Time, Space and the Measurement of the World – The Latest in Geodesy

We live in a world that demands ever greater precision in measurement and understanding. At every moment, we generate vast quantities of data and information, each piece of which has the potential to change our view of the world. One of the sciences concerned with this precise measurement of our world is geodesy.

Today we take a look at a group of scientists who are at the forefront of these efforts: the ‘Clock Metrology’ research group of the German Research Foundation (DFG). One of the participants is the German Research Centre for Geosciences (GFZ) in Potsdam – Germany’s national research centre for geosciences. Others include the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig and the Technical University of Munich (TUM).

What exactly do these scientists do? Put simply, it’s about surveying the Earth as precisely as possible. But it’s not just about simple surveying. It’s about how the Earth is changing – for example, through rising sea levels or the movements of the continental plates. To calculate these processes accurately, we need what are known as geodetic reference systems. These are like an invisible grid that spans the entire Earth and helps us to take precise measurements.

A key aspect of these measurements is time. Imagine you are measuring the distance between two points using light signals. You would draw a line from point A to point B and then measure the length of that line. As the speed of light is known, you would need an extremely accurate clock to do this. And what if point B is moving? Then you must also take into account the extra time it takes to get from point A to point B. This is a very simplified example, but it shows how important time is for accurate measurements.

The GFZ carries out the surveying and mapping of the Earth with the highest precision on a scientific basis. Thanks to Telekom’s Optical Time Transfer (OTT) solution, the GFZ in Potsdam receives the necessary time signal from the atomic clock in Braunschweig via a 230 km-long optical transmission link, including propagation delay compensation. 
“This enables the GFZ to achieve a level of accuracy that has never been seen before,” said Prof. Dr Karl Ulrich Schreiber of the Institute of Astronomical and Physical Geodesy at the Technical University of Munich and project leader of the ‘Clock Metrology’ research group.

Until now, the methods used to synchronise time between different measurement points have not been perfect. There have always been systematic measurement errors that have distorted the overall picture. The ‘Clock Metrology’ research group has found a solution to this problem. They have identified time as a new ‘observable’ – that is, something that can be measured and observed – and are using optical clocks and specialised compensation techniques for the distribution of time and frequency to create a coherent, i.e. consistent, time base.

For this research work, Deutsche Telekom has been commissioned to set up a new transmission link. The specialised transmission technology required for this – OTT-ELSTAB (Optical Time Transfer, ELectronically STabilised) – has been in use within Telekom’s network for four years and is now being deployed for the first time in a customer project. This transmission link is capable of transmitting time information and a frequency with a level of accuracy that has not been possible before. The time signal is transmitted via a 230 km-long fibre-optic link. Due to the required accuracy, all known negative influencing factors must be compensated for. These include, for example, the propagation time of light through the fibre, temperature changes in the fibre buried in the ground, the so-called Sagnac effect and chromatic dispersion.

The use of specialised hardware and software compensates for all these negative factors. This enables Telekom’s specialists to achieve the highest possible accuracy. The transmitted signal deviates by only a few picoseconds (1 ps = 10⁻¹² s = one trillionth of a second) from the original time in Braunschweig. The major challenge here is not only to maintain this level of accuracy during transmission for a short period, but to sustain this quality over several months or years.