Electronic position-fixing systems
Electronic position-fixing systems provide an automatic and continuous position update for ships fitted with a suitable receiver using either a terrestrial hyperbolic radio navigation system such as Loran C, or a global satellite system such as GPS.
Hyperbolic positioning systems
The use of hyperbolic positioning systems at sea is declining. Omega is no longer operational and Decca is being phased out. Loran C, as a back up to the global navigation satellite system, is to be retained for the time being.
The use of lattice charts showing hyperbolic lines of position has also declined, and most receivers convert the readings to latitude and longitude.
Loran C has a basic range of approximately 1200 miles using ground-wave signals, although extended range coverage is possible using skywaves.
Corrections need to be applied to Loran C signals to take into account variations in the conductivity of the earth's surface over which the signals pass. These are known as additional secondary factor (ASF) corrections. The corrections may need to be manually applied before plotting the position on the chart.
Global navigation satellite system
A global navigation satellite system (GNSS) is a satellite system that provides ships fitted with suitable receivers with a means of obtaining continuous worldwide position, time and speed information.
The Global Positioning System (GPS) operated by the United States and the Global Navigation Satellite System (GLONASS) operated by the Russian Federation are currently available for civilian use on ships.
GPS and DGPS
GPS offers commercial users a global positioning capability with accuracy of the order of 100 metres.
Differential GPS (DGPS) receivers apply corrections to raw GPS signals determined and transmitted by terrestrial monitoring stations. Differential signals can be transmitted to ships via satellites or using HF radio links. Within DGPS coverage, positional accuracy of the order of 10 metres at the receiver antenna is possible.
Date: 2016-04-22; view: 3668