Database Reference
In-Depth Information
11.1.2 Maritime Positioning Systems
Two of the most successful systems used in maritime navigation and positioning
are the Automatic Radar Plotting Aid (ARPA) and the AIS. Both are used
by vessels and Vessel Traffic Services on shore (VTS) in order to facilitate
navigation decisions and warn about possible collisions. Vessel traffic services
also take advantage of their higher computing and networking resources to store
data locally and share them at national and worldwide levels (e.g., program
SafeSeaNet of the European Maritime Safety Agency).
Marine Radar
with automatic radar plotting aid tracks vessels using radar contacts. A radar
transmitter generates very short pulses of radio waves. When the radio waves
of one of these pulses encounter any obstacle, such as a ship, shore line, or big
sea waves, part of the radiated energy is reflected and received by the emitting
radar. The reflected pulse constitutes a radio echo. The time between the pulse
and the echo can be accurately measured and used to calculate the distance
between the radar and the echo. The direction of the echo reflects the direction
of the pulse. When a target echo appears on a radar screen, an operator plots the
relative motion of the echo in order to determine the target's course and speed.
The maximum range of an object detected is affected by the height of the radar
antenna as well as the height of the object due to the curvature of the earth. In
the same way, mountainous sea lines cause blind areas, and objects behind these
areas cannot be detected. Bad weather conditions can also affect significantly
the effectiveness of radar tracking. Thus, any target should be acquired and
confirmed in at least five of ten scans over a period of 2 minutes in order to be
brought to the attention of the operator with an identifier and coordinates.
Automatic Identification System
has been recently implemented and made a mandatory standard on commer-
cial and passenger ships. This system, whose objective is to identify and
locate vessels at distance, automatically broadcasts location-based information
through self organised wireless communications (VHF). AIS usually integrates
a transceiver system, a GPS receiver, and other navigational sensors on board,
such as a gyrocompass and a rate of turn indicator. An AIS transponder runs
in an autonomous and continuous mode, and regularly broadcasts a position
report according to the ship's behavior. The information is broadcast, within a
range of 35 nautical miles, to surrounding ships and maritime authorities on the
ground. There are two different classes of AIS that can be found on ships, search
and rescue aircrafts, and base stations on ground: Mandatory AIS (class A) for
large vessels and low-cost AIS (class B), which has been introduced for smaller
