The
advances in maritime electronics systems over the last 10 years have
nothing short of spectacular with the world of marine communications
and navigation seemingly destined to a total reliance on
satellites.
NAVSTAR Global Positioning System
(GPS) service is fully operational, and GPS equipment prices have
literally plunged to consumer electronics levels. INMARSAT
has most commercial and large pleasure vessels equipped with satellite
communications. IRIDIUM although promising
is now in financial difficulties, and SSB shore stations have largely
ceased in most main western countries. The
Global Maritime Distress and Safety System (GMDSS) is now operational
with far reaching ramifications to all yacht owners. It is a current
subject of debate with respect to equipment suitability and prices for
yachts, and equipment is now available to allow yachts to plug into the
system. There are many commentators who
seem to misunderstand the magnitude and complexity of the system as it
is implemented, which unfortunately increases the confusion.
The one inherent danger of course is
that many people are sailing away without the knowledge to maintain the
equipment, understand how to use it properly, and worse still without
the skills to cope when the systems fail. All these high
technology systems are in the end only aids to navigational for use in
conjunction with properly corrected charts, good seamanship and a
seaworthy vessel. Always keep a plot going on a chart.
Double check your navigation receiver fixes with compass bearings on
coastal passages.
* * * * * Make sure to see the: GMDSS Trainer - Simulator
Software Page on here! * * * * *
GMDSS Basics for the Cruising
Yacht
The
Global Maritime Distress and Safety System (GMDSS) has been fully
implemented. For commercial shipping over
300grt and is now on a fully automated and satellite based
communications and safety system. The
problem is that yachts are now unplugged from this safety system. Already major radio shore stations are closed
for HF/SSB operations. The main elements
of GMDSS are INMARSAT C, Navtex, VHF and SSB/HF Telex and Digital
Selective Calling (DSC), EPIRBS and SARTS.
Under
the GMDSS the world is divided into areas for search and rescue and
these areas are based on the NAVAREA zones. The 121.5/243 MHz and 406
MHz EPIRB units are COSPAS-SARSAT compatible. This
is a world-wide satellite assisted SAR system for location of distress
transmissions emitted by EPIRB's. The 406
MHz beacons can have a unique identification code in the distress
message so that precise vessel data can be included. Search &
Rescue Transponders (SART’s)
are devices are designed for use
in search
and rescue. An EPIRB will put potential rescue vessels in the area, but
the transponder will accurately localize your position.
The EPIRB is generally the first GMDSS compatible item purchased
for the yacht, and if the budget allows and you go deep sea then a
406MHz unit is the way to go
NAVTEX
is controlled by the various navigation authorities within each of the
16 Navareas who participate in the provision of meteorological and
Maritime Safety Information (MSI) navigational safety information for
transmission. NAVTEX is the broadcasting
of weather and navigational information by teleprinter.
It is an integral part of GMDSS. Broadcast
frequency for NAVTEX is 518kHz and uses a dedicated receiver,
permanently tuned to the frequency, and an integral printer is used. In normal use, the unit is left on
permanently, and can be programmed to receive specific messages. Ranges are typically around 250nm. There are some excellent smaller units around
such as those from ICS Electronics, and as a second step towards GMDSS
it is a sensible purchase.
Digital Selective Calling (DSC). DSC is at the heart of GMDSS communications. DSC has a number of important features. It utilises a single VHF channel, specifically
channel 70. A digitally encoded message is
transmitted instead of a voice one. For
many the obtaining of a new compatible VHF is a vexed and expensive one. There are a number of new GMDSS units
available specifically for yachts and smaller vessel.
This makes a good third step in the switchover to GMDSS, so get
either a VHF that can be upgraded later or go for the full unit. Remember licensing requirements are different,
and a new course and certificate is also required.
For the Blue Water cruising sailor
the whole question of long range communications within a GMDSS context
is a very difficult one. Whilst Amateur or HAM radio is now
proving its value and worth to the primarily USA based cruisers, and
there are some useful commercial E-mail services on HF now available,
the whole question is still a hard one to address. Iridium is
literally in a satellite "crash and burn" situation, so
any salvation
on that front is seemingly limited, and INMARSAT services are somewhat
expensive for cruisers, although a range of marketing options are
becoming available.
GMDSS
Information from the US Coast Guard:
An Overview of the Global Maritime Distress & Safety
System
See USCG GMDSS Brochure provided here for
our visitors.
Download
the recreational vessel version (Always get the latest info from
the US Coast Guard, and the FCC.)
| Make sure to see the: GMDSS Trainer - Simulator Software
Page on here! Personal versions allow you to practice VHF DSC
(Digital Selective Calling) and Distress DSC without getting yourself
into trouble (It is illegal to transmit a DISTRESS DSC SIGNAL as a
test!!!!) Captain Schools Versions have Student and Teacher Systems
that allow full GMDSS scenario training. Both versions allow you to
practice DSC and Actual Voice Communications on very realistic software
simulated radio gear without transmitting illegal signals that could
get you in trouble. |
Since the invention of radio at the end of the 19th Century,
ships at sea have relied on Morse code, invented by Samuel Morse and
first used in 1844, for distress and safety telecommunications. The
need for ship and coast radio stations to have and use radiotelegraph
equipment, and to listen to a common radio frequency for Morse encoded
distress calls, was recognized after the sinking of the liner Titanic
in the North Atlantic in 1912. The U.S. Congress enacted legislation
soon after, requiring U.S. ships to use Morse code radiotelegraph
equipment for distress calls. The International
Telecommunications Union (ITU), now a United Nations agency,
followed suit for ships of all nations. Morse encoded distress calling
has saved thousands of lives since its inception almost a century ago,
but its use requires skilled radio operators spending many hours
listening to the radio distress frequency. Its range on the medium
frequency (MF) distress band (500 kHz) is limited, and the amount of
traffic Morse signals can carry is also limited.
Over fifteen years ago the International
Maritime Organization (IMO), a United Nations agency specializing
in safety of shipping and preventing ships from polluting the seas,
began looking at ways of improving maritime distress and safety
communications. In 1979, a group of experts drafted the International
Convention on Maritime Search and Rescue, which called for development
of a global search and rescue plan. This group also passed a resolution
calling for development by IMO of a Global Maritime Distress and Safety
System (GMDSS) to provide the communication support needed to implement
the search and rescue plan. This new system, which the world's maritime
nations, including the United States, are implementing, is based upon a
combination of satellite and terrestrial radio services, and has
changed international distress communications from being primarily
ship-to-ship based to ship-to-shore (Rescue Coordination Center) based.
It spelled the end of Morse code communications for all but a few
users, such as Amateur Radio. The GMDSS provides for automatic distress
alerting and locating in cases where a radio operator doesn't have time
to send an SOS or MAYDAY call, and, for the first time, requires ships
to receive broadcasts of maritime safety information which could
prevent a distress from happening in the first place. In 1988, IMO
amended the Safety of Life at Sea (SOLAS) Convention, requiring ships
subject to it fit GMDSS equipment. Such ships were required to carry
NAVTEX and satellite EPIRBs by 1 August 1993, and had to fit all other
GMDSS equipment by 1 February 1999. US ships were allowed to fit GMDSS
in lieu of Morse telegraphy equipment by the Telecommunications Act of
1996.
The GMDSS consists of several systems, some of which are new,
but many of which have been in operation for many years. The system
will be able to reliably perform the following functions: alerting
(including position determination of the unit in distress), search and
rescue coordination, locating (homing), maritime safety information
broadcasts, general communications, and bridge-to-bridge
communications. Specific radio carriage requirements depend upon the
ship's area
of operation, rather than its tonnage. The system also provides
redundant means of distress alerting, and emergency sources of power.
The GMDSS consists of many separate systems which are being
implemented in a coordinated and agreed-upon manner. Some of these
systems are discussed on the following page.
GMDSS
SYSTEMS
| Make sure to see the: GMDSS Trainer - Simulator Software Page on
here! Personal versions allow you to practice VHF DSC
(Digital Selective Calling) and Distress DSC without getting yourself
into trouble (It is illegal to transmit a DISTRESS DSC SIGNAL as a
test!!!!) Captain Schools Versions have Student and Teacher Systems
that allow full GMDSS scenario training. Both versions allow you to
practice DSC and Actual Voice Communications on very realistic software
simulated radio gear without transmitting illegal signals that could
get you in trouble. |
The
COSPAS-SARSAT System
COSPAS-SARSAT
is an international satellite-based search and rescue system,
established by Canada, France, the U.S.A., and Russia. These four
countries jointly helped develop a 406 MHz satellite emergency
position-indicating radiobeacon (EPIRB), an element of the GMDSS
designed to operate with COSPAS-SARSAT system. These
automatic-activating EPIRBs, now required on SOLAS ships, commercial
fishing vessels, and other ships, are designed to transmit to a rescue
coordination center a vessel identification and an accurate location of
the vessel from anywhere in the world.
NAVTEX
NAVTEX
is an international, automated system for instantly distributing
maritime navigational warnings, weather forecasts and warnings, search
and rescue notices and similar information to ships. A small, low-cost
and self-contained "smart" printing radio receiver installed in the
pilot house of a ship or boat checks each incoming message to see if it
has been received during an earlier transmission, or if it is of a
category of no interest to the ship's master. If it is a new and wanted
message, it is printed on a roll of adding-machine size paper; if not,
the message is ignored. A new ship coming into the area will receive
many previously-broadcast messages for the first time; ships already in
the area which had already received the message won't receive it again.
No person needs to be present during a broadcast to receive vital
information. See Practical
Instruction for the Use of a NAVTEX Receiver (Acrobat PDF).
INMARSAT
Satellite systems operated by the Inmarsat,
under contract to the International Maritime Satellite Organization
(IMSO), are also important elements of the GMDSS. Three types of
Inmarsat ship earth station terminals are recognized by the GMDSS: the
Inmarsat B, C and F77. The Inmarsat B and F77, an updated version of
the A, provide ship/shore, ship/ship and shore/ship telephone, telex
and high-speed data services, including a distress priority telephone
and telex service to and from rescue coordination centers. The F77 is
meant to be be used with the Inmarsat C, since it's data capability
does not meet GMDSS requirements. The Inmarsat C provides
ship/shore, shore/ship and ship/ship store-and-forward data and email
messaging, the capability for sending preformatted distress messages to
a rescue coordination center, and the Inmarsat C
SafetyNET service. The Inmarsat C SafetyNET service is a
satellite-based worldwide maritime
safety information broadcast service of high seas weather warnings,
NAVAREA navigational warnings, radionavigation warnings, ice reports
and warnings generated by the USCG-conducted International Ice Patrol,
and other similar information not provided by NAVTEX. SafetyNET works
similarly to NAVTEX in areas outside NAVTEX coverage.
Inmarsat C equipment is relatively small and lightweight,
and costs much less than an Inmarsat B or F77. Inmarsat B and F77 ship
earth stations require relatively large gyro-stabilized antennas; the
antenna size of the Inmarsat C is much smaller. .
In July 2002 IMSO notified IMO of the decision by Inmarsat
to withdraw provision of Inmarsat A services as from 31 December
2007. On that date, Inmarsat A can no longer be used for any
purpose. The last type approval by Inmarsat for a new model of
maritime Inmarsat A mobile earth station was granted in 1991, since
when no new Inmarsat A models have been approved.
Under a cooperative agreement with the National Oceanic
and Atmospheric Administration (NOAA), combined meteorological
observations and AMVER reports can now be sent to both the USCG AMVER
Center, and NOAA, using an Inmarsat C ship earth station, at no charge.
There is also no charge to register for this service and to receive the
necessary Inmarsat C software. For more information, see the NOAA Shipboard
Environmental (data) Acquisition System, or SEAS.
SOLAS now requires that Inmarsat C equipment have an
integral satellite navigation receiver, or be externally connected to a
satellite navigation receiver. That connection will ensure accurate
location information to be sent to a rescue coordination center if a
distress alert is ever transmitted.
High Frequency
The GMDSS includes HF radiotelephone and radiotelex
(narrow-band direct printing) equipment, with calls initiated by
digital selective calling. Worldwide broadcasts of maritime safety
information are also made on HF narrow-band direct printing channels.
To meet these GMDSS requirements, the Coast Guard has
improved high
frequency (HF) ship-shore radio safety services from our
Communication Stations to the maritime community, as well as narrow-band
direct printing broadcasts.
Search and Rescue Radar
Transponders (SARTs).
The GMDSS installation on ships include one or more search
and rescue radar transponders, devices which are used to locate
survival craft or distressed vessels by creating a series of dots on a
rescuing ship's 3 cm radar display. The detection range between these
devices and ships, dependent upon the height of the ship's radar mast
and the height of the SART, is normally about eight nautical
miles. Note that a marine radar may not detect a SART even within
this distance, if the radar settings are not optimized for SART
detection. For more information, see IMO SN/Circ.197,
Operation of Marine Radar for SART Detection. (PDF)
Digital Selective Calling
The IMO also introduced digital
selective calling (DSC) on VHF, MF and HF maritime radios as part
of the GMDSS system. DSC is primarily intended to initiate ship/ship,
ship/shore, and shore/ship radiotelephone and MF/HF radiotelex calls.
DSC calls can also be made to individual ships or groups of ships. DSC
distress alerts, which consist of a preformatted distress message, are
used initiate emergency communications with ships and rescue
coordination centers. DSC was intended to eliminate the need for
persons on a ship's bridge or on shore to continuously guard radio
receivers on voice radio channels, including VHF channel 16 (156.8 MHz)
and 2182 kHz now used for distress, safety and calling. A listening
watch aboard GMDSS-equipped ships on 2182 kHz ended on 1 February
1999. In May 2002, IMO decided to postpone cessation of a
listening watch on VHF CH.16 aboard ships. That watchkeeping
requirement had been scheduled to end on 1 February 2005.
IMO and ITU both require that the DSC-equipped VHF and
MF/HF radios be externally connected to a satellite navigation
receiver. That connection will ensure accurate location information is
sent to a rescue coordination center if a distress alert is ever
transmitted. FCC regulations actually require that ship's position be
manually entered into the radio every four hours on ships required to
carry GMDSS equipment, while that ship is underway (47 CFR
80.1073).The Coast Guard believes VHF, MF and HF radiotelephone
equipment carried on ships should include a DSC capability as a matter
of safety. To achieve this, the FCC requires that all new VHF and MF/HF
maritime radiotelephones type accepted after June 1999 to have at least
a basic DSC capability.
VHF digital selective calling also has other capabilities
beyond those required for the GMDSS. The Coast Guard uses this system
to track vessels in Prince William Sound, Alaska, Vessel Traffic
Service. IMO and the USCG also plan to require ships carry a Universal
Shipborne Automatic Identification System, which will be
DSC-compatible. Countries having a GMDSS A1 Area should be able to
identify and track AIS-equipped vessels in its waters without any
additional radio equipment.
A DSC-equipped radio cannot be interrogated and tracked
unless that option was included by the manufacturer, and unless the
user configures it to allow tracking.
U.S. shore-based radio stations currently exist to support
every element of the GMDSS, except for digital selective calling. The
United States intends to declare an Sea Area A1 in 2006 and a Sea Area
A2 for the contiguous U.S. will be declared once the 2 MHz
infrastructure has been upgraded.
Use of GMDSS for Routine
Telecommunications
GMDSS telecommunications equipment should not
be reserved for emergency use only. The International Maritime
Organization in COMSAR
Circ.17 encourages mariners to use that equipment for routine as
well as safety telecommunications.
What
is the
Automatic Identification System (AIS)? http://www.navcen.uscg.gov/enav/ais/default.htm
Picture a shipboard radar display, with overlaid electronic
chart
data, that includes a mark for every significant ship within radio
range, each as desired with a velocity vector (indicating speed and
heading). Each ship "mark" could reflect the actual size of the
ship, with position to GPS or differential GPS accuracy. By
"clicking" on a ship mark, you could learn the ship name, course and
speed, classification, call sign, registration number, MMSI, and other
information. Maneuvering information, closest point of approach
(CPA), time to closest point of approach (TCPA) and other navigation
information, more accurate and more timely than information available
from an automatic radar plotting aid, could also be available.
Display information previously available only to modern Vessel Traffic
Service operations centers could now be available to every AIS-equipped
ship.
With this information, you could call any ship over VHF
radiotelephone by name, rather than by "ship off my port bow" or some
other imprecise means. Or you could dial it up directly using
GMDSS equipment. Or you could send to the ship, or receive from
it, short safety-related email messages.
The AIS is a shipboard broadcast system that acts like a
transponder, operating in the VHF maritime band, that is capable of
handling well over 4,500 reports per minute and updates as often as
every two seconds. It uses Self-Organizing Time Division
Multiple Access (SOTDMA) technology to meet this high broadcast rate
and ensure reliable ship-to-ship operation.
Automatic Identification System
From Wikipedia, the free encyclopedia
The International Maritime Organization's (IMO) International
Convention for the Safety of Life at Sea (SOLAS) requires AIS to be
fitted aboard international voyaging ships of 300 or more gross
tonnage, and all passenger ships regardless of size. It is estimated
that more than 40,000 ships currently carry AIS class A
equipment.[citation needed]
Contents
1 Applications and limitations
1.1 Collision avoidance
1.2 Vessel traffic services
1.3 Aids to navigation
1.4 Search and rescue
1.5 Binary messages
1.6 Concern over web-based data
2 How AIS works
2.1 Basic overview
2.2 Detailed description: Class A units
2.3 Broadcast information
3 See also
4 References and Footnotes
5 External links
Applications and limitations
Collision avoidance
AIS is used in navigation primarily for collision avoidance. Due to the
limitations of radio characteristics, and because not all vessels are
equipped with AIS, the system is meant to be used primarily as a means
of lookout and to determine risk of collision rather than as an
automated collision avoidance system, in accordance with the
International Regulations for Preventing Collisions at Sea (COLREGS).
When a ship is navigating at sea, the movement and identity of other
ships in the vicinity is critical for navigators to make decisions to
avoid collision with other ships and dangers (shoal or rocks). Visual
observation (unaided, binoculars, night vision), audio exchanges
(whistle, horns, VHF radio), and radar or Automatic Radar Plotting Aid
(ARPA) are historically used for this purpose. However, a lack of
positive identification of the targets on the displays, and time delays
and other limitation of radar for observing and calculating the action
and response of ships around, especially on busy waters, sometimes
prevent possible action in time to avoid collision.
While requirements of AIS are only to display a very basic text
information, the data obtained can be integrated with a graphical
electronic chart or a radar display, providing consolidated
navigational information on a single display.
Vessel traffic services
In busy waters and harbors, a local Vessel Traffic Service (VTS) may
exist to manage ship traffic. Here, AIS provides additional traffic
awareness and provides the service with information on the kind of
other ships and their movement.
Aids to navigation
AIS was developed with the ability to broadcast positions and names of
things other than vessels, namely it can serve to transmit navigation
aid and marker positions. These aids can be located on shore, such as
in a lighthouse, or on the water, on platforms or buoys. The US Coast
Guard suggests that AIS might replace RACON, or radar beacons,
currently used for electronic navigation aids.
The ability to broadcast navigation aid positions has also created the
concept of Virtual AIS, also known as Synthetic AIS or Artificial AIS.
The terms can mean two things; in the first case, an AIS transmission
describes the position of physical marker but the signal itself
originates from a transmitter located elsewhere. For example, an
on-shore base station might broadcast the position of ten floating
channel markers, each of which is too small to contain a transmitter
itself. In the second case, it can mean AIS transmissions that indicate
a marker which does not exist physically, or a concern which is not
visible (i.e. submerged rocks, or a wrecked ship). Although such
virtual aids would only be visible to AIS equipped ships, the low cost
of maintaining them could lead to their usage when physical markers are
unavailable.
Search and rescue
For coordinating resources on scene of marine search & rescue
operation, it is important to know the position and navigation status
of ships in the vicinity of the ship or person in distress. Here AIS
can provide additional information and awareness of the resources for
on scene operation, even though AIS range is limited to VHF radio
range. The AIS standard also envisioned the possible use on SAR
Aircraft, and included a message (AIS Message 9) for aircraft to report
position.
Binary messages
The Saint Lawrence Seaway uses AIS binary messages (message type 8) to
provide information about water levels, lock orders, and weather in its
navigable system.
Concern over web-based data
In December 2004, the International Maritime Organization's (IMO)
Maritime Safety Committee condemned the use of freely available AIS
data published irresponsibly with the following statement.
In relation to the issue of freely available automatic information
system (AIS)-generated ship data on the world-wide web, the publication
on the world-wide web or elsewhere of AIS data transmitted by ships
could be detrimental to the safety and security of ships and port
facilities and was undermining the efforts of the Organization and its
Member States to enhance the safety of navigation and security in the
international maritime transport sector.
How AIS works
System Overview from US Coast Guard
Basic overview
AIS transponders automatically broadcast information, such as their
position, speed, and navigational status, at regular intervals via a
VHF transmitter built into the transponder. The information originates
from the ship's navigational sensors, typically its global navigation
satellite system (GNSS) receiver and gyrocompass. Other information,
such as the vessel name and VHF call sign, is programmed when
installing the equipment and is also transmitted regularly. The signals
are received by AIS transponders fitted on other ships or on land based
systems, such as VTS systems. The received information can be displayed
on a screen or chart plotter, showing the other vessels' positions in
much the same manner as a radar display.
The AIS standard describes two major classes of AIS units:
Class A - mandated for use on SOLAS Chapter V vessels (and others in
some countries).
Class B - a low power, lower cost derivative for leisure and non-SOLAS
markets.
Other variants are under development specifically for base stations,
aids to navigation and search and rescue, though they will all be
derived from one of the existing standards and inter-operate with them.
Detailed description: Class A units
Each AIS transponder consists of one VHF transmitter, two VHF TDMA
receivers, one VHF Digital Selective Calling (DSC) receiver, and links
to shipboard display and sensor systems via standard marine electronic
communications (such as NMEA 0183, also known as IEC 61162). Timing is
vital to the proper synchronization and slot mapping for a Class A
unit. Therefore, every unit is required to have an internal global
navigation satellite system (e.g. GPS) receiver.[4] This internal
receiver may also be used for position information. However, position
is typically provided by an external receiver such as GPS, LORAN or an
inertial navigation system and the internal receiver is only used as a
backup for position information. Other information broadcast by the
AIS, if available, is electronically obtained from shipboard equipment
through standard marine data connections. Heading information and
course and speed over ground would normally be provided by all
AIS-equipped ships. Other information, such as rate of turn, angle of
heel, pitch and roll, and destination and ETA could also be provided.
The AIS transponder normally works in an autonomous and continuous
mode, regardless of whether it is operating in the open seas or coastal
or inland areas. Transmissions use 9600 bit/s Gaussian minimum shift
keying (GMSK) modulation over 25 or 12.5 kHz channels using the
High-level Data Link Control (HDLC) packet protocol. Although only one
radio channel is necessary, each station transmits and receives over
two radio channels to avoid interference problems, and to allow
channels to be shifted without communications loss from other ships.
The system provides for automatic contention resolution between itself
and other stations, and communications integrity is maintained even in
overload situations.
In order to ensure that the VHF transmissions of different transponders
do not occur at the same time the signals are time multiplexed using a
technology called Self-Organized Time Division Multiple Access
(SOTDMA). The design of this technology is patented, and whether this
patent has been waived for use by SOLAS vessels is a matter of debate
between the manufacturers of AIS systems and the patent holder. In
order to make the most efficient use of the bandwidth available,
vessels which are anchored or are moving slowly transmit less
frequently than those that are moving faster or are maneuvering. The
update rate of fast maneuvering vessels is similar to that of a
conventional marine radar. The time reference is derived from the
navigation system.
Each station determines its own transmission schedule (slot), based
upon data link traffic history and knowledge of future actions by other
stations. A position report from one AIS station fits into one of 2250
time slots established every 60 seconds on each frequency. AIS stations
continuously synchronize themselves to each other, to avoid overlap of
slot transmissions. Slot selection by an AIS station is randomized
within a defined interval, and tagged with a random timeout of between
0 and 8 frames. When a station changes its slot assignment, it
announces both the new location and the timeout for that location. In
this way new stations, including those stations which suddenly come
within radio range close to other vessels, will always be received by
those vessels.
The required ship reporting capacity according to the IMO performance
standard amounts to a minimum of 2000 time slots per minute, though the
system provides 4500 time slots per minute. The SOTDMA broadcast mode
allows the system to be overloaded by 400 to 500% through sharing of
slots, and still provide nearly 100% throughput for ships closer than 8
to 10 NM to each other in a ship to ship mode. In the event of system
overload, only targets further away will be subject to drop-out, in
order to give preference to nearer targets that are a primary concern
to ship operators. In practice, the capacity of the system is nearly
unlimited, allowing for a great number of ships to be accommodated at
the same time.
The system coverage range is similar to other VHF applications,
essentially depending on the height of the antenna, but slightly better
due to digital VHF and not analog VHF. Its propagation is better than
that of radar, due to the longer wavelength, so it’s possible
to “seeâ€
around bends and behind islands if the land masses are not too high. A
typical value to be expected at sea is nominally 20 nautical miles (37
km). With the help of repeater stations, the coverage for both ship and
VTS stations can be improved considerably.
The system is backwards compatible with digital selective calling
systems, allowing shore-based GMDSS systems to inexpensively establish
AIS operating channels and identify and track AIS-equipped vessels, and
is intended to fully replace existing DSC-based transponder systems.
Shore-based AIS network systems are now being built up around the
world. One of the biggest fully-operational, real time systems with
full routing capability is in China, operated by China MSA and
delivered by Saab TransponderTech in Sweden. The entire coastline is
covered with approximately 150 base stations and 50 computer servers.
Hundreds of shore based users, including ca 25 VTS centers, are then
connected to the network and been able to see the maritime picture, but
also to communicate with the ship with SRM:s (Safety Related Messages).
All data is in real time and will full safety and security of ships and
port facilities.
Broadcast information
AIS transceiver sends the following data every 2 to 10 seconds
depending on vessels speed while underway, and every 3 minutes while
vessel is at anchor. This data includes:
The vessel's Maritime Mobile Service Identity (MMSI) - a unique nine
digit identification number.
Navigation status - "at anchor", "under way using engine(s)", "not
under command", etc
Rate of turn - right or left, 0 to 720 degrees per minute
Speed over ground - 0.1 knot resolution from 0 to 102 knots
Position accuracy
Longitude - to 1/10000 minute and Latitude - to 1/10000 minute
Course over ground - relative to true north to 0.1 degree
True Heading - 0 to 359 degrees from eg. gyro compass
Time stamp - UTC time accurate to nearest second when this data was
generated
In addition, the following data is broadcast every 6 minutes:
IMO number - a nine digit number that remains unchanged upon transfer
of the ship's registration to another country
Radio call sign - international radio call sign, up to seven
characters, assigned to vessel
Name - Name of vessel, max 20 characters
Type of ship/cargo
Dimensions of ship - to nearest meter
Location of positioning system's (eg. GPS) antenna onboard the vessel
Type of positioning system - such as GPS, DGPS or LORAN-C
Draught of ship - 0.1 meter to 25.5 meters
Destination - max 20 characters
ETA (estimated time of arrival) at destination - UTC month/date
hour:minute
