Blog Editor’s Note: One of the first questions skeptics ask is “When GPS isn’t available, why don’t aircraft just use the backup network of terrestrial aviation nav beacons?” – These beacons are called “VORs” and “DMEs,” by the way.
Well, some smaller aircraft don’t have anything but GPS to navigate with, other than the pilot using a map and looking outside. And finding your way with a map and looking out the window isn’t as easy as it seems. Especially if you have been relying on GPS to help you for 20 years or more. This is why, for example, light aircraft regularly accidentally violate restricted airspace around places like the Capitol and White House, and have to be escorted out before they get too close.
The FAA even allows some aircraft to fly in “instrument” conditions (restricted visibility due to clouds, fog, etc.) with just GPS for navigation. When GPS goes out and these folks are in the clouds, they need air traffic controllers to tell them which way to head to get into clear air. That increases workloads for everyone, controllers and pilots alike, and makes the system more dangerous.
Many larger aircraft can use VOR and DME information to navigate. BUT, their flight management systems aren’t able to convert that information to allow point to point routes. Without GPS they often have to go from one terrestrial beacon to another. So flights are longer, burn more fuel, and are overall more expensive.
Even for the largest and most sophisticated aircraft, GPS is important. In addition to being the primary nav sensor, it enables a wide variety of non-navigation cockpit services. Things like finding the closest landing field in an emergency, choosing the right frequencies to talk to air traffic control for the area the plane is in, coordinating arrival services with the company’s personnel and contractors at destinations, etc.
So, for the reasons above and many others, many companies and pilots choose not to fly when GPS equipment is not working properly rather than rely on backup systems and procedures.