GNSS (Global Navigation Satellite Systems)
Global Positioning System (GPS)
GPS is a satellite position-fixing system operated by the US Department of Defence (DoD) which is space-based with a constelation of 24 satellites. The system is used for general navigation on land, sea and air. It also has survey and timing applications. GPS provides world-wide three dimensional coverage.
GPS provides two services for position determination. The Precise Positioning Service (PPS) will provide predictable positioning accuracy of 22m horizontally but is limited to the US and allied military and federal government use. The Standard Positioning Service (SPS) is available to civil, commercial and other users at the highest level of accuracy that is consistent with the international interest of the USA. Up to 1st May 2000, the US DoD deliberately degraded the signal by use of "Selective Availability" and provided an horizontal accuracy of 100m with 95% probability. With SA removed, raw GPS accuracy is reckoned to be 20m. GPS signals are liable to interuption without prior notice.
Global Navigation Satellite System (Glonass)
Glonass is a space-based radio positioning system operated by Russia for world-wide use. The system is similar to GPS and may be used for general navigation by aircraft and ships, etc. The accuracy is 100m horizontal, 150m vertical and 15cm/s velocity (all 95% probability). After the breakup of the former USSR, the system became unreliable as satellites were not maintained and there were insufficient working satellites.
Galileo is the European Union equivalent of GPS and GLONASS but has been delayed due to funding and operational issues. Two test satellites are in orbit but shipping of the first two operational satellites ( IOVs - In Orbit Validation) has been delayed until early 2011, with two more due for launch 3 months later. Contracts have been signed for 18 satellites so far but full services requires 24 plus 6 spares and while Galileo may be able to provide initial services by 2014, the system is unlikely to be fully operational before 2016 -2019.
Compass (Beidou Navigation Satellite System)
China is also in the process of launching a satellite system. China launched its third satellite in January 2010 and intends to launch 3 each year for the next 3 years which will give coverage over the Asia-Pacific region. China intends to fullfill a constellation of 35 navigation satellites to provide a full global service by 2020.
Terrestrial Radio Systems
Decca Navigator System (DNS)The Decca Navigator System, generally known as Decca, is a hyperbolic radionavigation system which is now almost obsolete. The system uses groups of at least three ground transmitter stations called chains. Each chain comprises one Master and two or three Slave stations, 80 - 110 km from the Master station. The accuracy of Decca ranges from 50 - 800m and decreases as the distance from the baseline increases. The accuracy is also subject to night and seasonal effects which generally reduces the accuracy by a factor of 6 to 8. Irish and U.K. stations were closed down in Year 2000.
Loran-C is an all weather, highly accurate and reliable hyperbolic radionavigation system which covers most of the Northern Hemisphere. The system uses groups of at least three ground transmitter stations called chains. Each chain comprises one Master and two or three Secondary stations, several hundred kilometres from the Master station. Unlike Decca, Loran-C is unaffected by night and seasonal effects and coverage remains the same throughout the year and also by day and night. The Northwest European Loran-C System (NELS- which is now discontinued) was part of the European Union plan for an independent European Radionavigation System. A predicted accuracy of 463m (0.25 nautical miles) will extend up to 1000 km off the south and west coasts of Ireland while an accuracy of greater than 100m is predicted for the Irish Sea, the Celtic Sea and the seas to the north of Ireland as well as all Irish coastal waters. Loran-C accuracy of greater than 100m will also extend over the whole of Ireland for aero and civil users.
All Loran-C signals are constantly being monitored. A code within the signal which is known as Blink, will warn users of any abnormality.
Irish Loran C and FAQ
The Commissioners of Irish Lights have been providing radiobeacons since the first one was established at Mizen Head in 1931. With the implementation of the Global Maritime Distress and Safety System (GMDSS), the CIL radiobeacon service ceased on 1st February 1999.
A radiobeacon transmits a Morse coded signal, unique to its location, e.g. Tuskar Rock Morse Code TR (- ·-·). This signal is repeated in a set sequence. The signal allows receivers to take a bearing of the source transmitters to help determine position. The frequency of radiobeacons are in the band 285 - 315 kHZ. The emission type is A1A (simple keyed carrier signal) and the range of each radiobeacon is 50 or 100 nautical miles.
The Calibration radiobeacons transmit only on request and are of lesser range (nominally 5 nautical miles). They are used to calibrate ships' Direction Finding (DF) equipment.
Differential GPS (DGPS) is a system where the ground reference station is able to analyse the GPS signal and correct any errors. It then re-broadcasts this correction to suitably equipped receivers. This allows a far better accuracy than the 100m horizontal accuracy of raw GPS.
Radiobeacon DGPS Service
Marine radiobeacons are a suitable means of transmitting differential corrections inncluding integrity messages to suitable GPS receivers. Many authorities world-wide, including the General Lighthouse Authorities for UK and Ireland, have implemented this service. The accuracy is better than 5m at the present time. Such radiobeacons have a useful range of 100 to 150 nautical miles. The Commissioners Of Irish Lights currently provide differential signals from Mizen Head, Loop Head and Tory Island.
Eurofix is a method by which differential corrections to GPS, including integrity messages are transmitted within the Loran-C signals. It has been agreed in principle to provide Eurofix from the Northwest European Loran-C System thereby providing complete coverage of DGPS over Northwest Europe. A combined receiver would therefore allow a position fix be computed using 3 methods - Loran-C, GPS and Eurofix DGPS, with each system checking the others.
Radar Target Enhancers
A Radar Target Enhancer (RTE) is designed to respond to an interrogating radar with an amplified signal, which is transmitted on the same frequency with minimal time delay. The effect of this is to provide the structure on which it is mounted with a consistent radar return where otherwise, without enhancement, it would have become intermittent, difficult or impossible to detect.
The term Racon is derived from the first and last syllables of the words Radar beacon. This is an accurate description of the use and purpose of Racons.
In their basic form they receive signals from ships' radars which trigger the Racon to emit a characteristic signal which is in turn received by the ship's radar. This characteristic signal is in the form of a series of response pulses which will show up on a ship's radar as a Morse coded trace and allow easy identification of the particular Racon being interrogated, eg: Kish Bank Morse code T ( - ); Codling Lanby Morse code G ( --· ).
Racons can be placed on any navigational mark (eg: lighthouses, beacons, perches, buoys, etc). The return on the ship's radar will clearly identify the mark from surrounding targets and allow the mariner to accurately measure his range and bearing.
The Commissioners of Irish Lights have made major improvements in the provision of Racons on the Irish coast. Twenty one high specification Racons are in place. The Racons provided are state of the art and will respond on standard 3cm ships' radars (X-band 9300 - 9500 Mhz) and on 10cm radars (S-band 2900 - 3100 Mhz). They are strategically placed to serve through traffic, the approaches to major ports, and fishing and leisure interests.
AIS (Automatic Identification System)
IALA presented the first proposals of AIS to IMO. AIS is a new technical system that makes it possible to monitor ships from other ships and from shore based stations. AIS equipped ships will continuously transmit a short message containing information of position, course over the ground, speed over the ground, heading etc. Therefore other ships and shore stations will be able to receive and assess this information without recourse to radar.
AIS will be phased in over the coming years and will be of major benefit to maritime authorities to monitor shipping in their areas, and for inter-ship communication.