Users Guide to GPS

The Global Positioning System uses 24 active satellites orbiting the earth to transmit timing signals that can be used by receivers to calculate a person's latitude and longitude on the earth's surface. This system is operated by the US military and signals can be degraded or switched off at any time. The signals are very weak and prone to interference, both deliberate and atmospheric. In its basic form, a GPS set receiving signals from at least 5 suitably positioned satellites may have an accuracy of 15 meters, but this may be degraded to 100m or more and occasionally tens of miles.

Another set of satellites can transmit 'differential' signals that can interact with the GPS signals to give accuracies in the order of 3 metres. This system is Wide Area Augmentation System (WAAS) in the US and WAAS enabled GPS receivers are compatible with the European EGNOS system.

ERRORS:

Each map-making nation drew its charts to its own 'datum' according to their idea of the shape of the earth. The differences between the systems can lead to errors of several hundred metres. The accuracy of GPS gave rise to a single 'datum', namely WGS84. All GPS positions are given to WGS84, but users can set their GPS set to give positions using the datum of the chart in use. On the UK's south coast, using a WGS84 position on a Admiralty chart drawn to OSGB36 would give rise to a position error of about 135 metres.

All current UK Hydrographic Office charts of the UK are now made to WGS84.

Up-to-date paper charts may be drawn from surveys made in the 1800s using lead line and compass bearings and errors may be large. Unless you are aware of the date and scale of the original survey, you may be lulled into a false sense of security and be unaware of errors up to a couple of miles or so. UK Admiralty charts have a source diagram showing the dates and scales of the survey data.

GPS should NEVER be used as the sole method of deriving your position but it is a wonderful aid to navigation, and few yachtsmen venturing far from shore would be without one.

THE GPS RECEIVER

The GPS receiver has either a built-in antenna (for outdoor use) or a separate antenna, that can be fixed to the external structure of the boat, with an unobstructed view of the sky because the satellites in view may be anywhere from on the horizon to directly overhead. The more unobstructed the view, the more satellites will be seen, and accuracy enhanced.

On start up, the receiver will take a little while to determine where it is. This may be as little as 30 seconds if the set hasn't been moved since it was switched off, to many minutes. The set's screen will normally show a sky view of the satellites until the position is fixed and then will automatically switch to a screen showing the position in latitude and longitude.

That's all you need to plot your position, just transfer the lat & long to your paper chart and you know where you are but there can be a human error in transferring your position to your chart. Sets will also show the speed over the ground (SOG) and the course over the ground (COG) to show where and how fast you are going.

The most useful aspect of a GPS receiver is that you can insert the latitude and longitude of your destination and then not only will your set indicate position, SOG and COG, but also distance and bearing to your destination. This enables you to very easily proceed from where you are to where you want to be, but do remember that GPS should not be the only source of your navigation information. This system is prone to errors of reading the latitude and longditude from the chart and keying them into the GPS.

By stringing together a number of destinations (Waypoints) you can create a route and follow it. It is ESSENTIAL that you investigate each leg of the route for safety before you use it.

GPS CHARTPLOTTER

Many GPS sets now have a big enough screen to display an electronic chart. There are a number of different charting systems available and if you like a particular make of chart, this will restrict your choice of chartplotter. Remember that chart errors may be present, either because of the date and method of the original survey, or because of datum errors or production errors. I have seen 300 meter errors on a chart plotter which would have caused disaster if that had been the only method of navigation. The latest charts (C-Map MAX and Navionics Platinum) have a 3-dimensional capability, tidal flows and photographs! You could even stay at home and have a virtual sail with the chart plotter!

A major advantage of a chart plotter is that no plotting is involved and thus no errors arise from reading and keying lat and long. Another major advantage is that you can see exactly where you are, provided that the GPS position and charts are accurate.

A route can be constructed directly on the chart just by 'clicking' and this route can then be followed, either by keeping the boat symbol following the route or by following a pictorial director, such as a compass or 'rolling road'. Some charts now incorporated tidal currents and tidal height data.

A colour chartplotter is easier to use than a mono one and bigger is definitely better, but of course the depth of ones pocket is a very big factor.

	Pat Manley

Pat is an Associate Fellow of the Royal Institute of Navigation and has been sailing dinghies since 1959 and cruising yachts since 1984. He is a Yachtmaster Instructor (sail) and Yachtmaster (power) is the principal of 1 to 1 Navigation and sails a Westerly Oceanquest 35. A member of the Yachting Journalists Association, he is a regular contributor to Practical Boat Owner Magazine, he also writes for Fernhurst Books who have published his Diesel Companion, Electrics Companion and Radar Companion. Just published is his new hardback book, Small Boat Maintenance. www.1to1navigation.co.uk