GMDSS Handbook Annex-6

GMDSS Handbook

Annex - 6
COSPAS-SARSAT System Data

INTRODUCTION
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 (SAP) 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. The COSPAS-SARSAT system Is intended to serve all organizations in the world with responsibility for SAP 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.
The major advantage of the 406 MHz system described above is the provision of global Earth coverage using a limited number of polar-orbiting satellites in tow Earth orbit (LEO). Its basic limitation is that continuous LEO Search and Rescue (LEOSAR) coverage would require a large number of satellites which cannot be achieved at a cost acceptable to the COSPAS-SARSAT space segment providers, This consideration led the COSPAS-SARSAT participants to experiment with 406 MHz repeaters onboard geostationary Earth orbit (GEO) satellites, with a view to enhance the 406 MHz LEOSAR system while ensuring the full compatibility of the GEOSAR components with existing 406 MHz beacon designs.
The 406 MHz GEOSAR system currently comprises:
-406 MHz repeaters onboard three geostationary satellites, plus one spare; and
-6 experimented ground receiving stations (GEOLUTs) In five countries, linked to the COSPAS-SARSAT MCC network for the distribution of GEOSAR alert data.
GEOSAR satellites have the capability to provide almost immediate distress alerts using existing 406 MHz beacons, although without the distress location as there is no Doppler shift on the uplink frequency. To take full advantage of the GEOSAR alerting capabilities, new coding schemes have been defined allowing 406 MHz beacons with an in-built satellite navigation receiver, or an interface for external navigation data input, to transmit position data in the 406 MHz digital message.
BASIC CONCEPT OF THE SYSTEM
The basic COSPAS-SARSAT system concept is given in Figure 7, There are at present three types of beacons, namely Emergency Locator Transmitters (ELTs) (airborne), EPIRBs (maritime) and Personal Locator Beacons (PLBs) (land). These beacons transmit signals that are detected by COSPAS-SARSAT polar-orbiting satellites equipped with suitable receivers/processors. 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 Centra (MCC), either to a national RCC, another MCC or to the appropriate SAP 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 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 altitude of the COSPAS satellites' orbit Is approximately 1,000km while that of SARSAT satellites is about 850km. These low-altitudes result 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 (see Figure 6). See Chapter 9-8, for the current spacecraft availability status, the operational and planned Mission Control Centres (MCCs) and the status of the existing and planned Local User Terminals (LUTs).
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 can often be estimated 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, see Table 1 at the end of this Chapter. 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 realtime 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 over flight 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, or by an integral global navigation satellite system receiver.
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 over flight 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 (approx5n.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.
406 MHz EPIRB REGISTRATION
Because a beacon may be transmitting from anywhere in the world each beacon should be registered whereby the serial number of the EPIRB, together with any other relevant Information, is included in a suitable registration database. Information encoded in the 406 MHz EPIRB message includes information on the specific databases location. Details of specific registration centres are listed below. It is vitally Important that the registration authorities are Informed promptly of any changes affecting the information given on the registration cards eg: change of vessel, change of ownership, loss, theft etc. Owners of vessels registered outside of the following areas, are advised to contact the relevant flag state to establish the registration procedures.
SPACE SEGMENT
The SAR instrumentation on board the COSPAS and SARSAT satellites operates in the following modes:
- realtime mode: 121.5 MHz repeater;
- realtime mode: 406.025 MHz data processing and downlink; and
- global coverage mode: 406.025 MHz stored data transmission.
The equipment on board the satellite consists of the following basic sub-assemblies:
- 121.5 MHz receivers;
- 406.025 MHz receiver/processor and memory unit; and
- 1544.5 MHz downlink transmitter.
a. 121.5 MHz receiver This unit has a bandwidth of 25 kHz. Automatic level control (ALC) is provided to maintain a constant output level.
b. 406.025 MHz receiver/processor
The functions of the receiver-processor are as follows:
- demodulating the digital messages received from beacons;
- measuring the received frequency; and
- time tagging the measurement.
All these data are included in the output signal frame for downlinking to LUTs. The frame is transmitted at 2400 bps in the realtime mode and also stored in memory for later transmission. The on-board memory is dumped in the same format and at the same bit rate as realtime data. LUTs thus directly receive the stored beacon messages acquired during an entire orbital revolution. If a beacon signal is received during the stored memory dump, the dump is interrupted so that the signal can be processed and the resultant message interleaved with the stored data. Appropriate flag bits indicate whether the data are realtime or stored and the time at which full playback of the stored date was accomplished.
The average global response time for LUTs receiving stored and subsequently dumped data from 406 MHz beacon signals through the COSPAS-SARSAT satellites is 1 hr. The best case is about 30 mins and the worst 2h.
c. 1544.5 MHz downlink transmitter The 1544.5 MHz downlink transmitter accepts input from the 406 MHz receiver/processor and receiver(s) operating on the other COSPAS-SARSAT band(s), adjusts the relative power level in accordance with ground command, phase modulates a low-frequency carrier with the composite signal, multiplies the frequency to produce 1544.5 MHz, amplifies the power level and drives the satellite downlink antenna.
LOCAL USER TERMINALS (LUTs) AND MISSION CONTROL CENTRES (MCCs)
The configuration and capabilities of each LUT vary to meet the specific requirements of countries, but the COSPAS and SARSAT satellite downlink signal formats ensure inter-operability between the various satellites and all LUTs meeting COSPAS-SARSAT specifications.
There are two types of LUTs, those which process 121.5 MHz and 406 MHz signals and those that process 406 MHz signals only.
For the 121.5 MHz signal, each transmission is detected and the Doppler shift information calculated. A beacon location is then determined using this data. All 406 MHz data received from the satellite memory on each pass can be processed within a few minutes of pass completion.
MCCs have been set up in each country operating at least one LUT. Their main functions are to collect, store and sort the data from LUTs and other MCCs, and to provide such data to SAR networks. Most of the data handled consists of:
- Alert data: This is the generic term of COSPAS-SARSAT 121.5 and 406 MHz data derived from distress beacon (EPIRB) information. Alert data comprises the beacon location and (for 406 MHz satellite EPIRB) other information such as beacon identification data and coded information.
- System information: This is primarily used to maintain COSPAS-SARSAT operations efficiently and to provide users with as much accurate and timely alert data as possible. It consists of tabulated data (ephemeris and time calibration) used to determine beacon locations, the current status of all sub-systems, and co-ordination messages required to operate the COSPAS-SARSAT system.
The COSPAS Mission Control centre (CMC) in Moscow is responsible for co-ordinating all COSPAS activities and provides the link through the SARSAT MCCs for all interaction with the SARSAT system. The CMC computes and sends COSPAS satellite ephemeris data to other MCCs and LUTs, and receives, processes and transmits SARSAT ephemeris and time calibration data received from the SARSAT MCC to the COSPAS MCCs and LUTs.
A designated MCC in the United States acts as a focal point for the co-ordination of SARSAT satellite operations. It compiles many 406 MHz satellite EPIRB locations using stored data received from LUTs, distributes ephemeris data, processes time calibration data (required for use of SARSAT 406 MHz data), and forwards the appropriate results to other MCCs. This MCC acts as the main system operational contact point between SARSAT and the CMC.
PERFORMANCE PARAMETERS (OF THE SYSTEM)
The following parameters are particularly important for the user:
a. EPIRB detection probability: EPIRB detection probability for the 406 MHz satellite EPIRB is defined as the probability of detection by a LUT of at least one message with a correct code-protected section from the first tracked satellite.
b. EPIRB location probability: EPIRB location probability for the 406 MHz satellite EPIRB is defined as the probability of detecting and decoding at least four individual message bursts during a single satellite pass so that a Doppler curve-set estimate can be generated by the LUT. At 121.5 MHz, EPIRB location probability is defined as the probability of location during a satellite pass above 10░ elevation with respect to the beacon. EPIRB location probability relates to the two solutions ("true" and "mirror") and not to a single unambiguous result.
c. EPIRB location error: EPIRB location error is defined as the difference between the location calculated by the system using measured Doppler frequencies and the actual location.
d. Ambiguity resolution probability: Ambiguity resolution probability is defined as the ability of the system to select the "true" rather than the "mirror" location.
e. Capacity: Capacity is defined as the number of EPIRBs in common view of the satellite which the system can process simultaneously.
f. Coverage: The 121.5 MHz system operates in realtime only, while the 406 MHz system operates in both realtime and global (stored-data) modes. The overall coverage provided by the COSPAS-SARSAT system in realtime mode is determined by the number and positions of LUTs, each covering an area with a radius of approximately 2500 km. In the global coverage mode, using 406 MHz EPIRBs, complete world coverage is achieved.
g. Notification time: Notification time is the period from activation of an EPIRB (ie. first transmission) to reception of a valid alert message by the appropriate RCC.
Notification time depends on the following parameters:
- satellite constellation;
- LUT configuration; - EPIRB location relative to an LUT;
- EPIRB latitude; and - ground communication network. The system performance characteristics are given in Table 1.
It should be noted that performance at 121.5 MHz is highly sensitive to EPIRB spectral characteristics. The values given below were confirmed by statistical analysis of over 5000 beacons during the development and evaluation phase of the system.

Radio Officer й 2002 Edition