Satellite Navigation Systems
Global Navigation Satellite Systems (GNSS) is the name given to satellite based radionavigation systems. There are currently three systems in operation:
GPS – is owned and operated by the US military
GLONASS – is owned and operated by the Russian military
GALILEO – has been developed by the international community to operate without the inherent inaccuracies of military systems. Information will be provided on this system as it becomes more available.
GPS is a continuously transmitting 3-dimentional AID to navigation. A note is often displayed under the title block on a nautical chart. This is because GPS uses a different datum to refer positions, GPS always displays latitude and longitude in degrees, minutes and decimal minutes. Therefore, adjustments should be applied to readings before plotting a position on the chart. It is important to remember that GPS is an aid to navigation not a means. Positions obtained from GPS should be regularly checked by traditional methods. When navigating from one point to another a GPS will steer the shortest route, but there may be obstructions.
The system was developed and is controlled by the United States Department of Defence. The GPS unit tracks and receives data from at least 3 satellites to fix a position, time, and velocity.
One advantage of GPS systems is the facility to navigate by waypoints. A waypoint is any chosen position saved into the GPS, such as a good fishing spot or one’s home port. A waypoint can be saved in two ways: either by a save button, sometimes named ‘mark’, or by entering the latitude and longitude. When recalled a waypoint’s position is displayed with the bearing and distance from the current position. Also a number of waypoints can be pre-programmed to plot a passage from one place to another. Always refer to the manufacturer’s instructions on how to use a particular GPS.
Many GPS’s are set with a MOB (man overboard) button, which records the vessel’s current position when it is pressed. This button’s operation is similar to the ‘mark’ button for saving a waypoint. The MOB button also saves such information as the time the button was pressed. It does not calculate the set and drift of the man overboard, so this must be taken into account when retuning to look for a missing person.
Plotters are more detailed than GPS and often incorporate a fish finder as well. A plotter may show the lines of latitude and longitude, the course a vessel has travelled, steering and echo sounder at the same time. Again refer to the manufacturer for details.
Tips When Buying a GPS
Quality brands are more expensive.
High-energy interference from a nearby transmitter (TV or microwave towers) or harmonic interference from other systems is possible. Some military exercises in USA have been known to affect GPS receivers up to 300 nautical miles away.
Most GPS receivers have internal DC power supplies, some of which can use rechargeable batteries and/or be used as mobile units. Fixed (in-dash) receivers cannot be used as mobile units.
GPS displays can have a cathode ray tube display (CRT) or a liquid crystal display (LCD) in either colour or monochrome.
All GPS receivers indicate positions in latitude and longitude and have some waypoint navigation.
Chart plotters may have some in-built charts which cannot be corrected or they may allow input from chart cartridges or downloads from CD’s, computers or INMARSAT.
GPS signals are very weak. They are below the surrounding RF noise level, making them easy to jam or be interfered with by microwaves, radar, EPIRBs and motors. GPS can also be masked by other ships or buildings.
Not all GPS receivers are sealed adequately to be waterproof, they are classed as water-resistant.
Check whether the GPS calculates speed by measuring Doppler shift or the carrier wave’s time difference. Doppler shift is more accurate, but requires an additional circuit.
Take into consideration the unit’s ease of use.
Some GPS receivers are able to place an elevation mask to cut out low altitude satellites and so provide more accurate position information.
- Be aware that different units have different operating temperature limits, and may not be suitable for some area of operation.