# How does GPS measure any plane's ground speed?

How does GPS measure an equipped airplane's ground speed assuming that the plane in question actually uses GPS as opposed to an inertial navigation system or other method?

(This was originally part of another question)

• It's quite strange that the determination of the aircraft ground speed using a GPS is off-topic. It's a trivial question that may be closed, but not with the off topic criteria imho. VTR.
– mins
Jun 13, 2016 at 13:54
• Could be a duplicate of this more general question on ground speed though. Jun 13, 2016 at 14:39
• So, is SE beginning to institute the marking of duplicate answers then? This is not the same question. Jun 13, 2016 at 15:20
• @fooot It doesn't matter whether casey's answer applies here or not, we can't start marking things as dupes because of overlapping answers. See the comments on this question for a clearer example of why - in my opinion - it's a bad idea. Personally, I'd rather see 5 short, simple, obvious questions with equally simple answers than one 'generic' question that might answer everything if you read through all the answers. Jun 13, 2016 at 15:38
• @Pondlife per meta If you ask a question similar to another question and it is likely to get the exact same answer, you have yourself a duplicate question. imo, ground speed in general is not that much more complex and sufficiently covers the GPS part, there is no reason to answer it again. Jun 13, 2016 at 15:43

Therefore, GPS uses several satellites with exactly known positions which send very precisely timed signals. From the time difference between the signals when arriving at the receiver, the own position in space can be calculate, typically given in earths coordinate system.

Get the position two times, calculate distance, divide by time difference, and you have speed.

Another way is to measure the frequency of the signals from the satellites, since it slightly varies with speed due to the Doppler effect. This is similar to the shift in pitch of the horn of a police car passing you. This method gives you a speed value directly with your position.

As side note: GPS often is not that precise and gives your position with some offset. BUT the offset is mostly constant, so while the position is not correct, the distance traveled and so the speed is.

Exactly the same way that your car GPS measures your ground speed.

(Source)

The receiver can evaluate the location at two successive times, and simply compute the velocity vector from differentiation. This gives a precision of several meters per second.

This velocity can be derived from the Doppler shift of the L1 carrier frequency. This true measure is far more accurate, up to centimeters per second, but is less simple to implement, as the frequency shift must be smoothed over a period of time.

• I you want to relate the speed of the aircraft to the ground you have to taken into account that an aircraft is flying at 37000ft parallel to the earth. So with a bit of trigonometry you can come up with a ground speed. Jun 13, 2016 at 12:46
• @Brilsmurfffje: That's a good point! The difference between sea-level and 37,000 ft circumferences is about 70 km only, about 0.2%. Not that much, but you are right.
– mins
Jun 13, 2016 at 14:46
• Not quite, in the air the actual distance travelled is not the same as distance travelled on the ground. A correction has to be made for altitude. Jun 14, 2016 at 13:51