Should We Rely On GPS?

The chief mechanic and I lifted off into marginal yuck, and headed out toward Catalina Island, 26 miles across the sea. A pressurization check is standard procedure following an annual inspection, and accordingly, I dialed the pressurization system down to -2,000 feet, and we climbed up to 10,500 feet to check pressurization to max differential.

The coast quickly disappeared in the mist as we headed for the island. That was about the time I noticed the Garmin 530 had apparently gone inop sometime after takeoff. I tried a cold start, and there was still no joy.

Not a problem, I thought. I'll just whip out my trusty backup Garmin 496, position it on top of the panel and proceed.

That plan probably would have worked fine if I hadn't left my 496 in the backseat of my car. Okay, no problem, I knew I had a very tired but perfectly adequate Garmin 196 in my headset bag.

Of course, I did, and its power supply of AA batteries was almost totally dead with no backups at the bottom of the bag. Hmmm. I tried the standard trick of rubbing the AAs' positive terminals vigorously against my jeans to excite some static charge, but it was no joy.

Oh well, have to go back to VHF. I tuned the number-two box to 115.7 mHz for Seal Beach VOR, despite a vague memory of someone on the Long Beach ATIS saying it was out of service. There was no ADF in the airplane, and the ATIS was still advertising four miles. In addition to that, the CDI on number-two VHF box was doing weird things.

What a great preflight I'd done. Well, perhaps I could be forgiven, at least a little, as the big Cessna was destined for the avionics shop as soon as the annual was signed off.

Fortunately, I could see the mountains northeast of Los Angeles from 10,500 feet, and the weather began to lift as I approached the coast. I managed to sneak back into Long Beach, legally but somewhat chagrined.

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.

I'm one of those weird folks who bought a semi-sophisticated LORAN C receiver in the 1980s, a multichain ARNAV FMS5000, and it's still mounted on the panel of my Mooney. Fortunately, the FMS5000 also picks up GPS signals, so it serves as a backup to my Garmin 430 and 696. (When LORAN was up and running, the discriminator circuitry in the ARNAV unit evaluated the integrity of the GPS and LORAN signals and annunciated the system in use with a small "G" or a small "L." Contrary to what you might imagine, there were instances when the "L" was displayed for long periods of time.)

In some respects, comparing the old LORAN C to GPS is a little like contrasting a VW Beetle to a Porsche Panamera. Despite its antediluvian roots, however, LORAN C delivered nearly the same accuracy and was perhaps less susceptible to interference.

As many readers may remember, a decision was made that LORAN C be shut down in early 2009, suggesting that GPS had relegated LORAN to obsolescence. This allowed the U.S. Coast Guard to spend LORAN's annual $36 million on something else. It also effectively killed LORAN and destroyed any possibility of using it as a backup.

What no one mentioned was that we had already spent $160 million to upgrade the old LORAN C system to more modern eLORAN, an even more accurate and reliable navigation aid, and that project was about half complete. That $160 million was a total write-off.

And, oh yeah, the very act of decommissioning LORAN was scheduled to cost probably another $200-$250 million. At $36 million/year, we could have had another six years of LORAN C backup to GPS for the same price. As my ARNAV unit has so capably demonstrated for 18 years, it's no major engineering feat to incorporate both technologies in the same panel-mounted receiver.

That's all the more disturbing, since LORAN was equal to GPS in some respects, and we now have no precision navigation system to back up GPS in the event of a terrorist attack. The LORAN signal was tough to attenuate. It penetrated most buildings, double-canopy rain forests and urban and natural canyons, and the timing signal was just as accurate as GPS. LORAN used terrestrial rather than space-borne atomic clocks that actually tended to be slightly more accurate (one second every 6 million years?).

But of course, that's all academic now, as the promise of eLORAN is gone for good. For whatever it's worth, I'm still one of the world's strongest supporters of GPS. My Garmin 696 is a true wonder box with more capability than I'll probably ever be able to master. I recently flew Garmin's new 650/750 touch-screen systems in Florida, and they were nothing short of amazing.

Still, I can't help wondering what if!?