Global Positioning Systems (GPS) and Radar

GPS supplies accurate three-dimensional fixes 24 hours a day, in all weathers, anywhere in the world, even though for maritime purposes, two dimensions will do very nicely.

Given that your set does not break down, there is only one catch: in order to preserve U S national security, those in charge of GPS are `dedicated to the development and deployment of regional denial capabilities in lieu of global degradation'. This means that rather than downgrade the signals globally or turn off the satellites completely, sensitive areas can be blacked out from GPS signals.

A constellation of 21 satellites has been arranged in a `birdcage' around the Earth, together with 3 stand-by `spares'.

Their extremely high altitude keeps them clear of skywaves and other atmospheric interference. It also guarantees that four or more can be seen from a given location at any time. The system is completed by various control and monitor stations around the world. These maintain system accuracy, together with other forms of monitoring and updating of the navigation message of each satellite.

GPS works by knowing the satellites' position in space.

Each of them transmits coded data which include a radio signal timed to mind-numbing accuracy. This enables the receiver to calculate the distance of the transmitting satellite from its antenna, which gives it its position on the surface of a sphere centred at the satellite. The receiver chooses the three or more satellites offering the best `cut' of position lines (position spheres in reality) and works out its position from these .

Control of GPS accuracy

GPS is capable of producing fixes accurate to better than one metre, but this Precise Positioning Service (PPS) is kept by the military for themselves by encrypting the signals. The rest of us must be content with a potential accuracy of around five metres. In practice, fixes are rarely adrift by more than a boat's length or two.

Such discrepancies should be of no more interest to the marine navigator than his altitude, but with so high a degree of potential accuracy it's easy to get carried away, so we end users must remember that a fix is only as good as the chart it's plotted on. The survey data may not have allowed for such knife-edge potential, and in any case, failure to set up the receiver for the correct datum renders all such niceties a nonsense.

Differential GPS DGPS

Differential GPS (DGPS) upgrades the accuracy of the standard GPS position by comparing the GPS-fixed position of a static, ground-based receiver at any moment with its known geographic position. The ground station transmits signals to all GPS receivers in its vicinity, advising them of any error it has noted. The differential user's receiver then incorporates this data into the fix that appears on its screen. Accuracy Of a metre or so can then be expected.

Chart datums

The globe is not a perfect spheroid. Its surface has minor irregularities. In the days before the phenomenal accuracy of GPS, the various charting authorities were content to work within a generally agreed mean latitude and longitude grid.

If one Nation's convention varied from another by a few metres, as it did and sometimes still does, no practical navigator was interested. The difference was largely unobservable by conventional means. Because of the precision of GPS, all this has changed.

When any onboard GPS is reading out its fixes to the wrong `datum' for the chart in use, the plot could be in error by, at the extreme, up to several hundred metres. Datum shifts of a cable or so are commonplace. Fortunately, the majority of receivers will now read out in any one of a variety of datums.

Check the `title corner' of your chart. If it is reasonably modern you will find a statement of datum, eg `WGS84' or `OSGB (1936)'. The GPS default setting is WGS84, favoured by the US and rapidly becoming the world standard. Some charts do not declare their datum, but give an instruction concerning how to shift `satellite-derived positions' instead. If this is so, set your GPS to WGS84 and do what the chart says if ultimate accuracy is required. If it isn't, as is often the case, don't waste your time with the extra plotting duties.

Most electronic charts have been fine-tuned so that fixes displayed on their associated chart plotters are accurate. Whether this is done by redrawing the chart to WGS84 or by some other computer wizardry is not important to the practical user. Suffice to say that it generally works.

Until every chart aboard every boat is set in WGS84, the issue of datums will remain a serious one. Failure to address it can lead to serious errors in a world where so much accuracy is now assumed.

Beyond the fix

Some traditional navigators use GPS for nothing more than fixing their position on a paper chart. This is their choice, but it must be said that in doing so they are not reaping the full value from their investment. The additional functions offered by all GPS receivers have really given rise to what is in effect a modified and much simpler system of passage navigation.

Course and speed

These can be read out from most yacht navigation systems. The machine is, of course, working this up by comparing fixes. These relate to your actual position, so the course and speed are `over the ground', rather than `through the water' and the acronyms COG and SOG have entered the vernacular. Do not fall into the trap of imagining that because the GPS reads 9.3 knots, your 28-footer is logging anywhere near this velocity. Check the water log and the tidal stream atlas and re-enter the real world.

One use to which this information can be put is to check your set and drift against that predicted by the tide tables.

Waypoints

The idea of the waypoint forms the heart of modern navigation. You may choose to use few or many, but the concept must be understood and accepted so that you can take an informed decision based on the way you think and the circumstances.

It is standard practice to enter a number of predetermined positions along a predicted route into your navigator's computer. The theoretical ideal for a passage of any substance, offshore or along the coast, is to proceed from one of these waypoints to the next. This can be done informally, plotting them as you go, or as a pre-planned `route'. Waypoints are helpful on a chart plotter, although you may opt to use fewer of them, but with a paper chart they really do ease your task.

GoTo

By activating the `GoTo' function, the receiver will give a distance and bearing to next waypoint at any time. Distance and bearing to a known position is another form of expressing the yacht's position . This can often be a more convenient way of plotting a fix on the chart than trying to wrestle with lat/long over the folds of the paper with a plotter that is too short and a longitude scale that numbers from right to left in west longitude.

As an aside, the concept of a fix in terms of lat/long and range and bearing is useful when transferring a position from one chart to the next. It is all too easy to make an error over a question of scale, or by reading the longitude the wrong way. If you transfer the position by both means, they should marry up; if they don't, you've made a mistake. Using GoTo for comparing the bearing of the waypoint and the present track makes adjusting your course to take you to it a doddle.

Electronic chart plotters

Electronic chart plotters started life as computer screens depicting a caricature of a paper chart. Now they offer full-quality coloured chart coverage with a zoom facility to take the user from global scales down to the local marina pontoons.

The plotter's computer either interfaces with GPS or contains its own GPS facility, so that the yacht's real-time position is shown on a chart on its screen.

Types of chart plotter

The hardware chartplotters

These are single, dedicated instruments incorporating a screen, a processor and a GPS receiver. They will perform all the functions of a simple GPS set with the additional benefit of showing the boat on the chart in a moving, real-time position. Hardware plotters generally use a specific type of chart package so you need to decide which one gives the best deal for the charts you are going to want. It's also important to decide which charts appeal to your eye, because they are all vector charts and they all look a little different.

Hardware plotters often come as part of a larger package of instruments. As such, they can be interfaced in all sorts of directions. One such interface is with radar, and some can display the radar image as an alternative on the screen. Split screens may be available so you can access both tools at once, and some even overlay the radar image onto the chart.

The software chartplotters

A software plotter is a plotter program installed, together with a chart package, onto a boat's PC. Many people use laptops for this, but an inbuilt PC can be even better in certain applications. The PC is interfaced with an independent GPS receiver via the NMEA data protocol through a serial or USB port connection.

The combination is capable of doing most things a hardware plotter can do, and often much more besides. It has the advantage that since most people now have a laptop, the 'hardware' is already in place, so all that need be bought is the software. However, configuring and interfacing the system is often more of a challenge than hardware plotters, which you basically buy, set up the antenna and plug in. The rewards can correspond to the effort.

Leisure marine radar 

Radar is the most interactive aid to navigation. Using it properly demands far more expertise than an electronic fixing aid because its readout comes in the form of a picture which, to the uninitiated, is hard to interpret if not incomprehensible. Buying a radar set, therefore, does not solve all your navigation problems.

Nonetheless, the rewards are great for those who persevere, because radar indicates visually many of those things the navigator would like to see with his own eyes but cannot by virtue of darkness, range or poor visibility. These are not only navigational features, radar also shows the whereabouts of shipping and small craft so as to form a primary tool for collision avoidance in fog.