EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Coherent Time and Frequency for an Improved Global Geodetic Observing System

Karl Ulrich Schreiber1, Jan Kodet1, Thomas Klügel2, and Torben Schüler2
Karl Ulrich Schreiber et al.
  • 1Technische Universitaet Muenchen, Forschungseinrichtung Satellitengeodaesie, Geodaetisches Observatorium Wettzell, Bad Koetzting, Germany (
  • 2Bundesamt fuer Kartographie und Geodaesie, Geodaetisches Observatorium Wettzell, Bad Koetzting, Germany

The toolbox of space geodesy contains a number of measurement techniques, which are globally distributed. In this diverse network fundamental stations are playing an important role as they are forming the backbone of the global geodetic observing system. They provide the ties for the combination of the techniques.

Until recently these ties were only considering the spatial relationship between the measurement techniques. Upon closer inspection it turns out that clocks are also playing an important role. Variable delays within the main techniques of space geodesy, namely SLR, VLBI, GNSS and DORIS are limiting the stability of the measurements and hence the entire observing system. This leads to the rather paradox situation, that each technique has to adjust the clock offsets independently. Although all main measurements systems on an observatory are usually based on the same clock, each technique provides different offsets, thus weakening the local ties. This reflects the fact that the clock adjustments are also contaminated with (variable) system specific delays. Increasing the coherence of time on these GGOS observatories disentangles erroneous system delays from local ties, thus strengthening the entire observing system. 

We have designed and built such a coherent time and frequency distribution system for the Geodetic Observatory Wettzell. It is based on a mode-locked fs- pulse laser, fed into a network of actively delay controlled two-way optical pulse transmission links. This utilizes the ultra low noise properties of optical frequency combs, both in the optical and electronic regime. Together with a common central inter- and intra- technique reference target time can provide consistency for the complex instrumentation of SLR and VLBI systems in situ, which was not possible before. This talk outlines the concept and its potential for GGOS.

How to cite: Schreiber, K. U., Kodet, J., Klügel, T., and Schüler, T.: Coherent Time and Frequency for an Improved Global Geodetic Observing System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16586,, 2020


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  • CC1: Comment on EGU2020-16586, Axel Nothnagel, 05 May 2020

    I am very much surprised to see the figure on page 8 (VLBI signal delay) and love to see a rigorous study of this effect because this, if confirmed to be that systematic, has a tremendeous effect on our VLBI analyses. Can I please order such a study, i.e., multiple tipping and azimuth slew curves.

    Question: On which line is this effect observed, frequency distribution line to LO, receiver to backend line or phase-cal trigger line?

    • AC1: Reply to CC1, Karl Ulrich Schreiber, 05 May 2020

      The observations shown on page 8 are coming from an evaluation campaign during the definition of the fiber compensation requirements in the design phase of the elevation cabin time and frequency backends. We have now the first unit delivered to Wettzell, which will be set up at the RTW telescope over the next several weeks (lockdown permitting). After that we will certainly observe a whole series of such strain related delays and you are welcome to join the investigation. This is by far not the only critical spot. The other critical area is the digitizer.