How GPS works— or doesn’t

Apr 27, 2017 - 2:45 PM

The last thing you want your pilots to hear from a business jet’s GPS navigator is “recalculating”—especially on a dark and stormy night. How is it that we trust our lives in the air to the same navigation system that can be so schizophrenic on the ground?

If you drive a car, you probably know the frustration of hearing that calm, pleasant voice telling you to “turn left” on a divided highway or to “make a U-turn, when possible” just after you’ve passed a sign that says “Next Exit, 34 Miles.” But in its airborne role, GPS makes flying more reliable, efficient, and safe. And it forms the foundation of the NextGen platform that promises to transform the national airspace system. With a little understanding, the difference becomes clear.

The Global Positioning System used by both cars and airplanes has three elements: a receiver that employs satellite signals to locate where you are from moment to moment; a computer processor that interprets that information to determine direction and speed; and a database that tells you how to get from where you are to where you’re going.

The receiver picks up transmissions from 24 satellites, each in its own orbit above the Earth. Those signals tell the receiver exactly what angle it is from a satellite. Picture a thread running from the satellite to the receiver. That’s called a “line of position” and the concept dates back to when ancient navigators determined where they were by observing the angle of the sun and stars above the horizon—the original GPS “satellites.”

Knowing where you are in relation to just one GPS satellite might not help much. But if you stretch out the equivalent of those electronic threads to several of the 24 satellites, you can zero in on where they all converge. That’s your position in space (including your altitude), and GPS can nail it to within the proverbial gnat’s backside.

The processor then takes over. As you speed along, it measures where you are now…and now…and now, recording your exact location several times per second. From that data, it can calculate what compass direction you’re heading. And by comparing distance traveled with the time elapsed, it knows how fast you’re moving. Great stuff.

Your car’s GPS uses the same satellites that aircraft employ, and its receiver reads the signals just fine. Even the computer processor in an inexpensive GPS is comparable to an aviation-grade system. So why does it sometimes have such trouble getting you to that new dry cleaner?

It’s the database in the automotive GPS that falls short. And it has nothing to do with the quality of the hardware. The problem is that a car requires almost infinitely more complex data than an airplane.

Aircraft have to fly a carefully choreographed route only right after takeoff and just before landing. That’s when they have to avoid hills, cell towers, and skyscrapers. And since airports are to airplanes what switching yards are to trains, traffic control is critical, too. Especially in low visibility, every aircraft has to stay on the right track, and there are lots of them converging.

But taking off and landing usually represent just a few minutes out of a flight. The rest of the time, direct “airways” follow a series of straight lines between electronic waypoints that could be hundreds of miles apart. According to the FAA, our national airspace system includes 62,310 GPS waypoints. A waypoint can represent an airport, a navigation beacon, an intersection between two airways, or a strategic fix that’s part of a safe instrument approach to an airport.

While 62,310 might sound like a lot, it’s computer child’s play compared with the almost infinite number of pinpoints needed to map out the U.S. roadway system. Think of every traffic light, one-way alley, and exact street address. And remember, the airplane needs only to fly from waypoint to waypoint in a straight line. The roadway database has to track every twist and turn in every highway and byway. No wonder your car’s GPS sometimes gets “bewildered.”

So, at least when it comes to GPS navigators, flying halfway across the continent can be a lot simpler than driving to Grandma’s house once you land—especially if she lives on a blind cul de sac.