An Introduction to GPS

Once a sailor had to look to the stars in order to stay on his course – these days, sailors don’t need to check out the sky anymore, since their GPS is doing this job now. High-precision navigation toady is easier than every before – thanks to satellite support. In this article, I give a concise introduction to the Global Positioning System and its application for sailing.

Since the mid-1990ies, the “Global Positioning System”, generally called “GPS” has more or less revolutionized the practice of maritime navigation. The variety of electronic devices called “GPS” is of such high diversity, that sailing beginners often get confused about this technology. There is no reason to – in principal, all GPS function in the same way, only supplements make the device more or less elaborate in its “skills”.

GPS is based on a network of 24 satellites in motion, orbiting the Earth about twice a day. These satellites are constantly receiving and transmitting signals that easily penetrate even a thick layer of clouds or a heavily ionized atmosphere. This makes GPS highly practical, since it is widely independent of changing weather conditions. GPS was started for the US Army. It is still controlled by the US Department of Defense for strategic reasons.

Variety of GPS types

This is also why GPS kicked in so strongly after 2001. Until then, private GPS units have only limited access to the advantages of the system: The so-called “Selective Availability” practice of the Department of Defense artificially decreased the accuracy of civilian GPS devices. After this policy was abandoned, sales of GPS units increased tremendously, causing a significant diversification in models, types and brands.

If you plan to sail in a country that the United States decide to attack, you will experience one of the US’ strongest weapons: they can “switch off” GPS for selective areas in the World. This is also the main reason why the European Union is currently working on building up an alternative network of satellites with a similar purpose, called “Galileo”.

That is all very good – firstly, because it means that GPS units have become a lot cheaper in the past decade; secondly, because specific needs for sailors are met more directly; and finally, because the availability of high-precision GPS units helped to increase the safety at sea. The only disadvantage of the “GPS revolution” is, that the variety of devices available today can be confusing for the novice.

Handheld and Fix-Mount GPS

But that’s not that bad after all. On a functional note one can distinguish between handheld and fixed-mount GPS units. Handheld GPS units are very popular among daysailors. They are (fairly) cheap, easy to use and today, they come with more extras and functions that professional fixed-mount units had 5 years ago.

If you are looking for a handheld GPS, make sure that it has a water-resistant surface. They work with Alkaline or NiCad batteries; check, if the capacity of these batteries will be sufficient to keep the GPS going for the duration of your sailing trips. Backlighting of the LCD display can be a big advantage at low light, but is almost a standard extra with modern units.

Fixed-mount GPS units are significantly more expensive. They tend to be a lot more sophisticated and target professional racers or cruisers. Fix-mount GPS have larger, permanently lit screens and many resemble a classic navigational unit like those you find in big ships. They work with higher accuracy than their handheld little brothers and are – or at least should be – resistant against water and physical damages.

Extras and Additional Features of GPS devices

Some have internal antennae, which aims to make them more suitable for outdoor mounting, for example at racing dinghies. Very sophisticated devices, however, often come with external antennae that should maximize their “communicative skills”. But what makes a GPS “sophisticated”?

Now we are approaching the extras that make the market as wide as it is. As a beginner to GPS who needs it only for daysailing, you want your unit to be straight-forward, easy to use and with few extras. Good maritime GPS units come with a function to save and use waypoints. This is still a fairly basic feature and means, that you can program a course and define particular points on it – or simply hit a “store the current location under name X” type of button. Later, you can use these waypoints so sail a course from one to another.

Continue with “An Introduction to GPS Part II

Further Reading

Back to "gear"

Wikipedia on GPS and Navigation

Public Information on GPS