# How did planes measure their speed relative to the ground before GPS?

How did planes measure their ground speed before GPS?

If they used some sort of anemometer to measure speed, that would only measure the speed of the plane relative to the air through which it is passing.

Therefore, I'm wondering how speed relative to the ground was measured before GPS.

I'd be very grateful if someone could explain this to me!

• Your "plane staying over the same point in the earth" happens all the time, except "plane" is a helicopter. I've even flown a 172 with a zero ground speed (hovering), which was possible with a strong headwind. Yes it will show zero because GPS measures ground speed, not air speed. – Ron Beyer Jun 13 '16 at 1:40
• GPS satellites are on 12-hour orbits, much lower than geostationary. They always have very substantial speed with respect to the normal user. This is what makes initial fix difficult: the receiver needs to account for the Doppler shift in the frequency of each satellite, and if the constellation at this moment is not known exactly, the receiver needs to sweep through a range of frequencies rather than just tune to a 'station'. But for measurement it doesn't matter much: the system just applies some math to compensate for all these effects. It would have to anyway. – Zeus Jun 13 '16 at 2:04
• Major edit here: The question appeared more like a ramble, so I carefully picked through and clarified what is actually being asked here. Many answers are responding to the asker's general confusion due to unfamiliarity with the subject matter rather than the specific question being asked. The second question being asked: How does GPS measure a plane's ground speed? – Ryan Mortensen Jun 13 '16 at 6:01
• 100 feet between every telephone pole – Tyler Durden Jun 13 '16 at 17:29
• See en.wikipedia.org/wiki/… for an unfortunate example of what could happen if you got your ground speed estimation wrong due to wind. – Zarkonnen Jun 14 '16 at 13:17

The other answers seem to make a simple thing way too complicated. Yes there are DMEs or TACANs, that are sometimes used to get a ground speed, but it's only accurate if you're tracking exactly to or away from it. And it certainly wasn't used to calculate speed enroute unless one of your navaids was a VOR/DME or equivalent. Rather than using DME they'd be using INS, which delivers data more similar to GPS.

Calculating ground speed in the past (before INS, or in airplanes without one) was done using the simple method of having checkpoints along your route. These checkpoints where navaids or intersections determined by multiple navaids. Either way their location was known, divide the distance by the time spent travelling between them and you got your ground speed.

• "it's only accurate if you're tracking exactly to or away from it" is not true, trigonometry is possible, albeit annoying. – Sam Jun 13 '16 at 14:44
• @Sam if you want to split hairs, it's only accurate if you know your exact track and 1/cos(α) is defined where α is the angle between track and bearing. Similarly, for the checkpoint/time method, it calculates average ground speed, if you fly s-curves, it will also not be accurate. – falstro Jun 13 '16 at 14:48
• I'm an engineer, of course I want to split hairs :P I write ADCS software for spacecraft, you don't need to explain the math. – Sam Jun 13 '16 at 14:50
• @Sam so am I, I'm just returning the favor :) – falstro Jun 13 '16 at 14:50
• They still teach how to calculate ground speed and wind relative to the ground by timing flight between known references in private pilot ground school (at least the one I took recently based on the Jeppesen books). – Cody P Jun 13 '16 at 20:41

There's one method that has been successfully used since long-distance flights became available - you took out your map, and tried to match it to features under your plane. This allowed you to correct your course and have an average ground speed information. Obviously, this doesn't work over the ocean - it's yet another reason why ocean flights have always been tricky, to say the least.

Don't get me wrong - it isn't very reliable, it only works if you can actually see the ground and you must keep your navigation information accurate (if your navigator dozes off for an hour, it might be tricky finding your current location again!), but for a long time in aviation, it was the only thing you had. WWII strategic bombers still used this method when over enemy land (where they didn't have the assistance of a friendly ground radar).

As radar and radio communication became commonplace, it became quite trivial to track airplane positions and speeds - it's all in the "pong" you get back from the radio signal. Again, in WWII, this was used to great effect during the Battle of Britain and even later - but it only works if you have radar coverage, which usually means only over your own land and the border areas.

GPS only became available for commercial use in 1983, after a Korean Boeing strayed off course and was shot down over the USSR - GPS makes such a mistake much less likely to happen, especially over the ocean (again, oceans are tricky - usually no radar and/or features to track on your map; by the time the Korean flight could have corrected their course, they were already shot down).

According to a story I read about the SR71, planes used to be able to ask ATC for a "ground speed check", i.e. ask "What's my speed on the ground?" ATC would measure it using their ground-based radar and relay it to the pilot by voice/radio.

If you can provide some more material for us to read it would be great.

Here's a video of the pilot telling the story: Brian Shul, one of the few SR71 pilots, telling the LA Center speed story

Here's a written version (a transcript from a book): The Ultimate Ground Speed Check - Tales from the Blackbird

We listened as the shaky voice of a lone Cessna pilot asked Center for a readout of his ground speed. Center replied: "November Charlie 175, I'm showing you at ninety knots on the ground."

[...]

Just moments after the Cessna's inquiry, a Twin Beech piped up on frequency, in a rather superior tone, asking for his ground speed. "I have you at one hundred and twenty-five knots of ground speed."

. ...etc...

• Wow this is good. If you can provide some more material for us to read it would be great. Not LQP. – vasin1987 Jun 13 '16 at 12:16
• You can always ask ATC for a ground speed readout if you're in radar coverage (it's right there in their datablock), you don'y have to be in an SR71 to do so. – falstro Jun 13 '16 at 12:45
• Seems to be a story of who has the biggest one. – mins Jun 13 '16 at 14:04
• Or the fastest one, anyway. The point is though, I think this was an answer to the question. – ChrisW Jun 13 '16 at 14:24

How did planes measure their speed relative to the ground before GPS?

Assuming you ask for speed determination by on-board instruments.

The ground speed is best evaluated using an inertial navigation system. The INS knows where the aircraft is currently located (latitude, longitude, altitude). By associating two measures it's easy to compute the velocity of the aircraft. IRS (inertial reference system) has replaced INS. While INS relied on mechanical gyroscopes, IRS relies on laser gyros.

The problem with the inertial units is they drift with time, due to gyroscope imperfections, Schuler loop and Coriolis acceleration. This led to accidents in the past, before they could be realigned by GPS position determination.

Note the TK/GS position of the rotating switch, to display track and ground speed instead of latitude and longitude (source)

IRS are now included in larger units that process other sensors inputs, like an ADIRU.

ADIRU (and INS/IRS) are able to compute the wind direction and intensity by comparing the aircraft heading and the actual aircraft displacement. This drift information is usually displayed to the pilot.

Ground speed and aircraft drift in the left upper corner (source)

Note that determining the ground speed is not a critical need for the crew, the aircraft must be flown taking into account the relative airspeed and the air properties (temperature, pressure, density). However ground speed is useful to predict the time of arrival at the next reporting point, or destination.

Other means:

• Loran-C was a system used to compute a position and a ground speed over oceans where no DME or other VHF/UHF aids are available.

• The crew relied on a sextant and celestial navigation in "ancient" times, like ships did.

Aircraft use two fundamentally different speeds: airspeed and ground speed. (Each of them has its own sub-species, but it doesn't matter for now). There is virtually nothing common between them, and depending on wind, you actually can have zero and even negative ground speed, or can fly 'supersonic' on a regular airliner, or climb while trying to descent. The devices to measure these speeds are also fundamentally different.

For airspeed, the aircraft use, basically, a barometer, which chamber is attached to a pipe pointed forward (Pitot tube).

And for some time, that was it. The airspeed measured this way is all you need to fly an airplane. After all, the airplane flies in the air and doesn't care about the ground (until it hits it :) The pilot would then navigate visually, by matching the map with the landscape.

On aircraft without GPS and navigational equipment (say, 50 years ago), you could get an idea of the ground speed by

• Knowing the weather forecast and thus wind at every point of your intended route;
• Calculating airspeed at the ground from the indicated measurements. For that, you need to take into account altitude and/or temperature. Pilots have special tools and instruments for that.

Yes, this is not an easy walk in the park, but that's part of the pilot's trade. Early airliners often included a dedicated navigator position to do such calculations along the route. And generally, it is still considered a useful skill and you need to do similar calculations when planning the route.

The next step is to use ground navigational aids and/or an onboard system that measures ground speed from the Doppler shift of the radio signal reflected from the ground. On some aircraft, the radar can do this job.

And finally, we have GPS that... no, it doesn't really measure ground speed. But it calculates it from precisely known speeds and position (with respect to grouond) of multiple satellites.

• "The airspeed measured this way is all you need to fly an airplane." True, but knowledge of your groundspeed is useful if you want to do things like figure out whether you have enough fuel to reach your destination or conform to a time-based traffic separation scheme outside of a radar area. – Zach Lipton Jun 13 '16 at 6:43
• @ZachLipton or to appease all those pesky passengers who keep asking "Are we there yet?" ;) – falstro Jun 13 '16 at 6:47

Although it's fairly unusual, some aircraft (e.g., the KC-135) are equipped to do celestial navigation. This supports computation of ground speed without external support (e.g., LORAN/VOR/GPS).

On the down side, it is somewhat time consuming, and requires both equipment absent on most aircraft, and skills absent from most pilots' repertoire.

The Drift Meter has been used for a long time to measure ground speed.

https://en.wikipedia.org/wiki/Drift_meter

The usual method is to compute your True Airspeed (TAS) and then do the wind computation to calculate Ground Speed (GS). Computing TAS starts with pitot/static air pressure measurements, and takes altitude and air temperature into account. You have to get the Winds Aloft from someone on the ground, by radio.

There was at least one Ground Speed measurement device out there, that I THINK used low-lower Doppler radar, when I worked on the Tracor 7800 Omega/VLF radionavigation system, back in the late 1970s.

In the sense of "how fast are we going over the ground, right now?", its a measuring challenge. Some means of measuring successive distances to a point on the ground, or some means of measuring progress between points on the ground, is required. Before GPS, Doppler radar could read speed relative to the ground directly. DME, Distance Measuring Equipment, could give distance relative to a fixed point. Successive locations allow computing ground speed relative to the fixed point. LORAN long-range radio navigation equipment allowed location above the earth to be determined without star or sun sightings. In clouds, for example.

Before electronic tools, Celestial navigation could supply location above the earth, successive locations and times, giving ground speed. Sighting objects on the ground and measuring angles to them, or fixing the angle and timing progress between successive objects, was possible, if not very attractive.

All those give good to fair answers to "how fast are we going over the ground?" "

Just to add that there were (area) radionavigation aids before GPS and these could be used to determine velocity over ground (either manually or with a computer).

The hyperbolic navigation system Gee was used in WW2 and after by the RAF. The Decca Navigator was used commercially (primarily for ships but also for aircraft, vehicles and for one job I worked on, runners).

These systems operate in a similar fashion to GPS by measuring the difference in propagation time between signals from two stations - with terrestrial hyperbolic nav, the stations are fixed and the locus of the located vehicle is an ellipse. GPS follows the same principle, except the stations are moving and at a higher altitude.

The concept is very simple. We have an analog airspeed meter in the aircraft. Airspeed +/- wind is ground speed + if tailwind, - if headwind

Fly any radial towards a VOR. Turn left 90 degrees until you have intercepted another radial +5 degrees, and time the cross leg. Turn inbound again. Calculate distance from VOR. Fly 6 minutes. Repeat above procedure, except turn right back to original radial. Calculate new distance from VOR. Using 2 distances and 6 minutes inbound leg, calculate ground speed.

Several ways, though none quite as accurate as a GNSS ground speed calculation

TRUE AIR SPEED, WINDS ALOFT AND TRIGINOMETRY. Using the old, reliable E6B flight computers or by hand calcs, you can determine your true airspeed and, using the headwind or tailwind components of the winds aloft you can compute groundspeed from that.

RADAR TRACKING - ARTCC can track you aircraft's travel using a Doppler radar system. This along with the azimuth of the radar antenna can be used to compute a groundspeed for your aircraft.

INS - Inertial Navigation Systems ar used widely in civil and military aeronautical applications where terrestrial navaids are not available. These often use a periodic GPS update to refresh errors which slowly creep into their operation although the ring laser gyros are quite accurate and reliable.

DME - the use of Distance Measuring Equipment onboard could compute your distance to a VORTAC or VOR/DME station as well as your ground speed. As a DME computes a slant distance, this is only accurate at times when the distance from A/c to ground station was much higher than the a/c's height above the station. Groundspeed ccuracy was also affected if the aircraft was not flying directly to or directly from the station.

LORANS - Ah the late seventies and early eighties, when GA was in its prime and men were men - and everyone though that LORAN was the wave of the future. Well, it wasn't; but LORAN still remains popular for maritime navigation and is still very accurate, though it is slowly being phased out. You occasionally find older GA airplane's with a LORAN receiver in them, usually with an "INOP" tag on it.