Return Link Service, a major PLB & EPIRB improvement

Ben Ellison

Ben Ellison

Panbo editor, publisher & chief bottlewasher from 4/2005 until 8/2018, and now excited to have Ben Stein as very able publisher, webmaster, and editing colleague. Panbo is going to the next level in 2019 and beyond.

4 Responses

  1. Jim Hebert says:

    From what (little) I understand about the GALILEO Return Link Service (RLS), there are (or were) two tiers for Return Link Messages (RLM) . In the first tier, the acknowledgement message (RLM Acknowledgment Type 1) would be autonomously generated by the GALILEO satellite system, and would mean that the 406-MHz distress message had been received by the satellite. The GALILEO system would generate this message without involvement of SARSAT.

    A second tier message, called Acknowledgement Type II, would be initiated by a SARSAT Rescue Coordination Center (RCC), and would indicate the distress message had been received by the RCC. This would indicate that the distress alert had reached the proper authorities for initiating assistance. This message would require the RCC to be able to uplink to the GALILEO satellite system in some manner. I suspect that granting access like that will be a carefully controlled process.

    By the way, the inclusion of the 406-MHz bent-pipe transponder on GALILEO FOC space vehicles is not some little add-on feature. The receive antenna array is very significant. More details and some images are at

    SARSAT MEO 406-MHz System on GALILEO FOC Satellites
    http://continuouswave.com/forum/viewtopic.php?f=12&t=2951&p=23596

    • Ben Ellison Ben Ellison says:

      Thanks, Jim. You know a lot!

      I believe that RLM Type 1 was chosen, but that the message originates from the Galileo MEOLUT in Toulouse, France, not from the satellites. No one has been able to tell me if it’s a human or an algorithm that actually sends the Return Link Message, but the distress alert and location have gotten ashore and hopefully dispatched to the correct rescue center.

      The system diagrams have varied but I think the one above is correct and here’s some confirmation:

      https://www.oroliamaritime.com/solutions/galileo-returnlink-rescue/

  2. Jim Hebert says:

    There is a comment from Orolia that the return link message may take 15-minutes to be received by the PLB or EPRIB to which it is address. At first I thought perhaps this was due to some delay for the authorities to assess the nature of the distress or the location of the distress, and would indicate involvement of a human.

    On more (and clearer) thinking, perhaps the 15-minutes is due to the method of transmission of the message from the GALILEO FOC satellites. I believe the message will be embedded in the some manner (that is retro-fitted, that is, won’t break any current GALILEO receivers trying use the system) into the navigation message sent by the satellites. I am not expert of GALILEO, but if it is similar to GPS, the baud rate of the navigation message is very low, about 50-bps, and the complete navigation message is broken into segments for transmission. To receive the entire navigation message might take as long as about 15-minutes. This would account for the 15-minute latency, even if the acknowledgement was coming autonomously from the satellite.

    Also, there is no guarantee that the satellite that receives the 406-MHz transmission is going to be in view for the next 15-minutes. There must be some method to let other satellites in the GALILEO also encode the acknowledgment message into their navigation messages.

    I am interested to learn more about the Return Link Service. Maybe the engineers at Orolia can peel back another layer on this onion.

  1. November 26, 2019

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