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Celebrate (navigation and timing) diversity!

April 11, 2010

Most of us probably wouldn’t notice if all of a sudden the GPS in our car didn’t work – what with texting and changing songs on our iPods and eating breakfast, there’s barely time to look out the window anymore. “Besides,” you say, “I’m pretty good with a map and compass from my days in the Boy/Girl Scouts. And if that fails, I’ve got an old sextant my Grandpa had in the Navy…” All true. But GPS isn’t just for finding your way to the Kwiky Mart, anymore. It’s used extensively for time and timing applications.

“But I don’t look at my GPS for the time; I look at my cell phone.” Exactly: how does your cell phone know what time it is? “There’s a little clock inside, like everything else.” Well, almost: have you ever noticed you don’t have to ever set the time on your cell phone? Cell phones get the time from cell phone towers, which also happen to handle all of the data that flies back and forth between you and your spouse when you’re trying to find out which kind of toothpaste you’re supposed to buy: the cool mint, tartar control gel or the extra whitening, vitamin-D enriched gel with flavor crystals. The digital packets carrying your highly-important oral dentifrice data must be carefully timed, lest your voice be garbled or your call be dropped. You musn’t return with the wrong toothpaste.

The reason we get such spot-on navigation from GPS, enabling us to get within several feet of our favorite accident sites, is because it measures the difference in time it takes for the highly-ordered signals from each of the GPS satellites to reach us at our current location. Each GPS satellite has a cesium clock, which is even more accurate and expensive than that nice Swatch you got last Christmas. In fact, it’s so expensive that the fine folks who build cell phone towers and networks noticed that it would be much, much, much cheaper to use a GPS receiver and the totally free GPS signals than to install a cesium clock into each cell tower to ensure that your voice sounds like what you say. (You get the time displayed on your cell phone ‘cause it’s a nice feature and it’s basically free. And because the cell phone manufacturers are out to steal the personal timepiece market from Swatch.)

Cell phones aren't the only things that depend on GPS for timing information: financial institutions, power utilities and other communication networks do, too.

“So...?” So cell phones are so completely dependent on GPS signals that they don’t work within a few minutes if GPS isn’t available. “Well, geez, nobody’s going to destroy all the GPS satellites. That would require a highly sophisticated, coordinated attack using advanced rocket technology and…” No, probably not. But consider this: the GPS satellites are 12,000 miles up and they’re not very powerful. In fact, the signal that reaches the Earth’s surface is incredibly weak and delicate, like one of those prize-winning, elaborate sugar centerpiece decorations you see on the Food Network. If it weren’t for some clever slight-of-hand that most GPS receivers perform, you’d notice that if you stand under a decent-sized tree with a lot of leaves on it you don’t get a GPS signal there. That’s how weak it is. It’s not the satellites themselves, it’s the signal. GPS signals are ridiculously easy to jam with a nine volt battery and off-the-shelf electronics; occasionally it even happens accidentally. What if someone wanted to really disrupt, say, the entire city of Chicago? Or Los Angeles? Or New York City? Unthinkable? Think again.

Cell phones are not the only things that depend so heavily on GPS for timing information: financial institutions, power utilities, and other communication networks do, too. In fact, it’s hard to list all of the things that rely on GPS, because they’re everywhere: it’s so simple and cheap to integrate GPS into other products and instruments that we don’t really even think about it anymore.

Which is why I was bummed to hear that the U.S. government shut down the only viable backup to GPS for both navigation and timing in February this year. It was called Loran-C (or just “loran” for short), and it worked quite well. It provided signals that were good enough to be a backup for pilots to get to the runway and all but the most precise timing applications. The signals were also high-powered and very difficult to jam. Loran suffered from a problem, though, because it wasn’t GPS: it wasn’t quite as accurate, and it didn’t have worldwide coverage. The government threatened to shut down loran for years under the pretense of saving money. As a result, there were no sexy, little, modern loran receivers, either. (Go figure: which company is going to design a receiver the size of an MP3 player for a signal that the government might shut down at any moment?) So loran was under the sword of Damocles for a long, long time. Until 2008, when the government committed to keeping the loran system operational for another 20 years. Until recently when the government decided: “Well, not so much”.

So, when we consider the actual cost savings (loran can be operated and maintained for a year at a fraction of the cost to simply launch a single GPS satellite), we have an interesting situation: GPS is very accurate, global and very delicate; loran is less accurate, localized and very robust. Loran could be used as a backup to navigation and timing applications in the entire continental United States in the event of intentional jamming. The two technologies complement and enhance each other very well even under normal conditions because their signals are diverse. What’s that worth?