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Around seven years ago, I did airplane speed measurement. I activated the GPS on my tablet and I monitored the speed, especially when the airplane touched the ground (runway). I found that the airplane touched the ground with speed 420kph, which it was fast enough. The measurement was probably not 100% accurate, but I could feel that the airplane was landing and touched the ground quite fast.

My question, how accurate is measurement with GPS, especially using GPS installed on smart phone? If I need to calibrate, how to calibrate?

Edit: The airplane was Boeing 737. Not sure the series.

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closed as off-topic by a CVn, abelenky, Ralph J, David Richerby, Sean Jan 9 at 21:45

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "This question does not appear to be about aviation, within the scope defined in the help center." – a CVn, abelenky, Ralph J, David Richerby, Sean
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Is this really an aviation question, though? To me this reads as: "how accurate are GPS speed measurements?" which can be answered by a google search. The only cases where aviation would play a part in this is at altitudes where GPS hardware is inhibited or if you were asking if the precision is enough for some specific case. $\endgroup$ – AEhere Jan 9 at 13:10
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    $\begingroup$ This should be aviation as the context is in aviation. For example, how to calibrate due to the airline speed, etc.. $\endgroup$ – AirCraft Lover Jan 9 at 13:35
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    $\begingroup$ I don't see how this is about aviation. Yes, you say that "the context is in aviation", but the question is squarely about the GPS functionality of an unspecified tablet from back in 2011-ish. The exact same question can be asked without any mention of the aviation aspects with no loss of answerability; ergo the question is not about aviation. Now, if the question had been something along the lines of I checked the landing speed of some B737 on my tablet's GPS in 2011, and it showed as 420 km/h, is that reasonable?, that would be an aviation question. But that's not what you're asking. $\endgroup$ – a CVn Jan 9 at 19:23
  • $\begingroup$ @aCVn, I was probably wrong in wording my question. But however, airplane is also using GPS in flight. The different to my case is only the device: mine is tablet while the airplane is using dedicated device. $\endgroup$ – AirCraft Lover Jan 10 at 13:16
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    $\begingroup$ Actually, I would argue that this question is well within the scope of aviation. First, the question is about measuring the ground speed of an aircraft. Second it is about using GPS, which is now ubiquitous in aviation. Third, and perhaps most importantly it is about errors in the application of GPS, which are common, and the topic of substantial debate, often by poorly informed people talking about their experiences. Asking about calibration is a good question, unfortunately with the answer of you can't. The OP misunderstands many aspects of his question, but that is why we have questions. $\endgroup$ – mongo Jan 10 at 16:50
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It boils down to methodology used within the GPS receiver and processor. From general navigation perspective, GPS can provide extremely accurate velocity vectors. I'll spare the details as to why. But as an example, it can commonly be an order of magnitude more accurate than radar or the use of common ground based navaids.

However, if the GPS receiver and processor is designed to provide successive position information then the error budget goes up. The following factors come into play:

First, the position accuracy of GPS is not as accurate as the velocity information which could be available, and these errors are compounded with successive position determinations which might be used by the phone/tablet/device processor to determine an approximation of the velocity vector.

Secondly, the determination of velocity information with successive position processing may not utilize metadata which would be utilized with a dedicated navigation processor which has the goal of reliable velocity information. This means an increase in the error budget.

Thirdly, navigation devices which utilize GPS for aircraft, munitions and other applications, tend to perform continuous predictions as to the expected velocity, position, etc. in the future, so that the instantaneous readings from the device are a combination of prediction and historical data. In a tablet or smartphone, without knowing the attributes of the GPS and nav processor(s) one is likely looking at historical data, without the benefit of predictive models. This also increased the error budget.

So applied to the OP example, it is very conceivable that a reading inside the Faraday cage of a commercial aircraft, with limited sky view, multipath, and limited predictive modeling, and limited integration of sporadic readings, and without the benefit of strong predictive modeling, one may get readings that are substantially off from reality.

However, if one were using a GPS system designed for navigation, which exploited multi state Kalman filtering for velocity and position data, then I would expect a very highly accurate reading of velocity information. In the OP scenario, all of this does not exist.

So in the OP scenario, I would not expect a high level of accuracy and repeatability, and unfortunately, with the location/antenna/receiver/processor being used there is little that can be done to enhance the accuracy by calibration.

Addendum: One additional note. I don't know the current restrictions, but most GPS units have "speed restrictions" whereby they will not provide data above certain speeds (like Mach level speeds) which is done to limit their application to munitions. That may not be a factor in this case, but it is worth noting that all commercial GPS processors have this built in, if they are of US origin.

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    $\begingroup$ @Fattie, if it's involved we can go to chat, but I have developed nav systems on and off for aerospace, and my company has provided content for every GPS payload to date. If you want to get technical we can, but I tried to put my answer so it addressed the concerns of the OP. I have worked on other nav, including sat, aircraft, missile, etc. Rather than vote down, let's make it right. I can change my answer or delete it if appropriate. $\endgroup$ – mongo Jan 9 at 22:23
  • $\begingroup$ (Just FTR I don't downvote, @mongo - as a matter of fact I upvoted!!! To avoid a long-winded discussion I deleted my comment; I would just say I probably misread or skimmed your post or took it the wrong way. One could say, you're talking commercial devices and in the abstract; the shitty "gps apps" in phones ("seven years ago") either just have excellent coverage, and hence show a (perfect) speed, or, show nothing. There wouldn't be any mechanism where they would, consistently for minutes, show a speed that is off "by a factor of 2" which seems to be the case here. Cheers!) $\endgroup$ – Fattie Jan 10 at 12:17
  • $\begingroup$ @Fattie, yes one could consider it sporadic performance. But seriously, GPS is the cheapest way to get velocity information and it is extremely accurate. If not in a Faraday plane, and with reasonable processing, it is very very accurate. $\endgroup$ – mongo Jan 10 at 16:43
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    $\begingroup$ Anecdotal, but recently a friend and I used several different smart phones, including an iPhone 4S, a Motorola X 2nd Gen and a Samsung A tablet, non of which are "current" products, and checked out the GPS in the cockpit of various aircraft, also using a utility which showed the views of satellites. In the front office, the signals were a bit better, and at speeds between 300 and 450 k, we found that the inexpensive GPS displayed the steady state velocity within 2k of the FMS readout. Altitude readings were generally low. $\endgroup$ – mongo Jan 27 at 21:59
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That landing speed sounds rather high. There's a regulatory maximum landing speed for airliner type aircraft, and as I recall, it's typically not far from 150 kt (240 or so km/hr). Since it's unlikely the airplane you were in was actually flying at more than double normal speed at landing (and 180 kph will seem pretty fast to your own senses) I'm inclined to believe there was an error in your measurement.

The GPS in a tablet, reading from inside the airplane, is subject to a number of error factors. First, even in good reading conditions, the tablet's position is generally only good to (in my experience) around 50 m; second, from inside a metal fuselage, there may be path-length errors, satellite changes, and other errors on top of the accuracy limitation of your device.

This level of positional error can add up to a large percentage error in calculated speed.

The GPS installed in aircraft have better software and can fix location more accurately, as well as more regular position checks which give more accurate speed calculation. Those instruments make a good check against the pitot/static airspeed system. A smartphone or tablet GPS, probably not so much.

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    $\begingroup$ There are definitely some puzzles here: (1) A cheap two dollar GPS is , absolutely, perfectly cable of exactly measuring speeds around "420 kph" with total precision (2) I've always found that reception inside airliners is nonexistent or rubbish (3) if a GPS is getting weak reception, not enough sats seen, it won't show "wildly wrong figures" - it will just state "I can't see enough satellites". So, it's totally weird that the OP reports "420 kph" IMO. I do not agree that path-length errors / etc could result in a "meaningful bad reading" (so, one that's out by a factor of two). $\endgroup$ – Fattie Jan 9 at 17:34
  • $\begingroup$ I do not agree with the final paragraph. The advantage is (spectacularly) better antennas, viz a viz a phone gps held by the user in the cabin. $\endgroup$ – Fattie Jan 9 at 17:35
  • $\begingroup$ @Fattie It probably also has to do with what app is being used to read the GPS hardware in the device. My phone will routinely mis-estimate the distance to an intersection by a factor of three or four (either too far or too close), and works significantly better out of the car than in. This might be different if I ran dedicated GPS app instead of Google Maps. $\endgroup$ – Zeiss Ikon Jan 9 at 17:38
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    $\begingroup$ If you read an average speed over significant time, you might have included some period when you were cruising at appr. 850-900 kph... $\endgroup$ – Zeiss Ikon Jan 9 at 18:33
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    $\begingroup$ @AirCraft: Have a look at How does GPS work to gain some understanding. You will also find many YouTube articles. $\endgroup$ – Transistor Jan 10 at 16:15
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Measuring the speed of an aircraft with a GPS has several problems, and it's suitability will depend on what you are doing with that information.

A GPS typically uses a estimation and filtering algorithm to calculate position, time and velocity. A lesser known byproduct of these filtering algorithms are confidence intervals for each of the calculated parameters. Generally these are not shown to users unless they are looking at a diagnostics screen, and then, not always. Here I am talking about confidence of estimated state variables, such as position and speed.

These filtering algorithms are usually "tuned" to a dynamic model in order to intelligently weight historical measurements with current measurements. A GPS tuned for an oceangoing vessel can count on very small accelerations and altitudes near the geoid. I've seen models ranging from non-moving stations to airborne applications with up to 4g of acceleration. A smartphone is probably tuned for a dynamic model of an automobile, which can be expected to have dynamics < 1g. One consequence of this tuning is a trade-off between the noise and the latency of the output. Latency can also be variable and may be several seconds behind the actual motion of the receiver.

Dilution of Precision (DOP) is a useful metric, (distinct from a confidence interval on a state variable). DOP's come in several flavors and are a product of the geometry of the satellites relative to the receiver. DOP's are improved when the angles between satellites are as large as possible.

Some answers have mentioned numbers of satellites. In your case, your receiver's view was likely limited to satellites it could see out your, and perhaps a neighboring window, limiting the angles to a single side of the aircraft and probably less than a quarter sphere. Your DOP is poor in this situation as compared to having open sky above you.

One thing you have going for you is that speed for GPS is essentially derived from successive position solutions. While this makes the information inherently latent, it has the advantage of not being affected by steady position biases.

I suppose I'll end my answer by circling back to my opening statement: What are you hoping to use the speed information for?

  • In cruising flight, your phone's solution will be very good for indicating your speed over the ground.
  • Your question mentioned measuring landing: expect several seconds of latency if you are curious about speed at touchdown.
  • Another question may deal with airspeed, for which your phone's GPS cannot help you. GPS is often combined with an airdata system for a more complete navigation solution which may include indicated airspeed, true airspeed, wind speed, density and pressure altitudes, and other parameters used in aircraft control and performance calculations.
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Scott Kinder, a 737 pilot, answered this on Quora:

No way to give you a precise answer. Every aircraft lands at a different indicated speed based on flaps setting, their weight, and what the pressure altitude of the airport is. Also throw in wind corrections we make to the final approach speeds.

So say a normal 30 flap landing, with calm winds. My approach speed may be 150 knots (173 mph). At sea level, indicated airspeed is close to true airspeed, that means when I touch down it’s around 173 mph. Now throw me up to Denver International. At that altitude, my 150 knots approach speed is actually a TAS of 166 knots, so now I’m touching down around 190 mph.

173 mph = 278 kph 190 mph = 305 kph

S0 420 kph seems on the high side (260mph)

Tablet speed is questionable. Did you look to see how many GPS satellites it was receiving at the time? The external GPS antenna on the plane would see ~10 when near the ground (mine on the wing of my small plane sees 9-10 most of the time).

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  • $\begingroup$ If I was not wrong, there was 5 (as far as can I remember) satellites with good signal, probably more than 75% of the bar. 5 satellites with no other bad signal satellites. The weather at that times was clear sky. $\endgroup$ – AirCraft Lover Jan 9 at 13:37
  • $\begingroup$ I believe there was delay problem as the airplane is quite fast while the satellite signal had to travel two times, transmit and receive at the geo-stationer altitude, plus the processing time. And the calibration I am asking here is due to the delay. $\endgroup$ – AirCraft Lover Jan 9 at 13:39
  • $\begingroup$ 5? Thru an airline seat window? Wow. I thought you might have picked up 2 or 3 if you were lucky. Even with delay, most GPSs will update once a second. Aircraft (in the US) are prohibited from flying over 250mph below 10,000 feet, and then fly the descent for landing even slower, so I don't think delay is the problem there. $\endgroup$ – CrossRoads Jan 9 at 14:03
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    $\begingroup$ @RobertDiGiovanni, that was my experience at 3 January 2007. I remember the date as 1 January 2007, there was one airplane (Adam Air) crashed into a sea, 2km deep. Only the blackbox can be taken by the US Navy using robot due to it deep. That was the deepest sea in the world. At my flight, there was bouncing when landing, which I believed every hard landing will give more stress to the structure which could make damage and accident later. But GPS was not so advanve at that time, so I could not measure. And October 2011 I did the measurement. $\endgroup$ – AirCraft Lover Jan 9 at 17:43
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    $\begingroup$ @Fattie, I was not new to GPS. I was a surveyor, which always work with GPS to take coordinates and altitude meter to measure locations. Clearly, I understood what I did when I did the measurement. The problem that time was I did't have any idea about aviation. $\endgroup$ – AirCraft Lover Jan 9 at 17:46

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