Astronomers Measure Radio Emissions from Geostationary Satellites

Radio astronomy faces a significant challenge as satellites orbiting high above Earth increasingly interfere with the frequencies used by scientists to explore the cosmos. A recent study led by researchers at the CSIRO Astronomy and Space Science division has provided critical insights into this issue, specifically examining geostationary satellites located at an altitude of 36,000 kilometres. These satellites, which remain fixed in position relative to the Earth, manage various communications from television broadcasts to military data.

Historically, there has been considerable focus on the radio emissions from low Earth orbit satellites, particularly those from companies like SpaceX and their Starlink constellation. However, the impact of satellites in geostationary orbit has not been systematically assessed until now. The study utilized archival data from the GLEAM-X survey, captured by Australia’s Murchison Widefield Array in 2020, analyzing radio frequencies ranging from 72 to 231 megahertz.

Key Findings on Emissions from Geostationary Satellites

The research team observed up to 162 geostationary and geosynchronous satellites over a single night, employing a technique that involved stacking images at each satellite’s predicted position to identify any radio emissions. The results were promising: the majority of these satellites showed no detectable emissions that could interfere with astronomical observations.

For the most part, the study established upper limits on unintended emissions, with values better than 1 milliwatt of equivalent isotropic radiated power across a bandwidth of 30.72 megahertz. Notably, the best limits reached an impressive 0.3 milliwatts. The only satellite that indicated potential emissions was Intelsat 10-02, which recorded a possible detection of around 0.8 milliwatts. Even this satellite’s emissions were significantly lower than those typically observed from low Earth orbit satellites, which can emit hundreds of times more power.

The study’s methodology involved pointing the telescope near the celestial equator, allowing each satellite to remain within the instrument’s wide field of view for extended durations. This strategy enabled sensitive stacking techniques to uncover even intermittent emissions that might otherwise go unnoticed.

Implications for Future Observations

The Square Kilometre Array, set to be operational in Australia and South Africa, will be vastly more sensitive than current radio telescopes, particularly in the low-frequency range. What appears as background noise to existing instruments may become significant interference for the SKA. The findings from this research provide essential baseline data that can help predict and mitigate future radio frequency interference.

As satellite technology advances and the number of orbiting satellites increases, the pristine radio quiet that astronomers traditionally relied upon is gradually diminishing. Even satellites designed with frequency protection can inadvertently leak emissions through their electrical systems, solar panels, and onboard computers.

Currently, geostationary satellites are proving to be considerate participants in the low-frequency radio spectrum. Whether this trend continues as technology evolves and satellite traffic increases remains a critical question for the future of radio astronomy. The ongoing monitoring of these emissions will be essential to safeguarding the integrity of astronomical observations in the years to come.