What Could Go Wrong?
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. “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.