Time and Place and Tomatoes
WHEN GPS GIVES YOU A BAD POSITION, YOU MAY BE SURPRISED WHO’S TO BLAME.
o 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.
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 ( www.furunousa. com). “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
Ddimensions, 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
TAKEN FOR GRANTED