Peter Vollmair

With the increasing availability of colocated geodetic space techniques on ground and in space, particularly microwave and laser systems, combining these techniques is becoming more relevant. Future missions will extend this to colocation in time by connecting both techniques to the same reference clock. This applies particularly to applications like validating current GNSS, next-generation GNSS, and missions such as Genesis or “Atomic Clock Ensemble in Space”. A relatively new geodetic observation technique is the laser one-way observation technique. This technique is used, for example, in missions such as “Time Transfer by Laser Link” and “European Laser Timing”. The colocation of current and new techniques, and the combination of their observations, makes it possible to estimate parameters jointly. Least-squares parameter estimation is a suitable method for this combination, as it allows the relative weighting of observations from different techniques. Such a relative weighting can be performed, for example, based on the respective instrument noise, as is the case when combining GNSS code and phase observations. This study analyses the impact of additional laser one-way observations and of different relative weighting strategies. Therefore, we use a dataset created by a closed-loop simulation software. The dataset contains different combinations of one-way microwave, one-way laser, and two-way laser observations between the International Space Station and the geodetic fundamental station Wettzell. Two main results are obtained from our studies. Firstly, expansion of geodetic missions to include optical one-way observation techniques in addition to microwave one-way and optical two-way observation techniques enables relative microwave bias calibration and significantly improves the accuracy and precision of time transfer. The one-way laser observations determine the time parameters in addition to the geometry parameters determined by the two-way laser observations. Secondly, a high weighting of laser-based observations significantly improves accuracy and precision of estimated parameters. Although the tropospheric fluctuations in the microwave observations remain unmodelled in this study, the higher weight of the laser observations, dominated by white noise, acts as a geometric constraint that stabilizes the parameter estimation. White noise averages over time, so it does not introduce systematic effects into the solution. The results demonstrate the potential of additional laser-based one-way observations when combined with traditional geodetic space techniques and provide valuable insights for future missions.

All geodetic space techniques used today are based on measuring the propagation time of electromagnetic waves between a transmitter and a receiver. The atmosphere is one of the most limiting factors for the achievable accuracy of the observation techniques. The wet part of the troposphere, particularly water vapour, has a strong and highly variable influence on signals in the microwave range, as is the case with GNSS. By using current models, this high variability makes it difficult to sufficiently correct the influence of the troposphere on the signal propagation time. Neither weather models nor estimation of parameters can accurately capture the high tropospheric variability. An alternative method for determining the wet propagation delay is a water vapour radiometer.With the HATPRO-G5, the Geodetic Observatory Wettzell has a modern radiometer that is capable of measuring the wet part of the troposphere above the station not only at the zenith but also for various azimuth-elevation combinations. This allows the wet delay to be recorded as a function of azimuth, elevation, and time. The resulting data set will first be analyzed and then used in combination with GNSS observations. To make this data available for GNSS evaluation, the water vapour radiometer data must be smoothed and interpolated both spatially and temporally. To assess the impact of radiometer-based tropospheric correction, the estimated station height component is compared with a standard GNSS processing and with the height component of a station coordinate product. We performed a PPP to estimate the station coordinates. However, the results do not show a clear picture. On the one hand, the estimation of the station height component using radiometer-based correction appears to deliver better results than the classic approach for certain time periods. On the other hand, however, these improvements cannot be reproduced for the entire time period of the data set. The data set still exhibits systematic errors whose origin has not yet been clarified, which in turn negatively affect the accuracy of the height component estimate. Possible reasons for this could include rapid weather changes, rain events or also electronic-specific systematics. Nevertheless, radiometer-based corrections have the potential to positively influence the accuracy of parameter estimation in GNSS processing.

We present a project from the research unit (RU) "TIME" (Clock Metrology: A Novel Approach to TIME in Geodesy), which seeks to determine gravity potential or height differences between distant locations by comparing optical clocks. A strontium optical lattice clock at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig will be connected to the German Research Centre for Geosciences (GFZ) in Potsdam via a delay-compensated optical fiber link. Optical time transfer will then be applied between the geodetic observatories in Potsdam and Wettzell (hosting a second optical clock) through the Atomic Clock Ensemble in Space (ACES) using Satellite Laser Ranging (SLR) telescopes.Additionally, a third optical clock located in Grasse, France, will be included for comparison. This clock, assumed to have similar characteristics to the one in Braunschweig, will connect to PTB via microwave terminals and to Wettzell using both laser and microwave links, forming a triangular measurement configuration.The innovative aspect of this approach lies in utilizing time transfer, rather than frequency transfer, and employing free-space links over an extended period to measure physical height differences. Key challenges include managing clock and link variations, atmospheric disturbances, visibility limitations, and data gaps. The clock and link errors are modeled specifically for this constellation, involving the ACES system.This approach demonstrates the accurate determination of physical height differences via time transfer, particularly for Global Geodetic Observing System (GGOS) core stations such as the Geodetic Observatory Wettzell (GOW).We discuss the underlying principles, unique properties, and specific challenges of this measurement scenario. Additionally, we provide preliminary estimates of the expected accuracies for the various components and the resulting height differences, based on simulations with varying error budgets for comparison.We acknowledge the support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 490990195 – FOR 5456.

The future Atomic Clock Ensemble in Space (ACES) mission of the European Space Agency (ESA) will address the development of a time transfer concepts for tomorrow's technologies. The ACES configuration includes a new generation of high-precision atomic clocks, a microwave link terminal (MWL) on the ground and on the satellite, and an optical detector and reflector also on the satellite. Due to the fact, that the official launch date is in 2025, there is a lack of real observation data. For that, a full-scale simulation software has been implemented. The simulator produces MWL code and phase observations in downlink and uplink, as well as one- and two-way laser observations. To analyse the efficiency of a time transfer concept before launch, we used the simulator to generate a data set of 100 passes during July 2021. Investigations based on this data set showed that the colocation of the high-precision geodetic observation techniques of the ACES mission could better separate the individual error contributions of a measurement. Due to the colocation of optical and microwave-based geodetic observation techniques, also error parameters like orbit and troposphere correction can be estimated together. Estimation of a common troposphere for all observation techniques, improves the accuracy of the determination of the offset between ground and ACES clocks. Our further investigations focus on the common troposphere estimation of multi-color optical observations, together with microwave-based observations and the effects of different weighting methods. An extension to a network of ground stations will demonstrate the advantages of the ACES mission for synchronizing multiple ground clocks. The colocation of different high-precision geodetic observation techniques and estimation of common parameters will benefit timing and ranging applications and fundamental physics studies.

The ACES mission of the European Space Agency combines optical and microwave-based geodetic observation techniques with highly accurate atomic clocks to achieve a new level of accuracy for geodesy and fundamental physics applications. In addition, the combination of two high-precision measurement techniques provides an even more exciting insight into their errors. Fundamental physics is particularly interested in experiments that require high precision between the results of the successive passes of a satellite. An example of such an experiment is the determination of gravitational redshift. Geodesy applications, in contrast, require both high accuracy and precision. Especially for applications like precise ranging or time synchronization, all possible error influences must be characterized and determined with high precision. Therefore, electronic delays of microwave link terminals pose a challenge to achieving high accuracy. They must, therefore, be calibrated, and the stability of the electronic delays must be monitored. While optical observation techniques can be calibrated sufficiently on the ground, the calibration of microwave measurements before a launch is not precise enough, and continuous monitoring is also not possible. In this study, four calibration methods were tested, all based on colocating optical and microwave measurements onboard a satellite and on the ground. The results of two methods achieved the required accuracy of 100 ps for time synchronization, with a mean error and standard deviation of better than 4 ps and 55 picoseconds, respectively. Correlations between the measured parameters were identified, and the impact of the different approaches on accuracy was investigated. It will be shown that the satellite-based colocation of two different geodetic observation techniques has clear advantages, and the calibration results achieved the required accuracy for geodetic applications in this simulation study.