How does GPS help to find out where you are and where you are going?

GPS, short for Global Positioning System, is a space-based satellite navigation system that provides location and time information anywhere on earth as long as you are in range of four or more satellites. This information is then displayed on a stand alone GPS, for example next to the chart table or a handheld device, your car satnav or even on your smart phone or mobile device. Most GPS also provide you with a direction or course you are travelling and a speed, however both of those are calculated on historic data trying to predict where you are going if you do not change your direction and speed.

However, if there is no charting or route planning data associated, this will only be a number of digits that have to be converted into usable data using a map or chart.

Before you can start to work out your position – the point on the earth where you are at this moment, you have to “plot” or mark this on a chart (at sea) like this one of the Thames Estuary or a map (on land).

First, you need to note the position, in a format known as latitude and longitude.

In this case, the position is 51°20.55’N 1° 35.40’E

To plot or mark this position on a chart, you have to find the latitude (how far North of the equator you are) on the right and left hand side of your chart and the longitude (how far East of the West of the Greenwich meridian you are) on the top and bottom.





By joining up those to lines, drawing lines parallel to the edges of the chart, you can mark your position
(where the lines cross).

If your GPS also gives you a direction and speed, you can easily predict where you are going by using a method called “dead reckoning”. This is a rough method of predicting your movement, not taking into account any influences of wind or tide.

Find your direction of travel, shown above on the GPS as ‘Heading’ of '45°' Some GPS will also give your speed.










Using one of the compass roses and a parallel ruler, you can plot or draw this direction on your chart by transferring the direction from the compass rose through your position (also called fix).






If the GPS gives you a speed or if you can estimate it, just calculate the distance you will travel by multiplying your speed by the amount of time you want to ‘dead reckon’ or predict for.

For example, if you travel at 5 knots (that is 5 nautical miles per hour), you will travel 2.5 nautical miles in 30 minutes, 0.5 nautical miles in 6 minutes, and so on.







You can measure the distance by using the minutes on the left and right hand side of the chart. One minute is equal to one nautical mile.


Traveling at five knots, you will travel one nautical mile in a fifth of an hour which is equal to 12 minutes. So if you took your position at 9am and you intend to travel at 5 knots on a direction of 045°, you will be 1 nautical mile down that direction in 12 minutes time. This does not take into account wind and tide, but only your present direction (course) and speed. The mariner calls this a Dead Reckoning or DR position.

Watch Officers at MRCCs have to be able to plot a position and calculate a DR and are tested on this during their training.







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