Communication with the ISS
How do astronauts on the space station stay in touch with Earth?
If you’ve ever suffered patchy phone signal on a train journey, you’ll know that high-speed travel and consistent network coverage do not go together. As your phone tries to switch between the closest available network masts, signal can fluctuate and make communication unreliable. But what if instead of being on a train you’re orbiting Earth at over 27,500 kilometres per hour, and a loss of communication could be the difference between life and death? These are the stakes on the International Space Station.
Regular contact between the astronauts and ground control is vital for the proper running of the station and the safety of the astronauts onboard. However, signals can’t reliably be sent directly between mission control and the ISS via a single transceiver on Earth. Radio waves would not be able to travel through the Earth itself, so while the station travels over the other side of the planet, it would spend a significant proportion of its 90-minute orbit out of the transceiver’s range.
To solve this problem NASA uses the Tracking and Data Relay Satellite System (TDRS), a network of satellites that send and receive signals between the ISS and various ground stations on Earth. The seven active satellites that currently make up the TDRS are in geosynchronous orbit, positioned around the globe to provide continuous coverage to the ISS.
When astronauts need to contact mission control, their signals are sent from the ISS to the closest satellite in the TDRS constellation. The satellite then relays the signal to the ground station below, where it can be forwarded on to the relevant mission control centre.