RLS stands for Return Link Service, a Galileo service that sends information back to suitably equipped distress beacons. Its automatic acknowledgement tells the person in distress that Cospas-Sarsat has detected the alert and independently determined the beacon's location.
What the indication on the beacon means
A received RLS acknowledgement is shown by a light or text display, depending on the model. It reports a response received through the return link. A power or transmission indicator instead shows the device's operating state or its own transmission activity. The device instructions identify which indication corresponds to each function.
The automatic response is also called a Type-1 Acknowledgement or system acknowledgement. It is generated by the system after detection and location of the alert. The responsible rescue coordination centre assesses the emergency and coordinates the deployment of available resources. That operational decision is separate from the automatic response: Type-1 confirms detection and location, not an already organised or launched rescue operation.
The route back to the distress beacon
The beacon first transmits its digital distress alert in the 406 MHz band. Cospas-Sarsat receives it through satellites and processes it on the ground. For RLS, the alert also contains the protocol-specific request for an acknowledgement. Once the beacon has been located, the response passes through the ground infrastructure to Galileo and is broadcast by Galileo satellites within the navigation signal. This response is called a Return Link Message, or RLM, and is addressed to the originating beacon.
The forward path may use different parts of Cospas-Sarsat; the response is generated after ground processing and sent back through Galileo. Its defined message content serves the automatic acknowledgement function. A conversation or free-text exchange with rescuers requires an additional communications link.
Which devices can use RLS
RLS requires the appropriate transmission protocol and an embedded Galileo receiver that processes the response. An ordinary satellite navigation receiver determines its own position; RLS adds reception and processing of the acknowledgement message. PLB and EPIRB, by contrast, identify personal and maritime uses. Whether an existing device supports RLS depends on its specific design.
Missing acknowledgements and military relevance
A distress alert may already be present in the ground system while the beacon is still awaiting an acknowledgement. Before the indication appears, the request also has to be routed, the response broadcast through Galileo, and the message received and processed by the device. An absent indication leaves open which of these steps is still pending.
The beacon and return-message system use coordinated reception windows and repeated transmissions. Switching the beacon off and on again can disrupt that alignment. In a genuine SAR emergency, an activated RLS beacon remains on in accordance with its instructions, including after an acknowledgement, until the rescue is complete or a relevant authority directs otherwise.
For isolated military personnel, assigned communications procedures govern use of the beacon. RLS supplies the system response to the alert. Information about threats at the location and arrangements for the approach, pickup point and recovery time belong in communications with the responsible recovery forces or command authorities.