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How do pilots determine their location when flying a plane? Do they use GPS?

They do use GPS and a whole lot more.

In a modern airliner like the 737 that I fly, there are a lot of inputs that are used by a computer that can give us a very close approximation of our position.

The first is GPS, which as everybody knows is a satellite system, it is supplemented by WAAS which stands for Wide Area Augmentation System. It’s basically a system of ground stations that collect the GPS data and correct it for various errors, one of them being atmospheric. That information is then sent to the aircraft which uses it to correct any GPS errors.

The second is IRS or Inertial Reference System. This one is intriguing to me. It’s a set of sensitive gyros and accelerometers that does not receive outside input after it is turned on. We tell it the longitude and latitude of the airplane and from it’s internal sensors, it can tell us with surprising accuracy where we are during the flight. It’s older technology but it’s still fascinating to me.

Third is called “Selective Tuning”. There navigation stations across the world called VORs. These can tell us our distance and azimuth from the station. In the old days, we used these almost exclusively and would travel from one station to the next during the flight. The airplane system will tune in a VOR, and can give us our position. It does this every few seconds with various VORs as we fly.

Fourth is our compass. We have two “flux gate” compasses on the wingtips to provide our magnetic heading. It is useful in the lower latitudes. We also have what we call the “Whiskey Compass” which is an old fashioned magnetic compass in the cockpit that is immersed in a liquid to stabilize it. It’s probably the lowest tech piece of equipment in a modern airliner. It has barely changed since the dawn of aviation.

Flux Gate Compass

Whiskey Compass circled in red

So all of this information can be used by us as a single source to navigate. However, in most cases, we use a computer that takes information from all of these sources, compares them and then gives us a ‘’best guess” of our location. It’s usually within a few feet of our actual position. I’ve checked it on the ground and it’s always right on. There is a page on the computer that shows a map and has several dots on it, each labeled with our position from each of the navigation methods. I’m not sure exactly how the software decides our position but I’m sure a lot of thought went into this computation.

Beyond these methods, we also have weather radar. It’s not designed for navigation but we can tilt it down to where it hits the ground. We can often pick out landmarks. It’s pretty good at finding shorelines and big bodies of water. A little less so with mountains but still useable.

Lastly, although not a computer system, there’s the old fashioned looking out the window. We can see big cities, mountain ranges, rivers, sometimes highways and we can tell if we’re going in the right direction from that information as well. It’s not particularly accurate but we can figure out where we’re going

EDIT 1: I forgot one more method, Air Traffic Control can also give us a fix on our location if we ask them. In some cases, such as loss of all of our other navigation information, they can give us “no gyro” vectors. In that case, they will tell us to “turn right” and then “stop turn”. They’ll use those kinds of instructions to help us find our way to the airport when all else fails. There is also a PAR or Precision Approach Radar, that ATC can use to fly an approach to a runway. They have a specially tuned radar that can give the controller both glideslope and precision azimuth information. The instructions are similar but also include descent information. They’ll say, slightly left of course, correct to the right and slightly above glideslope increase rate of descent. There are not a lot of PAR approach facilities out there and our airline doesn’t even train to use them any more. In the old days, they were trained but even then they were rare. I flew only one in my professional life to an airport in Japan.

Modern PAR display

Vintage display

Source: Quora
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