The SAR/Galileo Initial Service forms part of the contribution to the COSPAS-SARSAT Medium-Altitude Earth Orbit Search and Rescue (MEOSAR) Programme through the provision of a Forward Link Alert Service. The forward link is the function that relays an incoming distress call from a beacon to the satellite and from there to the Mission Control Centre (MCC) and the Rescue Control Centre (RCC). It works with any “standard” distress beacon using the 406 MHz international distress frequency and can be picked up by any COSPAR-SARSAT satellite including Galileo.
The SAR/Galileo Initial Service is based on the infrastructure provided by the Galileo Programme which is composed of the following elements:
The SAR/Galileo Initial Service also utilises the signals relayed by the other available MEOSAR payloads (GPS/DASS S-Band and SAR/Glonass) to provide increased service availability to the COSPAS-SARSAT Mission Control Centres.
The signals relayed by the MEOSAR satellites are received by the three European MEOLUTs which ensures the provision of the service over a coverage zone called the ‘European SAR Coverage Area (ECA)’. Worldwide coverage is achieved through the COSPAS-SARSAT cooperation scheme.
The SAR/Galileo ground segment (SGS) is organised in Europe with the Centre National d’Etudes Spatiales in Toulouse (CNES) hosting the SGSC which manages the system operations, and ground stations receiving distress signals relayed by the first operational constellation SAR/Galileo or other MEOSAR. Reference beacons are disseminated in the ECA to monitor the system and service status. All the ground elements communicate via the dedicated network called SARN, which provides the ability to deploy the first extensive network of MEOLUTs.
The MEOSAR system provides independent distress beacon location information using a combination of Time Difference of Arrival (TDOA) and Frequency Difference of Arrival (FDOA) techniques. MEOLUTs calculate the beacon location by measuring and processing the time and frequency differences of the same beacon burst relayed by different satellites.
MEOSAR location accuracy is affected by many factors including the number of time and frequency measurements available at the MEOLUT for a particular beacon burst, the accuracy of the time and frequency measurements, and the geometry between the beacon and the satellites.
The time required for a MEOSAR system to produce independent location information is also affected by several factors. The most significant of these is the length of time required for multiple satellites to provide simultaneous visibility of the beacon and a MEOLUT.
Each MEOLUT consists of four reception antennas. The MEOLUTs’ role is to track SAR satellites in view, detect beacon distress messages, compute the beacon location, and provide alerts to MCCs.
The geographical position of the beacon is computed using TOA/FOA measurements and the position of associated satellites at burst time.
The nominal functions of a MEOLUT are:
In addition, the SAR/Galileo MEOLUTs have some specific features:
The advantage of networked MEOLUTs mode is to improve the probability and accuracy of localisation by exchanging the TOA/FOA measurements between MEOLUTs.The figure below shows the difference between the stand-alone configuration and the networked configuration.
The SAR/Galileo Service roadmap foresees the introduction of an additional service called "Return Link Service" to the currently provided Forward Link Alert Service. It would provide an acknowledgment capability to the distress beacons confirming that the alert has been detected and located by the system. This service, which will be provided worldwide, will rely on the Galileo L1 navigation signal to transmit the acknowledgment messages to the distress beacon and will only be available for RLS-enabled beacons.
This new service is planned to be operational by 2017.