COSPAS-SARSAT (COSPAS: Space System for the Search of Vessels in Distress; SARSAT: Search And Rescue Satellite-Aided Tracking) is a SAR (Search And Rescue) system utilizing both Polar orbiting and Geostationary (Geosynchronous) satellites, which provides Distress alerting and location information to SAR services for aviation, maritime and land-users. There are at present three types of Distress beacons which utilize the system:

• EPIRB – Emergency Position Indicating Radio Beacon (Maritime)
• ELT – Emergency Locator Transmitter (Aviation)
• PLB – Personal Locator Beacons (Land)
  

These beacons transmit radio signals on 406 MHz that are detected by COSPAS-SARSAT polar orbiting and geostationary satellites. Information is subsequently relayed, together with location and ID of casualty through Earth-based LUT (Local User Terminals) and MCC (Mission Control Centres) to the appropriate MRCC (Maritime Rescue and Coordinating Centre) which coordinates all SAR unit deployment in the SAR region where the casualty is located.

COSPAS-SARSAT space segment

The COSPAS-SARSAT system utilizes SAR packages on satellites in (LEOSAR) Low-altitude Earth Orbit and (GEOSAR) Geostationary Earth Orbit.

LEOSAR and GEOSAR satellites in the COSPAS-SARSAT System

LEOSAR

The LEOSAR system configuration comprises 4 satellites, 2 COSPAS (Russia) and 2 SARSAT (USA, Canada and France) in near North-South polar orbit, with an orbital period of approximately 120 minutes. The orbits of these satellites are arranged to scan the entire surface of the Earth. The satellites view an area of the Earth over 6000 km wide as they orbit the globe, giving an instantaneous field of view or footprint (like the illumination from a torch beam) about the size of a continent.

LEOSAR satellite path and footprint

The coverage is not continuous due to the orbital period of the satellite. By the nature of the polar orbits, the waiting time for detection, can be greater in equatorial regions than at higher latitudes, but on average it is 45 minutes.

When the LEOSAR system detects a Distress alert, it calculates the location of the Distress event using Doppler processing techniques. Doppler processing is based upon the principle that the frequency of the distress beacon, as "heard" by the satellite instrument, is affected by the relative velocity of the satellite with respect to the beacon. By monitoring the change of the beacon frequency of the received beacon signal and knowing the exact position of the satellite, the LEOSAR system is able to calculate the location of the beacon with an accuracy of within 5-10 km.

GEOSAR

The GEOSAR system configuration comprises 4 geostationary satellites located at approximately 36000 km above the Equator at different lines of longitude, giving an instantaneous footprint of the whole of the Earths surface nominally between 70⁰ North and 70⁰ South.

GEOSAR coverage areas

In contrast with the LEOSAR satellites the GEOSAR satellites provide continuous coverage only of a major portion of the Earth between 70⁰ North and 70⁰ South with near immediate alerting capability, but they cannot determine the location of the Distress event, because they are fixed with the respect to the earth so they cannot use Doppler-processing techniques. The location of the Distress event must be either:

• acquired by the beacon through an internal or an external position fixing system, for example GPS or Glonass, and encoded in the beacon message, or
• derived, with possible delays, from the LEOSAR System.
  

LEOSAR and GEOSAR capabilities comparison

	LEOSAR	GEOSAR
Location information	provided using Doppler processing techniques	available only if encoded in beacon message
Location accuracy	+/- 5 km	if acquired from GPS: within 10 m
Coverage area	the entire surface of the Earth	a major portion of the Earth between 700 North and 700 South
Distress event detection	the waiting time for detection is on average 45 minutes	near instantaneous

EPIRBs able to utilize the combined system are now available, having a built-in GPS position fixing system. They enable near instantaneous distress event detection with location information included so the SAR operation can start immediately.

COSPAS-SARSAT earth segment

LUT - Local Users Terminal

LEOLUT and GEOLUT Earth based ground stations, receive and process Distress alerts from 406 MHz EPIRBs relaying Distress information, comprising casualty ID, Position and UTC time, to MCC (Mission Control Centres).

MCC - Mission Control Centres

Their main purpose and function is to relay EPIRB Distress alert information to the appropriate MRCC who coordinates the deployment of SAR units in the SAR region in which the casualty is located.