Where am I?
Navigation is about getting yourself from somewhere to somewhere else, but for centuries human navigators have had to devote most of their efforts to answering the question “Where am I?” Electronic position fixing has changed all that, and chart plotters can relate our present position to the world around us easily, accurately and instantaneously.
Each of the past few decades has seen a major advances in navigation technology.
In the seventies, it was a radio position fixing system called Decca. Receivers were expensive and it was bedeviled by legal and political wrangling, but it was the beginning of a breakthrough. In the eighties, we saw the first civilian receivers for a satellite system know as ‘Transit’ followed in the nineties by GPS (Global Positioning System).
Now, GPS receivers costing as little as £100 offer better than 20 metres accuracy whatever the weather and whether you’re in the Solent or the Southern Ocean. We also have access to a supplementary system that offers 3-5 metres accuracy from equipment that costs less, in real terms than a fairly basic Decca Navigator ten years ago.
GPS is based on a constellation of up to 30 satellites, each continuously broadcasting a signal which translates as “I am here…” and “the time is now…”. Radio waves travel at a constant speed so by comparing the time as which the message was sent with the time it arrives; a GPS receiver can calculate its distance from the satellite. Just as you can plot your position on a chart by knowing your distance from two or three landmarks, it can work out its position by knowing the range of three or four satellites.
The fact that radio waves travel at about 162,000 nautical miles per second means that if the receiver’s clock is out of synch with the satellite clocks by even a fraction of a millisecond, the measured range will be miles out. That’s why the GPS receiver needs to listen to three of four satellites instead of two or three: comparing the results obtained from several satellites at once enables it to eliminate the clock error - so as well as telling you where you are, GPS is also the most accurate, self correcting clock you could wish for.
There are plenty of reasons for wanting accuracy better than 20 metres, so some very clever brains have applied themselves to achieving it. What they’ve come up with is Differential GPS (dGPS).
You can get some idea of how it works by imagining someone standing beside a lighthouse with a GPS set and a radio. Knowing exactly where he is means that he knows his GPS position error - so in theory, he could broadcast the information to anyone else in the vicinity, who could apply it as a correction to their own GPS positions.
In practice, it’s all automated, and instead of transmitting corrections to the displayed position, the reference stations transmit corrections, that are applied to the signals received from the satellites rather that to the position. The correction signals are quite different from those of GPS itself, so receiving them requires a piece of equipment called a Differential Beacon Receiver (DBR). The DBR may be integrated with the GPS, but many are separate “black boxes” designed to enhance the performance of existing GPS sets.
What else will it do?
To an instrument capable of working out its position using signals broadcast from satellites thousands of miles away, making routine navigational calculations is child’s play.
All GPS receivers, for instance, can store planned positions as “waypoints”: you can tell it where you want to go, and it will tell you the direction and distance you have to travel.
If you need to zigzag round obstructions, you can make up a route by stringing several waypoints together like a child’s join-the-dots picture.
By using what is know as the Doppler Effect and also comparing your present position with your position a few seconds ago, a GPS set can calculate your direction and speed of movement over the sea bed.
It’s probably in the last of these that dGPS really does come into its own. Small, random errors in an uncorrected GPS’s position don’t make much difference on their own, but when you compare two positions a few seconds apart, they can produce quite significant errors in speed and direction – a knot or two in a boat moving at six knots. By reducing positioning errors, dGPS makes the speed and direction displays, so much more accurate that racing sailors, for instance can use them to assess the true effect of tidal streams.
Knowing where you are is one thing: relating that to the real world is another.
Navigators have been doing the job with paper charts for hundred of years – probably since the magnetic compass was invented somewhere in the twelfth century. It was almost inevitable that charts and compasses should develop side by side, because without a compass, it would have been impossible to produce a chart, and without charts, the value of a compass would have been pretty limited.
GPS is as dramatic a development as the compass was in its time, and is making us look at new ways of storing, organising and displaying navigational information.
Setting up a computer to display a GPS position on a lat-long grid is no great problem, and it’s easy to make a few fixed points on the grid represent buoys or landmarks. The clever bit is in expanding this idea to show entire coastlines and contours.
In effect, a raster chart is an electronic photograph, produced by scanning a paper chart in much the same way as a fax machine scans a letter. The image is broken down into coloured dots, and information about the colour and position of each dot is stored on a computer disc or memory cartridges. When it’s needed, this mass of data can be reassembled to produce a picture on the screen.
A vector chart is more complicated to produce because it involves electronically tracing a raster chart to produce and image in which lines are stored as lines, rather than as strings of unconnected dots. The process is largely automated, but is still takes time, skill, and sophisticated equipment; in return, it produces charts that typically take up only a hundredth of the memory occupied by equivalent raster charts. This makes vector charts particularly well suited to dedicated chart plotters, whose memories and processor speeds may not match that of the latest personal computers, but whose internal organs are built to withstand life afloat and whose controls are more appropriate for the job.
Vector charts have other advantages, too. One is that they can be enlarged or reduced as much as you like: one minute you can be looking at a map of the British Isles, the next you can zoom in to individual pontoons in a marina. Another is that the chart can be “layered”, as though it were built up of a series of transparent sheets. Each sheet contains a different kind of information, so the picture can be de-cluttered by removing unwanted layers.
Of course there are drawbacks: the tracing process can introduce inaccuracies, or the chart editor may reduce “clutter” by omitting information. This was certainly common a few years ago, when the majority of chart plotters had small memories, slow processors and monochrome displays, but it’s changing fast: the latest generation of electronic charts is incomparably better than their counterparts five years ago.
At present, we have two broad classes of electronic carts – raster and vector – with several incompatible brands competing in each class. Each has its own particular strengths, so different hardware manufacturers have adopted different brands. It pays to look carefully at all the options and take expert advice before committing yourself to one particular system.
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