An Introduction to GPS Part II

Continued from “An Introduction to GPS Part I

Beyond these basic data-handling features, many GPS units can be programmed or come with integrated maps or cartographic data-sets. Some allow you to install programs or access information from the Internet (upload / download functions). The latter one can be extremely useful for cruisers. Different sets of data can be combined by utilizing “GIS” (Geographic Information System) technology: maritime maps that point fishermen to particularly promising spots, for example ("fishfinder"); information on landmarks; or even electronic charts and tide information.

Most of these features have existed for a while, but it is fairly new that they become available to the non-professional sailor. In that sense, we live in quite interesting times when it comes to GPS. To use charts, downloads and navigational functions in their full glory, you will probably want a very big screen with high resolution, good backlight and maybe even a color display.

If you feel attracted to a programmable GPS, look for compatibility of the software and electronic charts it comes with. Check the capacity, because it will limit the amount of data you can store on it; check if it will connect you with the Internet. Finally, check if you really need all of these fancy features for your specific type of sailing.

Improving GPS' Accuracy

One feature that should be part at least of all fix-mount GPS systems are emergency functions. They will allow you to automatically transmit a distress signal which will include precise positional information to Coast Guard and the “Global Maritime Distress and Safety System” GMDSS. Speaking of safety and emergencies: Never treat a GPS like a navigational aid of the kind that you might be used to in a car – especially handheld devices don’t know about obstacles and always guide you in a straight line from one point to another. There might be an island in between these points that the GPS does not know of!

Furthermore, the accuracy of GPS systems if often overestimated. In maritime applications, it can be as accurate as 300 feet and less – but not necessarily. More importantly, even 300 yards are insufficient to rely the navigation on it especially in difficult conditions such as fog.

To improve the accuracy of GPS units, many sailors use “Differential Global Positioning System” or DGPS. This works by utilizing a second signal from land-based stations that detect errors in the positional information and corrects them. This second signal generally comes from Marine Beacon Stations that operate on frequencies between 283.5 and 325.0 kHz. Using them is free of charge. DGPS has an accuracy of about 30 feet, meaning that it is more accurate by a magnitude of about ten with respect to conventional GPS.

DGPS and WAAS: Higher reliability

An alternative to DGPS is the “Wide Area Augmentation System” or WAAS. This system works with two additional satellites instead of land-based stations. On contrast to the 24 satellites that GPS uses, the two additional ones are not orbiting the Earth but are so-called “geo-stationary”. That means that their speed matches the rotational speed of the planet, which is why they are always in the sky above the same area. WAAS boosts the accuracy of GPS in a similar way as DGPS.

To improve the reliability of the GPS unit in general terms, you should look for systems with 12-channel receivers. This means, that they permanently scan for signals from the 12 satellites currently on the respective hemisphere. 12-channel receivers can receive information from all 12 satellites at the same time and therefore, have the most stable signal. They also tend to be widely unaffected by obstacles such as clouds and even a highly charged ionosphere. External antennae can help to improve your GPS’ reliability, too.

No matter if you have a plain hand-held GPS or a fancy, fix-mount device suitable for any circumnavigation – you should practice the use of your GPS system under non-serious conditions. Take a day aboard just to play with the GPS, match it with own observations of landmarks and chart information. Familiarize yourself thoroughly with all its features. The first time you are using a GPS unit, you might have to supply some basic information. Simply follow the instructions of the manual. Receiving a signal can take up to 30 seconds; after this you should be able to read your position in latitude and longitude.

Back to “An Introduction to GPS Part I

Further Reading

Back to "gear"

Wikipedia on GPS and Navigation

Public Information on GPS