Time and Place and Toma­toes

WHEN GPS GIVES YOU A BAD PO­SI­TION, YOU MAY BE SUR­PRISED WHO’S TO BLAME.

Power & Motor Yacht - - ELECTRONICS - Seem­ingly ac­cu­rate all the time, satel­lite nav­i­ga­tion may be spoil­ing boaters one in­for­ma­tion-rich screen at a time.

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

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