Flying before GPS

December 12, 2005 11:55 PM

How did airplane pilots figure out their speed in relation to the ground, when over water, before GPS?

Also, how did they compensate for the drift that wind, running perpendicular to their course, introduced into their flight path?

posted by 517 to science & nature (12 comments total)

Before LORAN as well.
posted by 517 at 12:01 AM on December 13, 2005

Take the difference of two sets of two-VOR position measurements, dividing by the time elapsed between measurements. This calculation equals velocity (speed). (Modern triangulation goes into the technical details.)

Once you have apparent velocity, you can compare this with the relative velocity as measured from the aircraft sensors. This will give you wind effect.
posted by Rothko at 12:12 AM on December 13, 2005

Doppler radar will give you your ground speed. As will an international guidance system.
posted by randomstriker at 12:24 AM on December 13, 2005

Disclaimer: I am not a pilot.

First, what Rothko said.

Also, there are DME receivers that give ground speed. Range is limited to line of sight (the link says ~60 miles from DME station.)

Further out at sea (and further back in history,) they could do what sailors used to do and employ celestial navigation -- many aircraft were equipped with sextants. Given two fixes they could apply Rothko's method for VOR, though probably with less accuracy.
posted by Opposite George at 12:38 AM on December 13, 2005

Oh yes, one more way: they had (still do) radio direction finders that could establish a bearing to beacons or broadcast stations. Get two fixes and then follow Rothko's two-VOR algorithm.
posted by Opposite George at 12:50 AM on December 13, 2005

Pitot tubes
posted by Pinback at 1:30 AM on December 13, 2005

(damn post button...)

Pitot tubes are/were the primary "aircraft sensors" mentioned in the second paragraph of Rothko's answer.
posted by Pinback at 1:34 AM on December 13, 2005

Even back before VOR's we used one of these (scroll down to the e6-b computer) - simply time yourself between two known points, and twirl the dial.
posted by pjern at 6:57 AM on December 13, 2005

On trans-oceanic flights, terrestrial radio navigation facilities aren't of any use. Instead, inertial navigation systems keep track of the plane's position by measuring gyroscopic precession.

Note that none of this technology mentioned above has been obsoleted by GPSes. GPS is still for convenience, and IIRC you still can't open an IFR (flying by instruments, not out the window) flight plan in which both the primary and alternate destinations are both GPS approaches, and while all IFR-equipped aircraft have VOR, ADF and ILS equipment not all have GPS. I'd bet the majority of aircraft used primarily for VFR (visual, checking your position against landmarks) flight still have no GPS. Airways tend to be between radio nav facilities (VOR-to-VOR, etc) anyhow, so there's a good chance that your next commercial flight is using "before GPS" navigation anyhow, or at least in combination with the GPS.

LORAN was never a major aviation navigation facility; being intended for marine use, LORAN stations cluster along the coasts. LORAN-C is only approved as a supplemental air navigation system; if you're flying on instruments there needs to be something other than LORAN guiding you.

(Of course, GPS won't be replacing ILS for instrument landings anytime soon either.)
posted by mendel at 9:26 AM on December 13, 2005

Dead reckoning was the method used before electronic aids. This depends on weather forecasts of winds aloft. You crank your heading, airspeed and estimated windspeed into the e6-b computer (an analog slide rule-like mechanical device) and out pops your true course. Forecasted winds aloft has a large error factor and the farther you travel, the greater the effect of the error, which could be hundreds of miles crossing an ocean. The idea is that you have a general idea of your course and make corrections when you can identify landmarks.

Charles Lindbergh used this technique on his flight across the Atlantic. He only had to end up somewhere in the British Isles, where he then was able to use maps to find his way to Paris. It turns out he was extremely lucky in that the weather conditions cancelled out the various wind drifts along his route so that the net effect was nearly zero.
posted by JackFlash at 9:33 AM on December 13, 2005

Others will know the physics and tools better than I, but I thought I'd add in that there's no "how did they BEFORE" about this question. Hobbyist pilots like my pops are still measuring the speed in their experimental (in the FAA definition, not dictionary) aircraft with instruments that are not driven by GPS and LORAN.

The majority are on Visual Flight Rules, not instruments, and often planes won't have GPS units that are sanctioned for instrument navigation at all. My father has an ipaq and garmin he mounts and uses as a supplement, but not a dash-mounted unit because the prices are a factor of ten higher if it's rated for in-flight navigational use.
posted by phearlez at 9:52 AM on December 13, 2005

I'm going to try to cram all of the stuff already said into one answer. I'm not positive of the timeline involved, especially since most of the advances in radio navigation were prompted by wars, not general aviation, and my range of experience is entirely in general aviation (student pilot in Canada, finished ground school and soloed but never finished up the license).

Before there were instruments at all, there was visual navigation: watching landmarks to figure out where you are according to a chart (map). Eventually instruments started to make this easier: if you knew your speed relative to the air and your compass bearing you had a much better time figuring out where you were on the chart. Today, flying like that is flying "VFR" or "under VFR"; VFR is Visual Flight Rules, and that's the sort of flying you do when all you have is your basic pilot's license. The E6B solopsist pointed out is a slide rule; pilots used it to quickly calculate the figures they needed to adjust their course based on where they were and where they expected to be.

Then came radios. The simplest sort of radio navigation is the ADF (automatic direction finder), which was basically a directional antenna; the dial on the aircraft's panel points to the transmitter relative to the airplane's nose. Pilots could plot courses that took them from transmitter (an NDB, non-directional beacon, or commercial AM radio stations) to transmitter, or could use landmarks and an ADF to triangulate their position on their chart.

The next level of radio navigation is the VOR (VHF Omni-Range). Unlike the ADF, every VOR station transmits on 360 radials, one per compass degree; where the ADF could only tune in the transmitter, a VOR receiver was tuned to a particular radial of a transmitter, so you knew your absolute bearing to the transmitter. Thus two VOR readings were sufficient to determine your exact position. VORs are still the primary mode of continental radio navigation. Modern VOR stations also tend to provide DME (Distance Measuring Equipment) as well, so a single VOR/DME receiver can tell you both which radial you're on and how far you are from the receiver -- that is, one radio can tell you your position.

With radio navigation came IFR ("Instrument Flight Rules") flying; it wasn't necessary to see the ground once you're up in the air anymore, which meant flying through and above cloud, which meant flights wouldn't have to wait for a sufficiently clear day to fly. But with IFR came a lot of regulation; IFR flights are managed by controllers throughout the flight (VFR aren't outside of control zones) and aircraft flying IFR have to meet redundancy requirements and so on.

It's all fine and dandy if you can fly somewhere without seeing the ground, but it doesn't do you much good if you still can't see the ground when you're getting ready to land. That problem was addressed by ILS (Instrument Landing System): basically a pair of VOR stations, one on its side, and an extra-sensitive receiver, which would show an aircraft's horizontal and vertical position relative to the normal path it should take to get to the runway. One more radio nav tool comes into play here, the marker beacon receiver, which simply beeps and flashes a light when the aircraft passes directly over a transmitter on the ground. Instrument approaches have three sets of beacons (outer, middle, and inner), which let the pilot know how far away from the runway he is.

The only problem with all of this is that sometimes you're not flying over land. That's where inertial navigation comes in per my previous comment.

So now we can get our position with a single radio with VOR/DME, or with two radios with VOR only; we know our altitude from the altimeter and our airspeed from the airspeed indicator and our heading from the directional gyro, which is essentially a gyroscopic "compass" that's readable through turns, and use a slide rule or an electronic calculator to pinpoint our position on a chart and figure out what corrections we need to make to get back where we expected to be, and what corrections to keep making after that to stay on course. When we're over the ocean, our nav equipment keeps track of our location on its own, and when we reach the other side we can double-check that with radio navigation over there. And when it's time to land, we can tell whether or not we're above, below, or to the right or left of the correct approach path, and we can tell how far away the runway is. Other than takeoff and the actual landing itself, we can fly the whole flight on radios alone.
And the same way we can do it, an autopilot can do it!

So what's GPS for, then? Convenience. For the military, GPS means radio navigation where there aren't any navaids on the ground to a level of accuracy that inertial navigation can't provide. For general aviation and airline pilots, GPS means moving maps and not having to deduce groundspeed and winds aloft to make course corrections. But GPS is too new; the FAA doesn't trust it yet, and you can't plan a flight that relies entirely on GPS. Every IFR flight plan needs to have two destinations -- the intended one and an alternate in case the conditions at the intended one are too severe to land even on instruments, and pilots can't plan flights where both destinations use GPS-based approaches. So even with GPS, IFR flights have to have approved VOR receivers.

The history of pre-GPS navigation will be around a long time, though; airline flights tend to stick to airways, and since airways were planned when VOR was the only way to get around, they tend to go between VOR stations. A pilot can always file "direct", but he needs to be ready for a controller to tell him to fly to such-and-such VOR anyhow, and so you end up with aviation GPSes that have the locations of VORs programmed in!

I've just done this up quickly; apologies that it's so long, but it's more complicated than it seems (especially since "before GPS" isn't really much different than "with GPS"). I'm sure I've made a bunch of little errors and lots of gross oversimplifications, so corrections and clarifications are welcome.
posted by mendel at 9:53 AM on December 13, 2005