Understanding GPS in 5 Minutes!

Written by Capt. Juan Watson

I would venture to say most of us have used a GPS on the water at some stage for navigation purposes. It is quite amazing to think around 20 working satellites (there are about 30 in orbit) are constantly providing positional information to millions of devices every second all over the globe.

This quick run down should at least arm you with enough knowledge to get your through a Jeopardy round.

So what is GPS?

GPS is a form of GNSS (Global Navigation Satellite System) a term used to describe any satellite derived positioning system. Currently only NAVSTAR GPS (US)  the most recognized and most relied upon system and the Russian GLONASS system have achieved global coverage. The European (Galileo) and the Chinese (Compass) systems are in development and due to be operational in the next few years.

How does it work?

Lets start by first looking at the set up.

GPS satellites orbit the Earth in groups of four on six orbital planes at various angles to the equator at some 11000 miles above us. This means at any time we should always have a minimum of six “line of sight” satellites from which to derive a position signal from. This is important as we can only receive signals from satellites in our visible sky. The satellites also pass over Earth Stations which monitor and provide updates such as time based on the atomic clock and orbital information to them.

So now we bring the math in! Each satellite continually transmits a radio signal containing their exact position based on orbit and time information and the exact transmission time of the signal. Given we know radio signals travel at the speed of light (186 000 miles per second) we have transmitted time and our receiver calculates an elapsed time we, have a distance ( time,speed,distance triangle) With four satellite signals we should obtain an accurate position.

The standard accuracy for GPS is 49ft. When using a receiver with dGPS (differential GPS) or WAAS (Wide Area Augmentation System) which use ground stations to provide error corrections and can provide accuracy up to 9ft

As with almost any navigational aid there are errors to take into account.  To give one some idea of how a slight error in your receiver clock for example can affect your position. An error of 0.001 seconds can have you 186 miles off your true position! What this serves to illustrate is that one should still confirm your position by other means. Also, certain paper charts will produce errors if plotting on them using information from a GPS receiver. Some will have corrections to be applied when plotting to compensate for this. The WGS84 datum however is the most common when plotting using GPS coordinates, so check on the bottom of your chart which datum it is based on.

We are very fortunate to be able to have this equipment so easily at our disposal and how much more enjoyable are trips now that we do not have to constantly be plotting. Still bear in mind that it is a great system but not perfect and if you are in unfamiliar areas use all available tools at your disposal to safely navigate.


Be safe!

Captain Juan Watson



Juan Watson is a 14-year mega yacht veteran. Currently, he owns and operates Pelorus Yacht Consulting, LLC in Annapolis, MD. PYC focuses on educating yachtsmen and owners.