# What kind of airspeed does a (3D) GPS measure?

Assuming a GPS receiver is advanced enough to compute 3D position/velocity with reasonable accuracy, it can indicate not only the aircraft's ground speed (which ignores altitude changes), but something akin to airspeed as well. If there was no wind, this would be equivalent to true airspeed. In reality there would be wind so it is "TAS minus wind". Since this completely ignores the actual air (the value would have been the same if there were no atmosphere), however, calling it anything related to "airspeed" seems like a misnomer to me.

Is there a standard terminology for this kind of velocity measurement?

EDIT: It seems my question was quite vague and providing more detail about what I'm trying to do would help. I'm writing the documentation for a GPS-based tracking device, which would be used in fixed-wing aircraft. The GPS is exceptionally good, with very high precision and quick sampling rate. It even provides very accurate speed, which is Doppler-shift based AFAIK. This data will be sent through telemetry. I'm trying to label the data channels in a way that would be useful for aviators down on the surface, so I'm trying to familiarize myself with the proper terminology. Therefore if I label this ("3D") velocity as "airspeed" (indicated/true airspeed), it will be misleading. If I label it "ground speed" (which is 2D), it will be misleading as well. My question is only whether a standardized name for this type of velocity exists; if there's nothing widely used, I'll just coin a name of my own (e.g. "3D velocity") and explain it well enough in a tooltip.

• You have confused terms. True Airspeed is not dependent on wind. TAS is airspeed adjusted for air density. It is adjusted from Indicated Airspeed factoring in altitude and temperature. Airspeed in general is based on the aircraft’s movement through an airmass regardless of wind. GPS calculates velocity based on the difference in position over time relative to the GPS transmitters. It does not factor in movement through an airmass nor air movement at all. Calling it’s readings with no data from air movement would technically not be any type of airspeed. Dec 27, 2020 at 16:35
• @DeanF., yeah, exactly, as I said it's tempting to call it airspeed (because it won't show as 0 if the airplane was falling straight down), however there's no air involved. It is velocity relative to the Earth's surface. Dec 27, 2020 at 16:45
• If you understand that in zero wind TAS = GS, then what are you actually asking?! Dec 27, 2020 at 17:09
• Do you really mean to ask, "What kind of velocity does GPS measure?" Dec 27, 2020 at 19:55
• @RalphJ, yes, and more exactly "how to you typically call this kind of velocity in aviation context" Dec 27, 2020 at 20:04

Q: "What kind of airspeed does a GPS measure?"

A: "None whatsoever."

The GPS does not sense or compute any type of airspeed, (True, indicated, calibrated, etc.) it only knows the airplane's velocity relative to the earth's surface. (Actually, relative to geosynchronous satellites...) GPS units currently in use can do this in 3 dimensions.

There is both a horizontal and a vertical component to movement in 3D space. For general navigation purposes the pilot is only concerned with the horizontal, i.e. ground speed. (How fast am I going, and when will I get there?)

On some systems you can display vertical velocity from the GPS, but it is instantaneous and very jumpy, so it isn’t particularly helpful. The pressure driven VSI gauge is much more useful.

While it would be possible for a 3D system to compute velocity along a diagonal line between vertical and horizontal, this information is of no use to the pilot. The vertical component comes into play mainly in the approach phase, to compute and display glide slope information during RNAV approaches.

In the approach phase the GPS system uses a blend of horizontal and vertical navigation information to allow us to fly the “hypotenuse”, but we don’t think of it in terms of a velocity vector, much less an “airspeed”. It is more an imaginary line in space we are trying to track along in 3D with the system providing cues to help us know where we are relative to the published course and glideslope.

ADDENDUM PER QUESTION EDIT: Wind will have a far greater effect on this velocity vector than the minor climb and descent angles encountered during "normal" (non-aerobatic) flight. Without knowing your customer or exact purpose of the device you reference, I can say confidently that no aviation person would care about earth referenced velocity other than vectors in the horizontal or vertical. If the diagonal, (not airspeed) velocity was desired for something like a mishap investigation, it could easily be calculated from others. If you need to assign a name I would consider "Velocity", "Magnitude", "Earth-speed", or a proprietary defined acronym like "VVGS" (Vertical Velocity plus GroundSpeed). Just avoid "air" unless that's what you actually mean...

• Isn't ground speed 2D? I.e. if the aircraft is in 90° dive, what would it indicate? Dec 27, 2020 at 17:19
• Ground speed would be zero. Vertical velocity, however, would be pegged! But thanks, that clarifies what you are asking about and I will edit my answer... Dec 27, 2020 at 17:22
• OK, so it seems a good call to request the speed indication to be split into ground speed (2D) and rate of climb, right? (I agree you can compute the 3D velocity easily from those) Dec 27, 2020 at 20:49
• Again, without knowing the customer or purpose I would say yes. Dec 27, 2020 at 21:24
• "Just avoid "air" unless that's what you actually mean" -- well put. Dec 28, 2020 at 17:56

Typically it is the Inertial based systems that give us this kind of data. A GPS with a suitable computing module could provide the same thing.

The closest thing we have is called the FPV, Flight Path Vector or the 'Bird'.

In the cockpit, generally, the FPV provides the pilot only a part of the information, and displays it in a particular manner, vertical path and Lateral path being separately shown.

The actual 3D speed component is not one of them, and neither would the 3D Speed be of direct value in flying, or navigating the airplane.

Typically it gives us:
(i)the instantaneous Track and Vertical angle of the actual flightpath with respect to the earth, ie the ground,
(ii)also often, it gives use a projection of where the airplane will be in 10, 20, and 30 seconds based on the current instantaneous flightpath.
(iii) resolving the East/West and North/South vectors gives us the instantaneous Track and Groundspeed.
(iiii) resolving the Vertical component of the FPV gives us the Vertical Speed, though it is normally only used to validate the Air Data based Barometric VertSpd, VS.

None of these systems actually measure any Airspeed. They measure GS, Ground Speed, though strictly speaking, it could not be called an act of measurement, but something arrived at by using position data, ie Lat/Long at suitable intervals and computing the difference to give an accurate estimation of Ground Track and GS (=TAS in still air).

The plethora of acronyms related to speed, direction, and vertical disposition were necessitated due to way we physically measure, then compute, then display them and our basic flying skills have developed around these - IAS, CAS, EAS, TAS, GS, ALT, FL, TRK CRS, HDG(M), HDG(T), ATT. There's no point introducing new measures such as 3D speed, though I'm guessing your intent maybe to have "one speed to rule them all" along with a suitable depiction of airplane and flightpath in a 3D EVS (Enhanced Vision System).

• Thank you for the detailed answer. It seems this GPS-derived velocity serves no use to a pilot, so an instrument showing it has no space in a cockpit. My question is not really about airplane instrumentation though, it's mostly about naming and terminology. My question didn't state that clearly enough. Can you see the edit I made to it? Dec 27, 2020 at 20:07
• Re your edit: it's not so much as "one speed to rule them all", rather than "the only speed we can get". The device has no access to the pitot tubes. Dec 27, 2020 at 20:24
• For completeness, since it doesn’t appear to be spelled out anywhere else, the speed indication from a GPS is typically the speed in the horizontal plane, vertical speed isn’t taken into consideration.
– Frog
Dec 28, 2020 at 0:22
• @Frog A few decades ago the vertical speed displayed in the Airliner cockpit was modified to include Inertial inputs and the nomenclature was changed to IVSI (Instantaneous Vertical Speed). The "Instantaneous" part has been dropped now, as it has become the 'new normal'. Pure baro VSI's suffered delays in displaying the correct VS. Whether such input corrections are specifically taken from GPS systems is not known to me, but the systems have the capability of providing it to us. My current airplane display can be switched to GPS altitude as an alternate when Air Data is unreliable. Dec 28, 2020 at 6:04
• To clarify: I’m referring to the NMEA feed from a GPS and specifically the RMC sentence. I don’t dispute that certain GPS-equipped instruments may indicate speed in 3 dimensions. GPS does not resolve altitude well and so it’s no surprise to hear that other methods such as inertial are used to determine vertical velocity.
– Frog
Dec 28, 2020 at 20:54