The Global Positioning System, aka GPS, is a satellite-based navigation system created by the United States Department of Defense for military applications—but most of us know them as the navigation system used by our cars and phones. Whether it’s the GPS in your car, your phone, or marine GPS units for boats, these devices use radio signals from several orbiting satellites to calculate your location.
There are reportedly 30 active GPS satellites in orbit and some spares in case one or more of the satellites experiences mechanical difficulties or is damaged. Your GPS receiver can pinpoint its exact location anywhere on Earth by knowing the position of at least three of these satellites and calculating the time gaps between the transmitted signals traveling through space at over 186,000 MPH.
You can find more info on installing a marine GPS as a boating accessory here.
How to Use a Marine GPS for Boating
The more signals the GPS receives, the more precisely it can calculate your position. While accuracy relies on various factors ranging from receiver quality to atmospheric conditions, a typical GPS receiver can usually put your location within a 16-foot radius when multiple satellites are in view. High-end GPS receivers with dual channels can improve accuracy significantly.
Once you’re on the road, the GPS will constantly update your location and provide speed and direction information.
GPS also lets you save positions colloquially referred to as “waypoints.” This can be useful for boaters to permanently document the location of a shoal, channel, or fishing hotspot. You can also connect several waypoints to form a route from point “A” to “Z.”
Originally, boating GPS units presented your position in longitude and latitude. While that information is still available, most modern marine GPS users use a digital chart to determine where they are, similar to how you can look at a street map that shows your position on your mobile phone. A “GPS/chart plotter” or “chart plotter” is a GPS that can display charts.
Digital charts have become increasingly comprehensive in the sectors they cover in recent years, with improved detail levels and accuracy. Indeed, many modern chart plotters let you improve the digital charts you see in real-time while operating your boat by aligning your GPS position with depth soundings obtained by your fishfinder.
GPS for Marine Navigation
GPS is the most efficient and easiest way to navigate a boat, but far too many individuals use it as a graphical representation instead of a numerical one. You can use your GPS unit to display a digital chart and steer the boat so that the boat icon in the middle of the screen points in the right general direction.
However, you’ll steer much more precisely if you use a dedicated “steering screen” that displays the numeric compass heading you wish to follow, the course you’re steering, and an indicator or arrow that indicates whether to steer to starboard or port (learn more nautical trivia and fun facts here!) to get back on the most direct course.
However, while most of us rely on GPS to navigate boats most of the time, nobody should rely on it completely. GPS, like any electronic system, is prone to failure, and you must know how to get home if your electronics fail.
Why Does Your Boat Need GPS?
Even boaters who never leave land should have a chart plotter/GPS on almost any boat. You don’t know when you’ll encounter fog or have to stay out late to navigate home in the dark due to a mechanical issue.
You’ll be glad you have your GPS on board at times like these. Plus, today’s units are so cheap (you can have a basic GPS/chart plotter for a couple of hundred dollars) that there’s no excuse not to have one.
But you already have GPS on your phone, so why get another for your boat? That’s a good question, especially since you can use various boating apps to transform your cell phone into a mini-chart plotter. However, cell phones have several disadvantages:
- They aren’t always tough enough to avoid injury when spray begins to fly, or they fall off your dash and onto a fiberglass deck.
- Their batteries may discharge faster than expected, and many boaters frequently travel beyond the cell coverage areas.
- They are not hard-mounted at the helm, and only a few add-on mounts can withstand the vibration and impacts of a running boat.
While having a phone with a navigation app loaded up can be useful and is a good backup to have aboard, it should never be relied on as your primary navigational tool.
Other Applications for GPS on Boats
Some modern systems also let you connect to your boat’s GPS from afar using an app on your cell phone, acting as a security system. You can set up a geofence around your boat, and if somebody tries to steal it, you’ll receive a text alert as soon as it moves out of place.
The ability to set an “anchor alarm” is a GPS feature especially useful for boaters. While your boat is at anchor, you can set up a geofence around it, and if the line comes loose or the anchor drags, and your boat passes through the geofence, your GPS will sound an alarm.
If the GPS is “networked” (connected to the other electronics on the boat), it can perform various additional functions. It can direct the autopilot and overlay additional data (such as boat radar) over the digital charts.
Most importantly, transmit the boat’s location to your VHF radio. You can use your VHF for DSC when it receives position data (digital selective calling).
If you do this, the local Coast Guard will receive your precise position data via radio transmission if you need to contact them. This is a possibly life-saving feature, so read How to Use a VHF Radio for more information on VHF communications and DSC.
How to Install a GPS on Your Boat
The antenna is the most important component of a GPS installation. It should be installed so that it has a clear view of the sky and is not in the shadow of any obstructions. The antenna should be placed where it will be least affected by multi-pathing or receiving unwelcome reflected signals from your boat’s structure.
A GPS antenna, on the other hand, should not be mounted on top of a sailboat’s mast. The boat’s rocking is accentuated at the top of the mast, making it unsuitable for the antenna since the GPS will move as the boat heels.
If your GPS or VHF receiver antennas are in the path of the radar’s transmission beam, they are very likely to receive interference. The powerful pulses from your radar can cause serious problems and interfere with your equipment in some installations.
Your GPS unit may have an integrated or separate antenna linked by a coaxial cable. Since the cable length must remain constant for maximum efficiency, the manufacturer might advise against cutting it. Follow the manufacturer’s directions.
Most recreational boats are small, and even if the antenna is located some distance from the GPS, the distance may be relatively short. On a large ship, the antenna may be located some distance away from the GPS display, and the readings displayed on its GPS relate to the antenna’s position rather than the location of the GPS unit.