optical time transfer via satellite between highly-precise optical clocks, we will measure the difference of elapsed time between Wettzell and Braunschweig for observation periods of a few days. From this observable, we will determine the physical height difference between both sites. Innovative research elements in this project are i) the use of time instead of frequency as basic quantity to determine the physical heights or gravity potential differences between distant sites and ii) the exploitation of optical free-space links to a satellite instead of optical fibres over a rather long timespan, i.e. integration over days to achieve the required high accuracy. This does not only pose challenges on the involved clocks and links, but also on the data analysis. We will investigate the dominant error sources (like different clock/link behavior, instrumental effects) as well as all relevant corrections and reductions to the data such as tidal effects or changes due to more local mass variations to finally determine height differences with an accuracy of 20 cm.