GPS Is Fabulous, but I’m Hedging My Bets

GPS is the most remarkable navigation system that aviation has ever had, and it's going to get even better with the decomissioning of Selective Availability and the launch of a new constellation of L5-band satellites. But don't yank the VORs and DME out of your panel until you've read the cautions by AVweb's avionics guru about some of the limitations and problems associated with relying on GPS as your sole means of navigation.


GPS triangulationAvionicsWithout doubt, GPS has been the greatest advance in aviation navigation since the invention of railroad tracks. It operates in just about any weather, uses a small antenna, and doesn’t necessarily have to be panel-mounted. In fact, some of the portable GPSs have more features than the panel mounts (because they aren’t hamstrung by the need to comply with complex FAA TSO requirements). The new GPS receivers are astonishingly small and low in power consumption. And the best ones are easy to operate because the avionics manufacturers have learned how to design user-friendly software. (Those of you that struggled with first-generation loran receivers know what I mean!)

An incredibly powerful handheld GPS costs between $600 and $1,200 including a moving map display, while panel mounted boxes run from $1,900 to $3,500. (In contrast, the first general aviation Loran C receiver I installed cost over $8,000…in 1982 dollars!)

More Satellites, Less Errors

GPS SVs in orbitIt appears that the next generation of GPS satellites (or “SVs” if you want to impress your friends) will operate on the “L5 band” which is a different frequency than the “L1 band” that current GPS satellites use. This change should help improve navigation accuracy by reducing errors when the signal passes through the ionosphere. This new “Block IIF” constellation is supposed to consist of 33 satellites, and “should” be launched by the first decade of the 21st century…but you know how that goes. The new satellites will work in conjunction with the old ones, although many of the satellites in the present constellation are getting long in the tooth.

The other good news is that the Defense Department has agreed to decommission Selective Availability (SA), the “wobulation” they intentionally designed into the signals that civilians use to mess up navigation accuracy. We’ve know for years that this was a dumb idea, and finally the military has agreed and pledged to do away with SA sometime in the next ten years.

GPS just gets better and better, and it’s revolutionizing navigation not just for aircraft, but for boats, cars, hiking, surveying, and all sorts of other activities as well. The potential uses of GPS seem almost endless.

But Is GPS Bulletproof?

With all this great news, you’re probably ready to yank out all of your panel-mounted nav radios, velcro a handheld GPS to your control yoke, and fly away into the sunset. Well, don’t be so quick about writing off those VORs and DMEs! You might be able to get away with GPS as your sole means of navigation if you fly strictly VFR. But according to my FAA inspector friend, you must have another means of navigation if you fly IFR and file an alternate on your flight plan. I’m not sure all FAA inspectors are preaching out of the same hymnal on this point, but I’m inclined to agree with the thought behind it.

Unfortunately, GPS is not bulletproof. Any receiver that depends on signals at -160 db (i.e., incredibly weak) will have problems from time to time. GPSs are susceptible to ionospheric distortion, especially around sunset and sunrise. Solar flare activity is on the increase again, and I predict that GPS reliability will decrease as solar activity increases. GPS is also very susceptible to radio frequency interferance (RFI) from other pieces of navigation equipment. Some DME frequencies can interfere with GPS receivers. VHF comm transmitters can shut down the GPS when you transmit on certain frequencies.

Another big problem with GPS is the satellites. When one goes on the fritz, it’s not easy to get it fixed. You don’t just call the Maytag repairman! Rumor has it that if a satellite goes down, the cost to build a replacement and launch it into orbit is greater than five years worth of maintenance on all the VOR and ILS stations in the United States. And get this: the early block of GPS satellites had a lifespan of only 7.5 years…most have been up for over five years now, and some have already failed. Let’s see a show of hands from those interested in going up one weekend to do some orbital repair work.

Redundancy Is Essential

Throughout our aviation training, we are taught to have backups for critical systems. Our engines have two magnetos and two spark plugs in each cylinder. Many of our aircraft have dual vacuum pumps and dual alternators…and some have dual engines. Most have an emergency exit door (or window) in case the main door jams. Most serious pilots have dual VOR/ILS receivers, plus DME and other navigation equipment. The basic idea is to make sure we can still fly safely, even in the soup, if any one critical device or subsystem fails.

I think the same philosophy has to be used with GPS. I’m happy to use GPS as my primary means of navigation, but never as my sole means. I’ll always back it up with loran and/or VOR/DME, preferably both. I’d suggest you do the same.