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RLS (Return Link Service)

As part of the Cospas-Sarsat System, Galileo satellites are capable of picking up emergency signals emitted on 406 MHz by distress beacons, and transmitting a signal back to the beacon via the E1 frequency to confirm receipt of the distress signal. This technology has been developed under the international Cospas-Sarsat program. It comprises the Galileo-enabled distress beacons, the SAR payload on the Galileo satellites, as well as ground-based receiving stations (LUTs) and Mission Control Centers.

If the option "Galileo" (Opt-GAL) is installed, GSG is capable of simulating the Return Link Message (RLM) transmitted by the Galileo signal to the distress beacon.

As a user you interact with the GSG by inputting the RLM parameters as a SCPI command into the computer; GSG will then take this message and simulate its transmission on the E1 frequency from the satellite to the distress beacon.

For more information on SCPI command syntax, and examples, see SOURce:SCENario:RLM.

SAR Data

Each Return Link Message encapsulated in an SAR data page contains the following data:

  • Beacon ID (60 bits): This field is used by the beacon to discern whether the RLM received is, indeed, addressed to itself or to some other beacon.
  • Message code (4 bits): Defines the message type.
  • Parameters field (16 bits for the short RLM, 96 bits for the long RLM). This field provides the information that SAR operators wish to send to the Galileo-equipped beacon.

Short RLMs are used to provide the activated Galileo-equipped beacon with a short acknowledgement of various kinds of commands (e.g., to reduce its transmission rate).

Long RLMs are intended for more complex commands in which several parameters may be required (e.g., to contain operational information or the coordinates of a location).

For more information on SCPI command syntax and examples, see SOURce:SCENario:RLM.


In order for the RLM simulation to work with GSG, the following pre-requisites must be met:

  • The Galileo-Option OPT-GAL must be present
  • GSG-5 or -6, with 8 channels (or 16 channels if GPS + GAL is required)
  • Firmware version 6.6.1 or greater
  • StudioView software Version or greater

Simulating RLMs

The following is a brief outline how to setup a basic test system:

  1. Using a GSG unit, and a GNSS receiver, start a scenario that is configured with GAL signals e.g., GPSGALStatic.
  2. Open StudioView on a Personal Computer (PC).
  3. Open the Console tool (Tools > Console).
  4. Connect the PC to the GSG unit by clicking the small GLOBE with PLUG icon on the left (or click Refresh if a connection has been established before).
  5. Issue the RLM command(s), using the Console e.g.:
    write SOUR:SCEN:RLM 0,8,711888,141509,1025,65536
  6. The submission should be confirmed: OK [no errors].
  7. View the received message on the receiver, see the following example:

For automated testing e.g., when integrating GSG into a larger test system, a SCPI program can be written. In this case, StudioView would not be needed.