Researchers at TU Graz have discovered a way to repurpose communication satellite signals for positioning and measuring Earth’s gravitational field, enabling more precise environmental monitoring and real-time weather tracking.
The visibility of Starlink satellites over Graz. Image Credit: IFG - TU Graz
Satellites orbiting Earth, particularly those in mega-constellations like Starlink, are traditionally used for communication. However, researchers at Graz University of Technology’s (TU Graz) Institute of Geodesy have found a way to repurpose these signals for Earth observation. This breakthrough, part of the FFG project Estimation, significantly expands the available data sources beyond navigation and dedicated research satellites, enabling more precise monitoring of environmental changes.
Success with the Doppler Effect
Satellite-based Earth observation works on the principle that variations in sea level or groundwater affect Earth’s gravitational field, which in turn influences satellite trajectories. Scientists can track these changes by analyzing satellite positions and orbits.
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. 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.
Philipp Berglez, Institute of Geodesy, Graz University of Technology (TU Graz)
Berglez added, “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 main challenges in executing the project was the lack of publicly available information on the signal structures used by satellite operators such as Starlink, OneWeb, and Amazon’s Project Kuiper. These signals are not only undisclosed but also continuously evolving. Additionally, the absence of precise orbital data and direct distance measurements to the satellites introduced potential errors in calculations.
Despite these obstacles, researchers found a way to achieve their goal by analyzing the Starlink signal. Within the signal, they identified persistent tones that remained consistently audible. By leveraging the Doppler effect, they tracked the frequency shifts of these tones as the satellites moved toward and away from the receiver. This approach enabled them to determine position with an accuracy of 54 meters.
While this level of precision is not yet sufficient for geodetic applications, the study successfully demonstrated the feasibility of the method using only a commercially available, fixed satellite antenna.
More Insight into How Our World is Changing
The next goal is to refine the accuracy to just a few meters. This will be achieved using antennas capable of either tracking the satellites or receiving signals from multiple directions. Additionally, measurements will be conducted at multiple locations to enhance precision and minimize errors.
With a larger dataset, researchers can improve orbital calculations, leading to more accurate positioning and better modeling of Earth’s gravitational field. The navigation working group also aims to develop advanced signal processing techniques to extract more precise data from signals that, until now, have not been widely used for geodetic applications.
Berglez stated, “By being able to utilize the communication signals for geodesy, we have revealed enormous potential for the even more detailed investigation and measurement of our Earth, 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.
He concluded, “Just to be on the safe side, I would like to make the following clear: we are analyzing 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.”