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