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Understanding How Your GPS Works

Time and Place and Tomatoes

When GPS gives you a bad position, you may be surprised who’s to blame.

Do you remember when you were just learning about boats—you may have been just a youngster at the time—and you thought boats and their equipment did what you wanted, just because. And then, as you got older you learned that they work because, well, you may not know exactly how they work, but you know it’s not magic. Just as an engine puts together fuel and air to make the boat go, so everything on the boat has a way that it works.

Glass helm

The Global Navigation Satellite System (GNSS) is the same way. It does its job and it can be explained without evoking witchcraft. And the more you understand how it works, the better off you’ll be when it works improperly, or stops working altogether.

First off, notice I didn’t say “GPS” for Global Positioning System. GPS is basically a brand (albeit a U.S. government-created, -funded, and -owned one) that once had the market cornered to the level of becoming a generic term, like Kleenex and Band-Aids. Now it’s one player in a market full of competitors around the globe. Except these competitors work simultaneously, covering the same market with many overlapping customers the world over, and they all put out the same product: a signal that creates positioning data.

“At this point the accuracy is very typical at 2 meters,” says Gil Passwaters, product manager for GPS for Furuno ( “In the maritime industry you can imagine the vessel’s moving up and down, so depending on wave action and so forth, that varies because it can move from one wave to the next.” Basically you can get a position if your electronic positioning unit can get a fix on three satellites—triangulation, get it? But because of that up-and-down wave action, it’s better to fix your position in three dimensions, and that’s where a fourth satellite is necessary. The more, the better is the rule for consistent positioning as you move around.

So you can see the challenges. “Initially GPS came out and we were pretty much running on the ground floor back in 1985 with that,” Passwaters says. “And now it’s progressed through the constellations where Russia has Glonass, the European Community will do Galileo very soon, and there are some other constellations trying to go up. Japan actually has a 2CSS system that’s strictly for use there. So accuracy-wise obviously more constellations mean more satellites, and the more triangulation we can use to generate more accuracy. Likewise the hardware is improved with more and more correlators and thus more virtual channels from those correlators to derive accuracy.”

What, you don’t know about correlators and virtual channels? Here’s how it really works: Basically the satellites, 32 for the GPS system (with a couple not operational right now), 28 for Glonass, 10 or 12 for Galileo (soon enough), and more for Japan, and don’t forget China’s Beidou system, which will eventually comprise 35 satellites but stands around 20 now—all these satellites are up in the sky, very far away. GPS satellites orbit at about 12,500 miles, and are solar-powered and send a radio signal. So your GNSS unit detects those signals and, based on the time stamp (kind of oversimplifying here) contained in the signal, your unit calculates how long it took the signal to reach it, and from that it determines how far it is from the satellite. At the same time it’s doing the same thing with another satellite. And another, and, for three-dimensional positioning, one more. Four satellites (and maybe more) all telling your GPS unit how large the spheres around them are (remember it’s happening in three dimensions, so it’s not a circle). Each satellite sits at the center of a sphere and you sit on the surface of a sphere, and (ultra-simply) where the surface of those four (or more) spheres meet, that’s where you are.

But here’s the funny thing. It’s all basically just one big set of sextant and chronometer problems with all the sights and math instantly taken care of for you. Instead of celestial bodies, it’s man-made satellites, but it’s still all about timing. Timing is everything, as was said once, perhaps by someone with bad timing.

Simrad NSS9 Evo2 Front Facing Flush Map with built in GPS

The Small Details of
Largemouth Bass Fishing

Bass fishermen on the tournament circuit can get very micro in their management of fishing data. That’s why they try to install a GPS antenna on their boat directly above anywhere there’s a transducer for a fishfinder. “A lot of fishermen place the transducer and the GPS basically in the same part of the boat,” says Lowrance’s Jeremiah Clark. “They know where they saw their target on the screen and that’s where they save their waypoint. We actually sell external antennas to consumers who will put them both on the front of the boat and on the back of the boat because they have sonars in both spots and transducers in both spots. Guys want that waypoint to be right on that structure. In our units we have the ability to choose our GPS source, so you can have several GPS sources on a boat to choose from. That’s how technical these guys have gotten.”

Why Is It So Reliable?

Over the years engineers have brought the power of technology to bear on the system, sped things up, and made it more reliable. “I think GPS is highly accurate,” says Scott Burgett, director of GNSS and software technology for Garmin ( “It’s really the gold standard amongst the satellite navigation providers that are available to us out there.Receiver design has improved over the years. Technology has advanced and the sensitivity has improved so receivers are able to operate on weak signals. At a high level I would say more sensitive electronics are able to receive weaker signals and do a good job of measuring the signals. Advancements in computing power have allowed GPS receivers to execute really sophisticated signal-processing algorithms. The GPS signal is very faint when it reaches the earth but that is by design. GPS receivers must be very sensitive to receive the signals because they’re so faint when they reach the earth.” (See the Garmin Quatix 3 smartwatch that manages both GPS and Glonass data here.)

The positioning unit on your boat has begun to incorporate the data from all the systems effectively. “It’s a computer,” Passwaters says. “That’s the way to put it simply, it’s a very powerful computer for what it’s doing—it’s a purpose-built computer.”

And it’s getting better all the time. “Since 2009, we’ve actually been through three generations of GPS receivers in that time, and each generation has become more and more sensitive and more and more accurate” says Jeremiah Clark, product manager at Lowrance ( “I think at some point you’re going to hit that PC processor thing when you start to max out on what benefits you get, but it has been a pretty steep curve on how fast GPS technology has advanced.”

As that powerful computer does a good job of incorporating all the data from the various systems, it also ferrets out the bad information and tosses it to keep the fix accurate. “If you run a measurement from a satellite that just doesn’t make sense when you compare it to other measurements available to you, you throw that measurement out,” Burgett says. “Many GPS receivers during the Glonass outage [in 2014] continued to work well because they threw out the Glonass satellite measurements and continued on the GPS measurements. Many people didn’t notice the Glonass outage because their GPS receiver had been designed in such a way that when the Glonass system started providing these erroneous measurements they were thrown out of the solution.” The outage lasted a reported 11 hours and was caused by bad data being uploaded to the Glonass satellites. The outage lasted as long as it did because new data had to be uploaded, and that had to wait until the satellites came back into view.

But the fact that we’re talking about an outage from two years ago is testament to the reliability of these systems. And you know what happens when something is reliable: People begin to rely on it. Another thing boaters want? Fast update rates. “When you’re using a trolling motor and you’re putting the boat in a cove and you maybe want to mark 50 waypoints in the cove, you want each waypoint to be right on the spot,” Clark says. “The big thing is update rate, measured in hertz. How many times per second the GPS information is updated. Our standalone, value products operate at 1 hertz. We get one position update per second, which sounds really good. Every second your GPS knows where you are and updates your position, updates your speed, updates the trail it draws behind your boat [on the plotter]. As you step into midrange or higher-range products we step up to 10 hertz or 10 times per second with the update. It may sound excessive but then you actually look at what you get out of it. Your speed over ground is very accurate so if you’re trolling and you wanted to make sure to stay right at 2 miles an hour, with a 10-hertz update any change in the vessel throttle is immediately seen onscreen. Those GPS trails that you leave behind the boat when you move with a 10-hertz signal are nice and smooth. With 1 hertz the curve would have sharp angles in it, and straight lines in the middle of turns.” Right now, it’s hard to say if the update rate will climb above 10 hertz. Think about it: There was probably a time when industry pundits said the standard wouldn’t go above 1 hertz.

Gamrin GPS-19x

What Could Go Wrong?

“One of the big sources of inaccuracy is something we call multipath,” says Burgett. “What that means is that the GPS receiver needs to receive in the direct line of sight the signals from the satellites in the sky. The GPS system works by measuring the distance from the GPS receiver to the satellites themselves. So if there is a big reflector nearby, like a metal structure, terrain, or various things like that, the GPS signals can bounce off the structures and then return to the receiver. That’s called a multipath bounce. Think about the distance between the receiver and the satellite. An inaccurate distance is going to be measured between the satellite receiver and the satellite because the signal bounced off a building that’s 100 yards away. So that causes an error in the range measurement—it’s actually too long. Sometimes the position solution that the GPS receiver presents to you can be inaccurate and multipath is one reason the boat mayshow up in the parking lot as opposed to in the slip.”

Many problems with GNSS on boats come from the confidence (you could say overconfidence) in the internal antennas of helm units. Most every MFD today comes with an internal GPS antenna, so why add an external one? Unbelievably, many boaters will leave off an external antenna for aesthetic reasons. “I guess because initially you used to have an extra antenna and then they started coming with patch antennas,” Clark says. “Then they went for internal. Now you see the products getting thinner and thinner and still working and more sensitive.” The point seems to be, that antennas these days work fine even under a fiberglass hardtop, or built into a helm console.

“We encourage people to install their GPS equipment in places where they have a clear view of the sky,” Burgett says. “You’re always well served if you can install the unit with a clear view of the sky without a lot of structure on top of it. Fiberglass doesn’t attenuate the signal too much. Metal certainly attenuates it completely. Honestly it’s a testament to the technological innovation and advancement in receiver design over the years that allows highly obstructed installations to work.” So the higher and less covered it can be placed, the better performance it will provide. And an external antenna, while an added complication, can ensure a good signal.

“To position an external antenna properly, keep it away from the radar,” Passwaters says. “If it’s in the path of a radar transmitter, that will really attenuate the signal. I like to use the tomato basket rule. Basically set that thing up and put it on an upturned tomato basket. If it works, great. But if it doesn’t work right, try somewhere else. But don’t just mount it and make it look really pretty … since it may not work in that spot. I made it a tomato basket to give the dealers something they could remember.”

Another good thing to remember: A good positioning signal is well worth the effort.

This article originally appeared in the March 2016 issue of Power & Motoryacht magazine.