Unveiling the Radio Silence Above: A Study of Geostationary Satellite Emissions
The vast expanse of space, once a pristine canvas for astronomers, is now facing a new challenge. As radio telescopes become more sensitive, the invisible pollution from satellites thousands of kilometres above is becoming a growing concern. While much attention has been given to the low Earth orbit satellites, a recent study has shed light on the radio emissions from a unique zone in space: geostationary orbit.
The Geostationary Orbit: A Special Zone
At an altitude of 36,000 kilometres, hundreds of satellites reside in a special zone known as geostationary orbit. These satellites move at the same rate as Earth's rotation, appearing stationary from our perspective on the ground. They play a crucial role in various communications, from television broadcasts to military operations. Unlike their low orbit counterparts, which swiftly cross the sky, geostationary satellites remain within a telescope's field of view for extended periods, making them a significant consideration for astronomers.
Measuring the Unseen Emissions
Until now, the potential radio emissions from these distant satellites were largely unknown. A team of researchers from CSIRO's Astronomy and Space Science division has taken on the task of measuring these emissions. Using archival data from the GLEAM-X survey, captured by Australia's Murchison Widefield Array in 2020, they analysed observations in the low frequency range of 72 to 231 megahertz. This frequency range is of particular interest as it will be utilised by the upcoming Square Kilometre Array (SKA).
The team's innovative approach involved tracking up to 162 geostationary and geosynchronous satellites over a single night, stacking images at each satellite's predicted position to search for radio emissions. The results were remarkable.
The Majority Remain Invisible
The study revealed that the vast majority of these distant satellites remain invisible to radio telescopes in the specified frequency range. The team established upper limits for most satellites, with an impressive low limit of 0.3 milliwatts of equivalent isotropic radiated power in a 30.72 megahertz bandwidth. Only one satellite, Intelsat 10-02, showed a possible detection of unintended emission at around 0.8 milliwatts, which is still significantly lower than emissions from low Earth orbit satellites.
The Importance of Distance and Geometry
The significance of these findings lies in the distance and geometry of geostationary satellites. Positioned ten times farther from Earth than the International Space Station, even relatively strong radio emissions fade to faint whispers by the time they reach telescopes on the ground. The study's observation strategy, focusing on the celestial equator, allowed for extended periods of satellite visibility within the telescope's wide field of view, enabling sensitive stacking techniques to detect intermittent emissions.
SKA's Sensitivity and Interference
The upcoming Square Kilometre Array in Australia and South Africa is set to be an order of magnitude more sensitive in the low frequency range. This heightened sensitivity could turn what appears as harmless background noise today into devastating interference for SKA. The new measurements provide crucial baseline data for predicting and mitigating future radio frequency interference.
The Evolving Satellite Landscape
As satellite constellations continue to proliferate and radio telescopes become more sensitive, the once-pristine radio quietness relied upon by astronomers is slowly disappearing. Even satellites designed to avoid protected frequencies can leak unintended emissions through electrical systems, solar panels, and onboard computers. For now, geostationary satellites seem to be respectful neighbours in the low frequency radio spectrum. However, the future remains uncertain as technology advances and satellite traffic increases.
Source: Limits on Unintended Radio Emission from Geostationary and Geosynchronous Satellites in the SKA-Low Frequency Range (https://arxiv.org/abs/2512.07341)
