COSPAS-SARSAT -Search and Rescue Satellite system

ABSTRACT:

      Countries use satellites in many areas such as military, navigation, search and rescue, etc. In this paper we have dealt with one of the newest search and rescue satellite system – THE COSPAS-SARSAT SYSTEM. It is an international search and rescue system made up of a network of satellites in space and control centres on Earth-ground stations, mission control centres and rescue coordination centres.

      Search and rescue satellites are designed to provide a way for vessels at sea and in the air to communicate from remote areas. These satellites can detect and locate emergency beacons carried by ships, aircrafts, or individuals in remote or dangerous places.

Satellites equipped with search and rescue equipment (transmitter) fly over a beacon that is releasing an emergency signal. Using mathematical calculations involving the Doppler Effect, scientists can translate that signal into coordinates and determine the location of the distress signal within four kilometers.

In the eyes of countries using the Cospas-Sarsat system, this system has very much helped the search and rescue efforts.

INTRODUCTION:

            The Sarsat system was developed in a joint effort by the United States, Canada and France. The Cospas system was developed by the Soviet Union. These four nations banded together in 1979 to form Cospas-Sarsat. In 1982, the first Cospas-Sarsat satellite was launched and by 1884 the system was declared fully operational. From there, the Cospas-Sarsat organization continued to grow. The four original members have now been joined by 25 other nations that operate 28 ground stations and 15 mission control centres worldwide.

The idea of search and rescue satellites comes from weather satellites programs. Meteosat, for example, monitors weather patterns by direct observation, but it also analyzes signals sent out from buoys floating in the ocean. Those buoys collect local weather information and transmit that to the satellite. The satellite then uses the Doppler Effect to find out where that particular buoy is. This mechanism is similar to the technology now used for search and rescue.

 Cospas-Sarsat is useful to private and corporate fliers as well as airline crews, pleasure boaters and small scale commercial fisherman and merchant shippers. Whether a plane has gone down in a remote mountain range or ship is floundering in the sub arctic Atlantic, satellites direct distress signals and alert rescue teams.

SYSTEM CONCEPT:

The basic Cospas-Sarsat concept is illustrated in the figure below:

The system is composed of:

Distress radiobeacons (ELTs for aviation use, EPRIBs for maritime use and PLBs for personal use) which transmit signals during distress situations;

Instruments on board satellites in geostationary and low-altitude Earth orbits which detect the signals transmitted by distress radiobeacons;

Ground receiving stations referred to as Local User Terminals (LUTs), which receive and process the satellite downlink signal to generate distress alerts; and

Mission Control Centers (MCCs) which receive alerts produced by LUTs and forward them to Rescue Coordination centers (RCCs), Search and rescue Points Of Contacts (SPOCs) or other MCCs.

The Cospas-Sarsat System includes two types of satellites:

Satellites in low-altitude

Channel Processing

LEOSAR

GEOSAR  

406MHz

Beacon identification information and location information provided

Global coverage, but not instantaneous

Beacon identification provided and location information available if encoded in beacon message (location protocol beacon)

Near instantaneous alerting in the GEOSAR coverage area

Earth orbit (LEO) which form the LEOSAR System

Satellites in geostationary Earth orbit (GEO) which form the GEOSAR System

Cospas-Sarsat has demonstrated that the GEOSAR and LEOSAR system capabilities are complementary. For example the GEOSAR system can provide almost immediate alerting in the footprint of the GEOSAR satellite, whereas the LEOSAR system:

Provides coverage of the polar regions (which are beyond the coverage of geostationary satellites);

Can calculate the location of distress events using Doppler processing techniques; and

Is less susceptible to obstructions which may block a beacon signal in a given direction because the satellite is continuously moving with respect to the beacon.

LEOSAR AND GEOSAR CAPABILITIES:

THE COSPAS-SARSAT 406 MHz SYSTEM:

The Cospas-Sarsat system detects and locates distress beacons operating at 406 MHz.

The 406 MHz system is composed of:

• 406 MHz radio beacons carried aboard ships (EPRIBs), aircraft (ELTs), or used as personal locator beacons (PLBs);
• polar-orbiting satellites in low-earth orbit from the LEOSAR system and geostationary satellites from the GEOSAR system; and
• The associated LUTs for the respective satellite systems (referred to as LEOLUTs for the LEOSAR system and GEOLUTs for the GEOSAR system).

406 MHz BEACONS:

Frequencies in the 406.0 - 406.1 MHz band have been exclusively reserved for distress beacons operating with satellite systems. The Cospas-Sarsat 406 MHz beacons have been specifically designed for use with the LEOSAR system to provide improved performance in comparison to the older 121.5 MHz beacons. They are more sophisticated because of the specific requirements on the stability of the transmitted frequency, and the inclusion of a digital message which allows the transmission of encoded data such as unique beacon identification.

Second generation 406 MHz beacons have been introduced since1997 which allow for the transmission in the 406 MHz message of encoded position data acquired by the beacons from global satellite navigation systems, using internal or external navigation receivers. This feature is of particular interest for GEOSAR alerts which, otherwise, would not be able to provide position information.

 LEOSAR SYSTEM:

      The Cospas-Sarsat 406 MHz LEOSAR system uses polar-orbiting satellites and therefore, operates with basic constraints which result from non-continuous coverage provided by LEOSAR satellites. The use of low-altitude orbiting satellites provides for a strong Doppler effect in the up-link signal thereby enabling the use of Doppler positioning techniques. The Cospas-Sarsat 406 MHz LEOSAR system operates in two coverage modes, namely local and global coverage.

 406 MHz LEOSAR LOCAL MODE:

         When the satellite receives 406 MHz beacon signals, the on-board Search and Rescue Processor (SARP) recovers the digital data from the beacon signal, measures the Doppler frequency shift and time-tags the information. The result of this processing is formatted as digital data which is transferred to the satellite downlink for transmission to any LEOLUT in view. This data is also simultaneously stored on the spacecraft for later transmission and ground processing in the global coverage mode.

      In addition to the 406 MHz local mode provided by the 406 MHz SARP instrument, a 406 MHz repeater, on Sarsat satellites only, can also provide a 406 MHz local mode operation. The difference between the SARP and the repeater is that the SARP performs some of the processing on board the satellite, whereas the repeater simply reflects the beacon signal to the earth, thereby requiring additional processing on the ground.

406 MHz LEOSAR GLOBAL MODE:

        The 406 MHz SARP system provides global coverage by storing data derived from on-board processing of beacons signals, in the spacecraft memory unit. The content of the memory is continuously broadcast on the satellite downlink. Therefore, each beacon can be located by all LEOLUTs which track the satellite (even for LEOLUTs which were not in the footprint of the satellite at the time the beacon was detected by the satellite). This provides the 406 MHz global coverage and introduces ground segment processing redundancy.

       The 406 MHz global mode may also offer an additional advantage over the local mode in respect of alerting time. As the beacon message is recorded in the satellite memory by the first satellite pass which detected the beacon, the waiting time is not dependent upon the satellite achieving simultaneous visibility with the LEOLUT and the beacon. Consequently, the time required to produce alerts could be considerably reduced.

406 MHz GEOSAR SYSTEM:

       Cospas-Sarsat has demonstrated that the current generation of Cospas-Sarsat 406 MHz beacons could be detected using search and rescue instruments on board geostationary satellites. The GEOSAR system consists of 406 MHz repeaters carried on board various geostationary satellites and the associated ground facilities called GEOLUTs which process the satellite the satellite signal.

      Geostationary satellites orbit the earth at an altitude of 36,000 km, with an orbit period of 24 hours, thus appearing fixed relative to the earth at approximately 0 degrees latitude (i.e. over the equator). A single geostationary satellite provides GEOSAR uplink coverage of about one third of the globe, except for Polar Regions. Therefore, three geostationary satellites equally spaced in longitude can provide continuous coverage of all areas of the globe between approximately 70 degrees North and 70 degrees South latitude.

          Since GEOSAR satellites remain fixed relative to the earth, there is no Doppler effect on the received frequency and therefore, the Doppler positioning technique cannot be used to locate distress beacons. To provide rescuers with position information, the beacon location must be either:

Acquired by the beacon through an internal or external navigation receiver and encoded in the beacon message, or

Derived from the LEOSAR system Doppler processing.

INDIA’S PARTICIPATION:

India is having international cooperation in Satellite Based Search and Rescue Operations Service. The government of India has launched a program to promote, participate and contribute to the development of international Satellite-Aided Search and Rescue system (SAS&R) for maritime, airborne and land distress alert detection and position location

This national (SAS&R) is a multi pronged plan involving:

• Use of the COSPAS-SARSAT Space Segment and establishment of Local user Terminal (LUT) in India.
• Indigenous development of 406 MHz Emergency Locator Beacons (ELBs).
• Contribution to the future international SAS&R space segment by carriage of SAS&R payloads on future Indian satellites.
• Securing first-level participation for India in the international SAS&R system.