Galileo pseudolite calibration target used to consolidate time between space geodetic techniques

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The Global Geodetic Observing System (GGOS) is the metrological basis for all global change research and for essential questions dealing with global deformation and mass transport within the System Earth consisting of the solid Earth, hydrosphere, atmosphere, and cryosphere. The stability and resolution of this observing system are of paramount importance for all sorts of applications, reaching from monitoring of global change to delicate tasks in navigation. The ultimate accuracy limit in realizing a global reference frame is given by the systematic errors in Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), and Global Navigation Satellite System (GNSS) measurement methods. They are connected by local geometric ties established by fundamental stations like the Geodetic Observatory Wettzell (GOW). These ties assume that the geometrically based instrumental reference point (e.g. a mechanically accessible antenna reference point) coincides with the technically effective reference point (e.g. the antenna phase center plus hardware delays accessible via the GNSS or VLBI measurements). However, unaccounted propagation delays may appear and cause systematic variable offsets between both points. These variable delays shall be investigated by utilizing time as a probe signal. In particular, we will use the clock signal from an active delay-compensated optical fiber line to generate and transmit an external time reference signal for GNSS over the air. This technique will establish a reference for the variable propagation delays, which will eventually be removed from the GNSS measurement process. The research unit builds on the existing optical timing system recently developed and operated at GOW, including VLBI telescopes. The work program is structured into four major work packages, which are focused on the development of the local pseudolite system. With the help of this system, we will remove the variable bias from the GNSS measurement process and align the GNSS receiver clock to the optical timing system. In this way, we reduce the uncertainty between GNSS and VLBI measurements. The first three tasks are developing the Galileo pseudolite system, which transmits a signal similar to Galileo E1BC from the station calibration target. This signal must be synchronized to the optical pulse train, representing time markers. The reason for this is that the most precious drift-free output of the timing system is the optical pulse train. While the ultimate goal is to remove the variable delays from the receiving system, this synchronization must occur on picosecond systematic uncertainty. The fourth work package verifies the developed calibration concept with a closure-based measurement concept utilizing a local GNSS network. Furthermore, as the goal is to unify the clock parameter between GNSS and VLBI, we proposed to do an additional experiment that includes GNSS and VLBI networks.