Co-location of Space Geodetic Techniques on Ground and in Space

根据国际大地测量学协会的全球大地测量观测系统，对海平面变化、冰后期反弹和无声地震等过程进行量化所需的陆地参考框架，在几十年内应符合1毫米的精度和0.1毫米/年的稳定性。不同空间大地测量技术（VLBI、SLR、DORIS和GNSS）之间联系的质量仍然是实现这种精确和独特的地球参考框架的主要限制因素之一。这些联系的不足之处来自（a）许多基本地点的地方联系不准确或不完整，以及（B）个别空间大地测量技术中的系统性仪器偏差。该项目侧重于后一个方面，主要目标是改进地面和空间空间技术之间现有的联系并建立新的联系，评估和减少特定技术的偏差，这是为全球地球观测系统建立独特的高精度地面参照框架的一个基本先决条件。为实现这一目标，该项目将利用大地测量观测台Wettzell（3台VLBI望远镜和2台SLR望远镜，超过5台全球导航卫星系统接收器，30年来进行地面局部联系测量）作为世界上独一无二的实验室，研究局部联系、系统性技术内和技术间偏差以及局部环境影响。特别是，将在所有主要仪器之间建立一个主动稳定的双向补偿光学时间和频率分配系统，以便监测和评估系统延迟，并在仪器之间建立一个额外的纽带，即所谓的时钟纽带。这将通过结合不同仪器和技术的时钟校正在数据处理中被利用。两台甚长基线干涉测量望远镜、两台激光测距望远镜和全球导航卫星系统天线将同时用于记录全球轨道导航卫星系统和伽利略卫星的观测结果，从而使这三种技术在空间实现真实的协同定位，即，在GNSS卫星上，以及一个独特的数据集来研究所有常见参数的严格组合。此外，极短基线的数据集将有助于对卫星的VLBI观测进行彻底验证，并从（准）同步SLR观测中形成单差和双差，以减少和更好地了解卫星和台站的具体偏差。在地面上，将通过开发一个共同的基准目标来补充空间的同一地点，以便在地面上的一个单一点校准和连接所有技术。将根据多天线全球导航卫星系统站点评估当地环境影响。该项目的成果是详细评估地面和空间新联系、新处理战略和实现参照框架质量的严格共同参数化所带来的惠益，这对项目合作伙伴至关重要，更重要的是，对监测不断变化的地球至关重要。

According to the Global Geodetic Observing System (GGOS) of the IAG, the terrestrial reference frame, as required to quantify processes like sea level change, postglacial rebound and silent earthquakes, should comply with an accuracy of 1 mm and a stability of 0.1 mm/y over decades. The quality of the ties between the different space geodetic techniques (VLBI, SLR, DORIS, and GNSS) is still one of the major limiting factors for the realization of such a precise and unique terrestrial reference frame. The deficiencies in the ties arise from (a) inaccurate or incomplete local ties at many fundamental sites as well as from (b) systematic instrumental biases in the individual space geodetic techniques. This project focusses on the latter aspect, with the main goal to improve existing and to establish new ties between the space techniques both, on ground and in space, and to assess and reduce technique-specific biases as an essential prerequisite for the realization of a unique high-precision terrestrial reference frame for GGOS. To achieve this goal the project will make use of the Geodetic Observatory Wettzell (3 VLBI and 2 SLR telescopes, >5 GNSS receivers, terrestrial local tie measurements over three decades) as a laboratory that is unique worldwide to study local ties, systematic intra- and inter-technique biases, and local environmental effects. In particular, an actively stabilized two-way compensated optical time and frequency distribution system will be established between all major instruments, allowing the monitoring and evaluation of system delays and realizing an additional tie, a so-called clock tie, between the instruments. This will be exploited in the data processing by combining the clock corrections of different instruments and techniques. Two VLBI telescopes, two SLR telescopes and the GNSS antennas will be used simultaneously to record observations of GLONASS and Galileo satellites, providing a real co-location of the three techniques in space, i.e., at the GNSS satellites, and a unique dataset to study the rigorous combination of all common parameters. In addition, the dataset from extremely short baselines will allow the thorough validation of the VLBI observation of satellites and the forming of single- and double-differences from (quasi-)simultaneous SLR observations to reduce and better understand satellite- and station-specific biases. The co-location in space will be complemented on the ground by the development of a common reference target for calibrating and linking all techniques in a single point on the ground. Local environmental effects will be assessed based on multi-antenna GNSS sites. The outcome of the project is a detailed assessment of the benefits from new ties on ground and in space, new processing strategies and a rigorous common parameterization for the quality of the reference frame realization, that is essential for the project partners and, more importantly, for the monitoring of the changing Earth.