By utilising communication signals for positioning and calculating the earth’s gravitational field, real-time observation of weather phenomena is now also possible.
Useable data are one of the most valuable tools scientists can have. The more data sources they have, the better they can make statements about their research topic. For a long time, researchers in the field of navigation and satellite geodesy found it regrettable that although mega-constellations with thousands of satellites orbited the Earth for communication purposes, they were unable to use their signals for positioning or for observation of the Earth. In the FFG project Estimation, the Institute of Geodesy at Graz University of Technology (TU Graz) has now conducted research on ways of utilising these signal data and thus tapping into a large reservoir of additional data sources alongside navigation satellites and special research satellites, which will help to observe changes on Earth even more precisely.
Success with the Doppler effect
Earth observation using satellites is based on the principle that changes in sea level or groundwater levels, for example, influence the Earth’s gravitational field and therefore the satellite trajectory. Scientists use this to utilise the positions and orbits of satellites as a data source for their research. “The increasing availability of satellite internet in particular means that we have a huge amount of communication signals at our disposal, which significantly exceed those of navigation satellites in terms of number and signal strength,” says Philipp Berglez from the Institute of Geodesy. “If we can now use these signals for our measurements, we not only have better signal availability, but also much better temporal resolution thanks to the large number of satellites. This also allows us to observe short-term changes. This means that, in addition to determining the position and changes in the Earth’s gravitational field that are relevant for climate research, weather phenomena such as heavy rain or changes in sea level can also be tracked in real time.”
One of the challenges in realising the project was that the satellite operators, including Starlink, OneWeb and the Amazon project Kuiper, do not disclose any information about the structure of their signals and these signals are constantly changing. In addition, there are no precise orbit data or distance measurements to the satellites, which represents potential sources of error for calculations. By analysing the Starlink signal, the researchers nevertheless found a way to enable the desired applications. They detected sounds within the signal that were constantly audible. They then utilised the Doppler effect and investigated the frequency shift of these constant tones as satellites moved towards and away from the receiver. This allowed the position to be determined with an accuracy of 54 metres. Although this is not yet satisfactory for geodetic applications, for the investigations that were carried out so far, only a fixed, commercially available satellite antenna was used to test and verify the basic principle of the measurement method.
More insight into how our world is changing
The aim now is to improve the accuracy to just a few metres. This will be made possible by antennas that can either follow the satellites or receive signals from different directions. In addition, measurements are to be taken at several locations in order to increase accuracy and reduce the influence of errors. And with more measurement data, the researchers can calculate more precise orbit data, which in turn makes determining positions and calculating the Earth’s gravitational field more accurate. The navigation working group also wants to develop new signal processing methods that filter out more precise measurement data from signals that have so far been rather unusual for geodetic applications.
“By being able to utilise the communication signals for geodesy, we have revealed enormous potential for the even more detailed investigation and measurement of our Earth,” says Philipp Berglez. “Now it’s all about improving precision. Once we have succeeded in doing this, we will be able to understand even more precisely what changes our world is undergoing. Just to be on the safe side, I would like to make the following clear: we are analysing communication signals here, but we cannot and do not want to know their content. We only use them for positioning and observing orbits in order to determine the Earth’s gravitational field.”