Terrestrial Communication
Long Range Service Terrestrial communication
HF provides long-range service for use in the ship-to-shore and shore-to-ship directions. In areas covered by Inmarsat, it can be used as an alternative to satellite communications and outside these areas, it will provide the only long-range communications capability.
DSC (kHz) Telephony (kHz) Radiotelex (kHz)
- 4 MHz 4297.5 4125 4177.5
- 6 MHz 6312.0 6215 6268.0
- 8 MHz 8414.5 8291 8376.5
- 12 MHz12577.0 12290 12520.0
- 16 MHz16804.5 16420 16695.0
Medium-Range Service Terrestrial communication
A medium-range service is to be provided on frequencies in the 2MHz band (MF). In the ship-to-shore, ship-to-ship and shore-to-ship directions, the following distress frequencies are used:
* DSC – 2187.5 kHz
* Radiotelephony – 2182 kHz
* Radiotelex (NBDP) – 2174.5 kHz
* NAVTEX – 518 kHz Short-Range Service
Terrestrial communication VHF will provide a short-range service on the following distress frequencies:
* DSC – 156.525 MHz (channel 70)
* Radiotelephony – 156.8 MHz (channel 16)
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Satellite Communications
The Inmarsat System
Inmarsat provides the space segment necessary for instant and reliable distress and safety accessed by users on board vessels fitted with either Inmarsat –A, B and C terminals. These terminals can be used to satisfy most of the medium and long-range communications functions specified in the GMDSS.
Inmarsat Concept
Ships fitted with Ship Earth Stations (SES) can also be utilized for originating and receiving communications with other ships involved in distress cases. When multiple ships are involved, the Enhanced Group Call (EGC) system can be advantageous for operational updates and planning actions from RCCs. These ships can also be directly contacted by RCCs via telephony or telex. An RCC may also be equipped with an Inmarsat terminal (SES) for SAR communications. Inmarsat Enhanced Group Calling System (EGC) Inmarsat also provides capability for an L-band satellite EPIRB service called Inmarsat-E. This satellite EPIRB will float free from a sinking ship and automatically transmit the distress alert including the position of the incident. The COSPAS-SARSAT System A satellite distress alerting system based on low altitude near polar orbiting satellites designed to locate distress beacons (EPIRBs).
Concept of the COSPAS-SARSAT System
The COSPAS-SARSAT EPIRBs transmit signals that are received by their satellites. These signals are then relayed to ground receiving stations called Local User Terminals (LUTs), which process the signals to determine the location of the EPIRB. Each LUT is, in turn, linked to a Mission Control Center (MCC) and alert messages are forwarded to the appropriate RCC for action.
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Digital Selective Calling (DSC)
Basic description of DSC
Technical Characteristics
1. The system is a synchronous system using a ten-unit error-detecting code. The information in the call is presented as sequence of seven-unit binary combinations.
2. The classes of emission, frequency shifts and modulation rates are as follows:
(a) F1B or J2B 170Hz and 100 band for use on HF and MF channels. When frequency-shift keying is effected by applying audio signals to the input of single-side band transmitters (J2B), the center of the audio frequency spectrum offered to the transmitter is 1700 Hz.
(b) Frequency modulation with a pre-emphasis of 6 dB/octave with frequency shift of the modulating subcarrier for use on VHF channels: - the frequency shift is between 1300Hz and 2100 Hz, the subcarrier being at 1700 Hz; - the frequency tolerance of the 1300 Hz and 2100 Hz tones is ± 10Hz; - the modulation rate is 1,200 baud; and - the modulation index is 2.0 ±10%.
Operational procedures
1. The content of a DSC call includes the numerical address of the station (or stations) to which the call is transmitted, the self-identification of the transmitting station and a message which contains several fields of information indicating the purpose of the call.
2. Various types of DSC calls are available, being broadly either distress and safety-related calls or "commercial" calls (to indicate that a commercial communication, e.g. a telephony or telegraphy call, etc., is required). In the case of VHF, automatic connection to the public network can also be established through suitably equipped coast stations.
3. The receipts of a DSC call by a receiving station is accompanied by a suitable display or print-out of the address, the self-identification of the transmitting station and the content of the DSC message, together with an audible or visual alarm or both for certain categories of calls (ex. for distress and safety-related calls).
4. The transmission speed of a DSC call is 100 baud on MF and HF and 1,200 baud on VHF. Error-correction coding is included, involving the transmission of each character twice together with an overall message-check character. The duration of a single DSC call varies between 6.2 and 7.2 seconds on MF and HF or 0.45 and 0.63 second on VHF, depending on the type of DSC call transmitted.
5. For distress and safety operation, simplex frequencies are used. For commercial operation at MF and HF, paired frequencies are used, but at VHF, the simplex channel 70 is used for both distress and safety calling and commercial calling.
In order to increase the probability of a DSC distress call or a DSC distress relay being received it is repeated several times to form a distress call attempt.
Methods: MF/HF – 5 consecutive DSC distress calls on one frequency (single-frequency call attempt)
MF/HF – up to 6 consecutive DSC distress calls dispersed over any of the 6 frequencies(multi-frequency call attempt)
VHF – only a single-frequency call attempt
VHF and MF/HF distress calls may be transmitted simultaneously.
EPIRB and SART
The 406 MHz satellite EPIRB transmits a 5W radio frequency (RF) burst of approximately 0.5s 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 new satellite EPIRB 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 satellite EPIRBs code in accordance with the maritime location protocol, the ship's position as determined by its navigation equipment. Satellite EPIRBs are dual-frequency 121.5/406 MHz beacons. This enables suitably equipped SAR units to home in on the 121.5 MHz transmission and permits over flight monitoring by aircraft. Depending on the type of beacon (maritime, airborne or land), beacons can be activated either manually or automatically.
System Performance and Operations Performance parameters
The following parameters are particularly important for the user:
* EPIRB location probability;
* EPIRB location error;
* ambiguity resolution probability;
* capacity;
* coverage; and
* notification time.
1.EPIRB detection probabilityfor the 406 MHz satellite EPIRB is defined as the probability of detection by LUT of at least one message with a correct code-protected section from the first tracked satellite.
2. EPIRB location probability for the 406 MHz satellite EPIRB is defined as the probability of detecting and decoding at least four individual messages 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.
3. EPIRB location accuracy is defined as the difference between the location calculated by the system using measured Doppler frequencies and the actual location.
4. Ambiguity resolution probability is defined as the ability of the system to select the "true" rather than the "mirror" location.
5. Capacity is defined as the number of EPIRBs in common view of the spacecraft which the system can process simultaneously.
6. Notification time is the period from activation of an EPIRB spectral characteristic. The values given below were confirmed by statistical analysis of over 5,000 beacons during the development and experiment phase.
121.5 MHz EPIRB - Average 6 Hour Notification
406 MHz EPIRB - Average 1 Hour Notification
406 EPIRB with GPS - Average 5 minute Notification
Search and Rescue Radar Transponder (SART)
Operational and technical characteristics of SART
1. The SART can be activated manually or automatically when placed into the water so that it will thereafter respond when interrogated.
2. When activated in a distress situation the SART respond to radar interrogated by transmitting a swept-frequency signal which generates a line of blip code on a radar screen outward from the SART’s position along its line of bearing. This unique radar signal is easily recognized on the radar screen and the rescue vessel (and aircraft, if equipped with suitable radar) can detect the survivors even in poor visibility or at night.
3. The SART provides a visual or audible indication of its correct operation and will also inform survivors when it is interrogated by radar.
4. The SART will have sufficient battery capacity to operate in the stand-by condition for 96 hours and 8 hours in the transmission mode and will be able to operate under ambient temperatures of -20°C to +55°C.
5. The vertical polar diagram of the antenna and the hydrodynamic characteristics of the device will permit the SART to respond to radar under heavy swell conditions. SART transmission is substantially omnidirectional in the horizontal plane.
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