Clock Ties: A novel approach for the reduction of systematic errors

The techniques of space geodesy, comprising the four techniques, Global Navigation Satellite Systems (GNSS), Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Doppler Orbitography and Ranging Integrated by Satellite (DORIS) are currently reaching a measurement resolution in the range of 1 part per billion for the terrestrial reference frame. However, a small set of discrepancies remain evident within each of the techniques as well as in the combination of different techniques. Systematic measurement errors are causing this and problems in the local ties between the reference points of the various measurement systems and biases in the atmospheric refraction correction have long been suspected as the main contributors. However, it turns out that errors in the internal delay compensation of the measurement systems are also a significant contributor. They are extremely hard to detect, since they are small and come with different characteristics. It is understood that the experienced delay variation is related to a complex pattern of ambient temperature variation inside of the electronic devices. These changes relate to the micro-climate of the electronic signal path and can both be slow and highly variable. With the advent of high bandwidth mode-locked lasers and active delay compensation in the optical domain, it is now possible to utilize coherent time as an independent probe for instrumental signal delays. The research unit FOR5456 of the German National Science Foundation (DFG) has been formed in 2022 in order to apply and investigate active delay compensation to the techniques of space geodesy. This talk introduces the application of coherent time in space geodesy and its potential to act as a novel tie in fundamental stations.