• Home
• Theory
• 1 Navigation
•      1.1 Position, speed &            distance
•      1.2 Direction
•      1.3 How to navigate
•      1.4 Fixing your postion
•      1.5 Position lines
•      Glossary
•      Questions
• 2 Tides
• 3 Charts and Nav Aids
• 4 COLREGS
• 5 Weather

1.4 Fixing Your Position

In the previous section we described how we can calculate an estimated position, from distance logged, course steered, leeway and tide. But the estimated position is only an estimate. Its accuracy depends on how well we can steer the ship in a straight line; the accuracy of the compass and log; and how well we have estimated the effects of tide and leeway.

The inaccuracies may accumulate over time. The navigator will therefore wish to check the ship’s position regularly, if possible. This is done by fixing the position, and (luckily) there are quite a number of ways of doing this.

Broadly, you can fix your position by means of:

• electronic devices

• observation of landmarks on shore or navigation aids (bouys) anchored at sea (or objects like the sun and moon, but that's generally for ocean sailors).

Global Position Fixing (GPS)

This is the most widely used electronic device for fixing a position. It works by picking up signals from a set of satellites continuously orbiting the earth. By knowing where the satellites are (information they continually broadcast) and the time delays of receiving signals from each satellite, the GPS receiver can work out the distance from three or more satellites and from that work out its position on the surface of the earth.

The user simply reads the latitude and longitude from the display, making these devices very simple to use.

GPS receivers are excellent position fixing devices. They are relatively inexpensive and generally reliable, although a number of them failed during the 1999 “week number roll-over” event (which will be repeated every 19 years). GPS is a military system, so theoretically the military may decide to shut it down, and from time to time there are local jamming exercises; but hopefully this is a small risk.

Despite its reliability and accuracy there are dangers, however, in becoming over-reliant on any electronic device, particularly where electric power and salt water are closely associated.

The accuracy of the position varies, and is generally close to 20 metres: adequate for most purposes, but remember that the accuracy is slightly less than the precision of the latitude/longitude readout (often three places of decimals, i.e. 2 metres) might suggest.

A higher order of accuracy is provided by Differential GPS or DGPS. This system relies on a ground station, which senses the GPS signals and works out the error in the GPS position (the ground station knows where it actually is!). It then transmits this information to nearby GPS receivers, which can then refine their own positions by applying the error. There was a time when ordinary GPS accuracy was quite severely restricted for civilian users, and DGPS was useful for mariners: now, however, ordinary GPS is quite accurate enough for our purposes, and DGPS is more relevant to people like surveyors.

GPS receivers are able to be set up to a number of different chart datums, which have been used for making charts in different parts of the world. Basically, the definition of Latitude and Longitude is not identical on all charts, largely because the Earth is not a perfect sphere. Differences are relatively small, but large enough to cause problems if your GPS is set up on the "wrong" chart datum. So CHECK that it is set up for the datum of the chart you are using.

A final point: GPS can be MORE ACCURATE than the chart you are using, particularly if you are navigating in a remote part of the world, where the person who drew the chart (probably in the 19th century) was using a sextant. So you may know your position very accurately, but unfortunately the person who drew the chart didn't know the latitude and longitude of the rock to nearly the same accuracy. Be warned!

Hyperbolic Systems: Decca, Loran

Before the advent of GPS a number of areas of the world had other electronic navigation systems, which relied on ground-based transmitters. Some of these still operate, although they are now being phased out in favour of GPS, which has so much wider coverage and is cheaply available.

In principle these systems are just as easy to use as GPS receivers, with a simple readout of latitude and longitude (on modern sets - earlier ones needed specially marked charts) and all the usual route and waypoint functionality. However they are subject to slightly more exotic errors. Advice here is: if you are using such a system (and you are now a minority) study the handbook carefully so that you fully understand the way the system should be used, the kinds of error that can happen, and how to correct the instrument if they do occur.