Getting the Most From The Marine GPS System

At the heart of modern marine navigation is the Marine GPS System, which is a network of 24 dedicated Global Positioning System (GPS) satellites spread out in geo-stationary orbit some 20,000km (12,500 miles) above the earth.

As the earth rotates below, at least four of the satellites are above the horizon at any one time and 'visible' from anywhere on earth. Each satellite knows precisely where it is, and kindly broadcasts this information at very accurately predetermined times.

The active antenna of our GPS set records the times taken for the transmissions to reach it and computes a three dimensional fix in terms of latitude, longitude and altitude, accurate to within a few metres.

As I write this, there's only one marine GPS system in place and it's owned and operated by the US military, who could presumably choose to block or degrade its signal to the general public - us, that is. But although they've been heavily involved in various scrapes around the world in recent years, they haven't yet done so, which is a good sign.

The European Community is currently building its own completely independent satellite-navigation system. Scheduled to be operational by the end of 2013, 'Galileo' as it's known, will be an orbiting system of 30 satellites giving even greater accuracy and dependability than the current marine GPS system.

Update - Galileo is now fully operational!

But there's a lot more functionality to satellite navigation than position fixing...

What the Marine GPS System Can Do

As long as you give it a clue as to your intended route by entering a start and a destination waypoint, along with as many intermediate waypoints as may be required your GPS unit can perform a number of other useful tricks, including displays of:

Course Made Good (CMG) - the bearing from the starting point to the current position;
Cross Track Error (XTE) - the distance off either side of the desired course, often shown on a 'rolling road' graphic;
Estimated Time of Arrival (ETA) - the date and time of arrival at the destination based on current speed over the ground;
Estimated Time on Route - the time to go to arrival at the destination based on current speed over the ground;
Ground Speed - the velocity in knots relative to the sea bed;
Velocity Made Good (VMG) - the closing speed towards the destination.

Be Alarmed!

Should you so wish alarms can be set to activate when various events occur, including:

When your anchor drags. This one will really get you out of your bunk in a hurry;
When the cross track error exceeds a preset distance;
When you're within a preset distance of your destination;
When you're within a preset distance of an identified hazard, ie a rock, shoal or reef;
If the GPS loses its fix.

All GPS sets have a dedicated Man Overboard (MoB) button. Pressing this will record the current position (as a waypoint) and displays the course and distance to get back to the point at which the MoB button was activated.

A Handheld Marine GPS Unit or a Fixed Unit?

Garmin GPSMAP 78sc Marine Handheld GPS Receiver with Compass, Barometer and BlueChart g2 Coastal Charts

With the functionality of handheld marine GPS units being much the same as fixed units, many sailors of small boats opt for the AA battery powered handheld GPS due to benefits to be gained by its portability.

But fixed GPS units are hooked up to the boat's 12v domestic supply, which providing your battery charging regime is up to scratch, will be more dependable than AA batteries.

And there's no denying that the larger screen size of the fixed GPS units makes them a lot easier to use than the handheld marine GPS units.

But there are real benefits in having both types:~

System redundancy - If one fails you have a backup
Emergency use - Keep the handheld GPS unit in the emergency grab bag along with a handheld VHF radio. Now if you're unlucky enough to have to take to the life raft, you can not only broadcast your predicament, but tell your rescuers precisely where you are.

But basic fixed GPS units are no where near as popular as they once were - most of us want combined GPS/chartplotters today, either at the navigation station or in the cockpit.

Next: The Chartplotter as Part of the Marine GPS System

You are here: Sailboat Cruising > Sailboat Instruments > Marine GPS System