Plane & Pilot
Tuesday, July 26, 2011

Should We Rely On GPS?


Is GPS too good to be true?


Lesson learned—or maybe not. I've been one of the strongest boosters of GPS since its introduction in the early '90s. I jumped on the GPS band wagon in 1991, employing one of Garmin's first experimental battery-powered portable systems, a GPS 100AVD, to ferry the first prototype Swearingen SJ-30 biz jet back from the Paris Air Show to the States. (Both of the airplane's VLF/Omegas had burned up on the outbound trip to France.)

Since then, I've used GPS all over the world, from the Middle East and Africa to Australia and Japan, South America and Alaska to Europe and Scandinavia. I learned to fly back in the point-and-shoot days, so I remember how that works, but GPS has become my bible.

Indeed, the system has been almost ridiculously accurate, and even more so with the introduction of differential and WAAS upgrades. It's also been fairly consistent and seemingly unaffected by atmospheric conditions in the 20 years it has been up and running.

Yes, I've witnessed "blinks," total dropouts of GPS information, but those have usually been deliberate, initiated by the USAF for engineering test purposes.

In addition to its obvious talent for pinpointing position to as little as one centimeter (in surveying applications), the atomic clocks in GPS serve as a source of accurate time for government and industry. The accuracy of those atomic clocks is phenomenal, and it has to be. Time signals must be very precise indeed when the 28 Navstar satellites are orbiting the earth 10,800 miles out at 8,000 knots. Specifically, the signal manifests accuracy on the order of 1x10-E14, roughly one to 100 trillion. (To put that number in perspective, GPS atomic clocks may be expected to gain or lose one second roughly every 3.1 million years.)

GPS signals are utilized by the vast majority of communications companies for synchronizing telephone lines and cellular phones, securing ATM transactions and helping to track a variety of overnight letters and packages. Banks and other time-sensitive industries around the world also rely on GPS for accurate time and position.

Despite all the above (or perhaps, perversely, because of it), GPS isn't invincible. In fact, it can be derailed, as recent tests of the proposed LightSquared communication system in Nevada proved. Other instances of interference have suggested it's possible to confuse the signal in a local area for hours or even days without much effort. One 2007 incident in San Diego caused inaccuracies for two weeks before the source was located.

For that very reason, I can't help but wondering why we shut down our only backup system two years ago.

Until early 2009, America had a viable backup in LORAN C (LOng RAnge Navigation), a ground-based system that had been around in various forms for 70 years. Though originally confined to marine use, later versions were adapted to help aircraft navigate over both oceans and land. LORAN C employed a number of four- to five-station chains, and like GPS, established position using a simple system of triangulation.



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