The Galileo Search and Rescue (SAR) service is the contribution from the European constellation to the international Cospas-Sarsat system. This system uses a variety of space and ground infrastructure to detect and localize distress signals from beacons on the 406MHz frequency. Satellites in different orbits detect the signals coming from the Earth and transmit them back to Earth stations that route them to the appropriate government authorities.

On top of the standard detection and rely service, the Galileo constellation is the first to offer a Return Link Service (RLS) that acknowledges the processing of the distress signal with a return link message (RLM) back to the originating beacon. This RLM is transmitted in the SAR field of the E1 signal I/NAV message, which allocates 20 bits every 2-second page. Therefore, transmitting a short RLM (80 bits) takes four consecutive pages or eight seconds. Moreover, each RLM is transmitted in parallel from two Galileo satellites. The RLS has been active since 2020, avoiding the spotlight of the GNSS community.

This paper presents an analysis of the SAR return link messages extracted from more than 3 months of signal-in-space data to investigate the current bandwidth use, detect anomalies in the service, and briefly propose authentication ideas for the messages. To extract and parse the return link messages, we have developed and published an open-source Python library called GalileoSARlib on GitHub, which is also detailed in the paper.