Navigation
GPS Unit on British Trader
9 October 2009
by Cadet Cameron Scott, onboard British Trader
It is essential that the Officer of the Watch on a ship is able to establish accurately the ships position. But how can he do this? He is not in the same situation as someone driving a car that can just follow a road. One method is to use landmarks. But these landmarks must be used accurately. It’s not a case of saying we are near a certain landmark. The ships position must be pinpointed accurately.
A visual bearing and distance are used as follows to pinpoint the ships position. For example, if the bearing of a lighthouse from the ship is 210°, a line will be drawn in this direction away from the lighthouse on the chart being used. This line is known as a position line because the ships position is known to lie on it.
To complete the position and to pinpoint our position on the line we must take a range from either a piece of land or a landmark. This is normally done by using the ships radar. For example, if the ship is lying 5 miles from the landmark on the radar, you would then measure 5 miles with your chart work dividers and plot this line across the position line. Where your position line and your range line cross, this is your ships position.
Close up of GPS on British Trader
On the open ocean, it's a different story, as there are no landmarks. We can't take a bearing from an object as distant as the sun or a planet, because the compass is too inaccurate an instrument. It measures in degrees, while a sextant measures in degrees, minutes, and seconds. The sextant does not give us a bearing, or azimuth, to a celestial body, but gives us information that helps us find the azimuth.
The stars pretty much stay in the same place - that's why they were known as the "fixed stars", except they rise and set; the sun, moon, and planets move, but predictably, and so with the aid of almanacs that tell us precisely where each body is at every second of every minute of every hour of every day of the year, and the practice of "sight reduction", we can take a position from two or preferably three stars, or planets, or the sun and moon when both are visible, or the sun at different times of the day, and where the lines of position cross is where we are.
Using a sextant for sun-sight
But navigation today is all about practicality and accuracy. And just as drivers ashore use GPS systems now to navigate in unknown territory, so is the GPS receiver a staple of navigation on board all deep-sea ships. It’s quick, it’s direct, there are no calculations involved, and it is nearly always correct nearly all the time. Once you know its limitations (and it has plenty of them) you know to watch out for these and you are generally ok.
So why would anyone bother with star-sights or sun-sights or the sextant at all, if GPS is so accurate and so handy?
The answer lies mainly in the prudent navigator’s precautionary approach to anything electrical. If something has a fuse, it can fail. There are no fuses in sextants, and nobody needs electricity to operate a set of nautical tables – all you need is a pencil, and a sheet of paper, and a little bit of practice – and a clear sky for your observation.
You can also cross-check the readings on the GPS wholly independently by using the traditional sextant method; and safe navigation is all about cross-checking. But most of all, you are keeping alive the true work of the self-sufficient navigator, and taking great satisfaction in being able to accurately fix your position in the knowledge that if the GPS unit failed completely it would not cause undue panic or inconvenience.