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GPS

Introduction

GPS ConsoleKnowing your boat's position at any given time is essential for safe navigation. A satellite navigator, best known as a GPS (receiver), calculates an accurate latitude and longitude position, which can be plotted on a paper chart or fed electronically to a chart plotter.

The Global Positioning System (GPS) uses satellites provided by the United States Department of Defense. It's a free service that, for marine leisure users, can give a position that is accurate (most of the time) to within 10m.

Handheld GPSThere are also new systems currently being developed by the Russians (Glonass) and by the European Space Agency (Gallileo) but for the time being GPS is the principal satellite navigation system.

It is advisable not to rely on your GPS receiver since, as with any device, it can fail at the most inopportune time. Consequently, make sure you have an alternative method of finding your position. We highly recommend all who go afloat to attend navigation classes.

Can your crew use it?

All GPS receivers show several 'pages' of navigational information. As part of the safety briefing, make sure the crew know how to find your boat's position on the GPS receiver or chart plotter in case the skipper/navigator becomes ill or falls overboard. They will need to provide this information to the search and rescue (SAR) services. On many sets, this may require no more than repeatedly pressing the 'page' button until eventually the boat's latitude and longitude are shown on the screen.

Chart Datum

Because the Earth is not a perfect sphere, cartographers have, in the past, used a number of geodetic datums to give the best approximation for the area that they were charting. For many years, in the UK, the Hydrographic Office used the Ordnance Survey Great Britain datum drawn up in 1936 (OSGB36), while many European agencies used the European Datum of 1950 (ED50). The datum is shown on the chart.

Chart showing different positions given by different datums

GPS bases its position calculations on a theoretical sphere for the whole Earth that is called World Geodetic Survey 1984 (WGS84). This provides a latitude and longitude that is in a slightly different position to that of OSGB36 and ED50. Consequently, if plotting a WGS84 position on, say, a chart drawn to the OSGB36 datum, it will be in the wrong place. The difference varies from place to place but around the UK OSGB36 to WGS84 will give an error in the region of 100-150m.

Modern charts are now usually drawn to the WGS84 datum but before plotting a position on a chart, check to see which datum the chart has been drawn. Most electronic charts (chart plotters) use WGS84.

GPS receivers can be set to provide latitude and longitude to different datums. The default setting is usually WGS 84. In the set-up section of your GPS receiver there will be a page that allows you to select either an appropriate datum for the chart being used or to input manually the north/south and east/west differences that will be shown on the chart.

Man Overboard

Man over board button on gps

Nearly all marine GPS receivers have a dedicated man overboard button (MOB).

Pressing this button when somebody falls overboard will:

  1. override the current navigation plan
  2. save the MOB position in the receiver's memory
  3. provide the bearing and distance to that position and, in some cases, swap the display to a 'highway' page for returning to the MOB position.

The MOB position stored in the receiver's memory provides an excellent starting point for the SAR services to start a search for the MOB. However, be aware that due to the effect of the tide and wind, the MOB is unlikely to be exactly at that position. Make pressing the button part of your MOB drill and practise using it. Some personal locator beacons (PLBs) can be linked to the GPS to trigger the MOB button when the wearer falls into the water.

Anchor Watch

When at anchor, it is advisable to take a visual bearing or align a transit to ensure your boat is staying in position. At night or in poor visibility this may not be possible.

Anchor watch range shown on chart

Some GPS receivers have the option of setting an 'anchor watch' that will sound an alarm if the position of the boat moves outside a set distance from the original anchoring position. Take into account how much the boat will move as it swings about the anchor and the range from potential hazards when setting the distance.

Even with the GPS anchor watch set, it is still advisable to check the anchor at regular intervals during the day and night.

Linking with DSC

GPS linked to DSC radio

A GPS receiver can output its position data to other instruments. To make the most of a DSC radio, connect the GPS set's NMEA (National Marine Electronics Association) output to the DSC radio's NMEA input. This usually only requires two wires to be connected. Most GPS sets also require the correct output to be activated. This is usually achieved by entering the set-up menu and finding the appropriate NMEA output function. If in doubt, ask a service engineer to make the connections for you. When purchasing a GPS receiver to go with a DSC radio, ask the salesman if the two sets are compatible.

Once connected, the position data calculated by the GPS receiver will also be displayed and updated every second on the DSC radio. To ensure that the radio is completely ready to be used in an emergency, remember to switch on the radio and GPS before departure. The GPS may require several minutes to calculate its position.