The MEOSAR (Medium Earth Orbiting Search and Rescue) satellite system has achieved Early Operational Capability (EOC) in the Cospas-Sarsat system. This means that the MEOSAR system isnít complete, but system data is being used to locate distress beacons. It joins the existing LEOSAR (Low Earth Orbiting) and GEOSAR (Geostationary) SAR satellite systems.
LEOSAR and GEOSAR receivers are mounted on weather satellites. LEOSAR determines beacon location using Doppler and can relay position data (encoded solution) from beacons. GEOSAR cannot determine beacon location, but can provide alert notification and encoded solutions from the beacon.
The MEOSAR receivers are located on global navigation system satellites (GNSS) - US GPS, European Galileo and Russian GLONASS. MEOSAR determines beacon position by measuring the time difference of arrival (TDOA) and frequency difference of arrival (FDOA) of the beacon signal at the satellites. Reception by three satellites is necessary for a 2D (lat/lon) position and four satellite reception adds altitude to the beacon position (3D).
This is similar to how a cell phone system can determine a cell phone location. The towers measure the TDOA of the signal from the phone. Itís the reverse of how a GPS receiver works. The GPSr determines its position using the TDOA of the signal from the GNSS satellites. Since MEOSAR also measures FDOA, it has two independent means of determining beacon position. Like LEOSAR and GEOSAR, MEOSAR can relay encoded position data from beacons.
The current numbers of operational satellites are 5 LEO, 9 GEO and 32 MEO. When complete, there will be 75 satellites in the MEOSAR system.
The advantages of the MEOSAR system (some not until the system is fully operational) include:
1. Determination of beacon location with only a single beacon burst. LEOSAR and GEOSAR can provide single burst location only if GNSS location is encoded in the burst. There are many cases where a beacon was able to send out a single burst without location before it stopped transmitting due to the incident environment.
2. Much less likely that the beacon signal isnít detected due to terrain blockage, due to the large number of satellites in the constellation.
3. More data for locating the beacon - TDOA, FDOA and the GNSS position from beacons so equipped.
4. Near real time detection and location.
5. Expected position accuracy much better than LEOSAR Doppler accuracy.
6. Continuous, complete coverage of the Earth.
7. Return-link capability between SAR and beacon (through Galileo satellites only)
MEOSAR info: https://www.cospas-sarsat.int/en/search-and-rescue/transition-to-meosar