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Cospas-Sarsat System

30-04-2020, 09:25 AM
The COSPAS-SARSAT (COSPAS: Space System for Search of Distress Vessels, SARSAT: Search and Rescue Satellite-Aided Tracking) system is a satellite-aided search and rescue (SAR) system designed to locate distress beacons transmitting on the frequencies 121.5 MHz or 406 MHz. Certain beacons also transmit on 243 MHz, but this signal is relayed only by SARSAT satellites and not all LUTs are equipped with 243 MHz receivers, it operates in the same manner as 121.5 MHz system. It is intended to serve all organizations in the world with responsibility for SAR operations whether a distress occurs at sea, in the air or on land.COSPAS-SARSAT is a joint international satellite-aided search and rescue system, established by organizations in Canada, France, the United States and Russia.The COSPAS-SARSAT system has demonstrated that the detection and location of distress signals can be facilitated by global monitoring based on low-altitude satellites in near-polar orbits. It has been used successfully in a large number of SAR operations world-wide.Unless, as an alternative, a ship is provided with a L-band satellite EPIRB operating in sea areas A1, A2 and A3 only, the carriage of a float-free satellite EPIRB operating on the 406 MHz in the COSPAS-SARSAT system is required on all SOLAS ships.
Basic Concept of the System
The basic COSPAS-SARSAT system concept is given in Figure below. There are at present three types of beacons, namely ELTs (airborne), satellite EPIRBs (maritime) and Personal Locator Beacons (PLBs)- land. These beacons transmit signals that are detected by COSPAS-SARSAT polar-orbiting satellite spacecraft equipped with suitable receivers. The signals are then relayed to a ground receiving-station, termed a Local User Terminal (LUT), which processes the signals to determine the beacon location. An alert is then relayed, together with location data and other information, through a Mission Control Centre (MCC), either to a national RCC, another MCC or to the appropriate SAR authority to initiate SAR activities.
Doppler shift (using the relative motion between the satellite and the beacon) is used to locate the beacons. The carrier frequency transmitted by the beacon is reasonably stable during the period of mutual beacon-satellite visibility. The frequencies currently in use are the 121.5 MHz international aeronautical emergency frequency and the 406.0 - 406.1 MHz band. The 406 MHz beacons are more sophisticated than the 121.5 MHz beacons because of the inclusion of identification codes in the messages, but complexity is kept to a minimum. To optimize Doppler location, a low-altitude near-polar orbit is used. The low-altitude results in a low uplink power requirement, a pronounced Doppler shift, and short intervals between successive passes. The near-polar orbit results in complete world coverage over a period of time. The nominal satellite system configuration comprises four satellites, two supplied by COSPAS and two by SARSAT.
The Doppler location concept provides two positions for each beacon: the true position and its mirror image relative to the satellite ground track. This ambiguity is resolved by calculations that take into account the earth's rotation. If the beacon stability is good enough, as with 406 MHz beacons which are designed for this purpose, the true solution is determined over a single pass. In the case of 121.5 MHz beacons, the ambiguity is resolved by the results of the second pass if the first attempt is unsuccessful. Location accuracy is also significantly better with 406 MHz beacons. The improved performance of 406 MHz satellite EPIRBs is the reason these devices were selected for the GMDSS and included in the 1988 amendments to the 1974 SOLAS Convention.
Coverage Modes

The COSPAS-SARSAT system implements two coverage modes for the detection and location of beacons, namely the real time mode and the global coverage mode. Both the 121.5 and 406 MHz systems operate in the real-time mode, while only the 406 MHz system operates in the global coverage mode.

a. Real-time 121.5 MHz repeater data system

If an LUT and beacons are in view of the satellite, a repeater onboard the satellite relays the 121.5 MHz EPIRB signals directly to the ground where it is received and processed. For this reason, world-wide real-time mode coverage is unlikely to be achieved.

b. Real-time 406 MHz processed data system

Once the satellite receives the 406 MHz EPIRB signals, the Doppler shift is measured and the beacon digital data is recovered from the beacon signal. This information is time-tagged, formatted as digital data, and transferred to the downlink repeater for real time transmission to any LUT in the satellites view. The data is simultaneously stored on the satellite for later transmission in the global coverage mode.

c. Global 406 MHz coverage mode

The 406 MHz system provides global coverage by storing data onboard for later dumping and reception by LUTs. Each satellite EPIRB can therefore be located by all operating LUTs.

Satellite EPIRBs

a. 121.5 MHz EPIRB

EPIRBs operating on 121.5 MHz are already in widespread use. They are used onboard light aircraft and ships and must meet national specifications based on International Civil Aviation Organization (ICAO) standards. The EPIRB signals also provide for homing by SAR units and overflight monitoring by aircraft.

b. 406 MHz EPIRB

The development of 406 MHz satellite EPIRBs has been undertaken to overcome certain shortcomings of the 121.5 MHz system.
The new EPIRBs were specifically designed for satellite detection and Doppler location and include the following features:

-improved location accuracy and ambiguity resolution;

-increased system capacity, ie. a greater number of beacons transmitting simultaneously in the field of view of a satellite can be processed;

-global coverage;

-unique identification of each beacon; and

-inclusion of distress information.

The 406 MHz satellite EPIRBs transmit a 5 W radio frequency (RF) burst of approximately 0.5 second duration every 50 seconds. Improved frequency stability ensures improved location accuracy, while the high peak power increases the probability of detection. The low duty cycle provides good multiple-access capability, with a system capacity of 90 activated beacons simultaneously in view of the satellite, and low mean power consumption.

An important feature of the 406 MHz EPIRBs is the inclusion of a digitally encoded message, which may provide such information as the country of origin of the unit in distress, identification of the vessel or aircraft, nature of distress and, in addition, for EPIRBs coded in accordance with the location protocol, the ship's position as determined by its navigation equipment.

Most satellite EPIRBs are, as recommended, dual-frequency 121.5/406 MHz beacons, though the inclusion of the frequency 121.5 MHz is not mandatory. This enables suitably equipped SAR units to home in on the 121.5 MHz transmission and permits overflight monitoring by aircraft. This type of homing facility, if provided, is indicated to the rescue authorities by the message. As Search And Rescue Transponders (SART) have limited range of operation (approx. 5n.miles), consideration is being given to requiring all maritime satellite EPIRBs to operate on the frequencies 121.5 MHz and 406 MHz.

Depending on the type of beacon (maritime, airborne or land), beacons can be activated either manually or automatically.

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