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

根据IAG的全球大地测量观测系统（GGOS），作为量化海平面变化、冰川后反弹和无声地震等过程所需的地面参考框架，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）之间的联系质量仍然是实现这种精确和独特的地面参考框架的主要限制因素之一。联系的不足是由于(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的系统仪器偏差。本项目的重点是后一方面，其主要目标是改善地面和空间空间技术之间的现有联系和建立新的联系，并评估和减少技术特有的偏差，作为实现地球观测系统独特的高精度地面参考框架的必要先决条件。为了实现这一目标，该项目将利用Wettzell大地观测站（3个VLBI和2个SLR望远镜，bb50个GNSS接收器，30多年来的地面局部联系测量）作为世界上独一无二的实验室，研究局部联系，系统的技术内部和技术间偏差，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，允许监测和评估系统延迟，并实现仪器之间的额外联系，即所谓的时钟联系。这将通过结合不同仪器和技术的时钟校正在数据处理中加以利用。两台VLBI望远镜、两台SLR望远镜和GNSS天线将同时用于记录GLONASS和Galileo卫星的观测数据，提供三种技术在空间（即GNSS卫星）的真正共定位，并提供一个独特的数据集来研究所有共同参数的严格组合。此外，来自极短基线的数据集将允许对卫星的VLBI观测进行彻底验证，并从（准）同步单反观测中形成单差和双差，以减少和更好地理解卫星和台站特定的偏差。在地面上，将制定一个共同参考目标，以便在地面上的一个点校准和连接所有技术，从而补充空间上的共同定位。将根据多天线GNSS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的新联系、新的处理战略和实现参考系质量的严格共同参数化所带来的好处，这对项目伙伴至关重要，更重要的是对地球变化的监测至关重要。

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.