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.
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.
Use of electronic position-fixing systems
Care should be taken when using electronic position-fixing systems.
Watch keepers need to understand the cap.abilities and limitations of the systems they are using and continually monitor and validate the information given.
Use of electronic position-fixing systems in integrated bridges
When position-fixing systems transmit data to other navigation systems, the integrity and quality of the data transmitted need to be safeguarded.
Techniques used should include:
· using pre-set quality limits to monitor the fix quality of each position-fixing system connected to the integrated bridge;
· comparing all positions to identify and reject any rogue positions or positions that are clearly incorrect;
· comparing electronic positions with the ship's estimated position (EP) calculated using direct inputs from the log and gyro;
· checking the status of the data transmitted and ensuring that only valid data messages are used.
Route monitoring
Route storage and cross track error (XrE) monitoring are common GPS features. By entering the passage plan in the GPS as well as the navigation system, the GPS can provide an integrated bridge system with an independent route monitoring capability.
