Ground speed. The speed of an aircraft relative to the surface of the earth.

This is from ICAO doc 9426 AIR TRAFFIC SERVICES PLANNING MANUAL PART V TERMS AND REFERENCES.

I think it's pretty clear isn't it?

Unfortunately no, it isn't. If you look at the way what we call SOG is calculated (and some details have been posted above) you will realise that strictly it is not what the ICAO document describes, however, the difference is irrelevant for air traffic services (the document's subject matter).

Kudos to the OP for posting what must be the most interesting question I've read here so far this year. Although aware of it, I'd never consciously thought about it.

Folks,
What we have here is a clash of "Airplane Navigation of Dummies" and something a bit nearer to an engineering solution.

For the average pilot, the simplest definition is all that is required, a bit like the simplistic diagrams we use to illustrate the "forces" acting on an aircraft in cruise, climb and descent ( thrust, drag, lift and the CAA) --- but for an engineer it is not "that simple".

Try arguing with an engineer for an airframe or (particularly) an engine manufacturer about in-flight performance analysis versus contract guarantees --- right down to latitude and Coriolis effect --- that's really getting into the fine print.

Tootle pip!!

Another general situation that may affect the answer is whether the "distance" an aircraft travels **according to its flight plan** is dependent on the altitude of the aircraft. Consider 2 aircraft on long range great circle tracks, zero wind, one at 3000 feet and one at FL450, both to the same destination. Will the flight plan distances be different?

Lead Sled, the forces are in fact, MONEY, lift drag and thrust, the first one being the most critical!

ATCast asked for an official definition of groundspeed. Genghis the Engineer thought it is horizontal component of absolute velocity but could not pin a reference.

Pugilistic Animus thinks the two definitions are equivalent. Wizofoz thinks it is not the "horizontal component of absolute velocity" because "absolute velocity" is not defined, although I understood perfectly well what ATCast was asking, as did Genghis.

Tmbstory thinks it is "speed over the ground", and Checkboard "speed over that surface", so FEHoppy "speed relative to the surface of the earth". None of these contributors seem to have picked up that their "definitions" are ambiguous in just the way ATCast explained.

BOAC sees the difference, I think, but considers the discussion "twaddle".

I did what ATCast wanted. I looked it up in Kayton and Fried, Avionics Navigation Systems, 2nd edition, Wiley-Interscience 1997, otherwise known as The Book, when I had the chance.

Ground speed is the horizontal (i.e. tangential to the reference ellipsoid) component of the resultant vector of the air-velocity vector with the wind velocity vector (Figure 2.4, p30, notation Vg). Genghis is right.

As for PA, we may take it he doesn't build inertial nav instruments for a living. Serious nav takes altitude into account, for reasons which I hope I indicated in my earlier post.

PBL

"Ground speed. The speed of an aircraft relative to the surface of the earth."

An aircraft that's diving vertically at 500 knots clearly has a speed of 500 knots "relative to the surface of the earth". Is its ground speed 500 knots?

Tim, his ground speed will be about zero, {just watch your GPS next time you are doing aeros} I said about zero so as to avoid those who will find that it wont be quite zero due to many obscure factors, none of which amount to a hill of beans, however if he doesnt pull out it will indeed be zero!

I dont agree clunckdriver, the ground speed would be about 3 regardless of other factors

Wouldn't that be "perpendicular to the local vertical" or equivalently, "perpendicular to the local gradient of the gravity field"? Is the parenthesised phrase part of the quote from the book, or is that yours?

Bumpy Flight, I dont know what you fly, but in my world three knots is just about zero,{and about 100kts under my stall speed} even our Jet Ranger ASI aint too precise under five knots!

I fly a dash 8

Really, it was an A321 the other day! :ugh:

tranfered mcgoo

I fly the dash 8 for private hire and the A321 for thomson

What all two of them whilst you don't fly any of their 5 A320s?

Intriguing!! :rolleyes:

Nope, perpendicular to the geodetic latitude. The "local gravity field" is known as the astronomic latitude, apparently, and it is not necessarily the same. Indeed, it is explicitly distinguished in Figure 2.2 of the reference I gave. (The third possibility is "perpendicular to the geocentric latitude", referenced to the "center of the ellipsoid and mass center of Earth", which is different again.)

No, it's not a quote from the book, it is my words saying what Figure 2.4 shows as a vector diagram (triangle with arrows and labels).

PBL

Look, I fly as a private pilot for a buisness I wish not to state the name of and I fly for thomson

Folks,

is it too much to ask that we try to have a half-way serious conversation about ground speed - and only that - on this thread, and discuss other stuff elsewhere?

PBL

PBL, I agreeI would like to take this chance to say I am not a troll, I do not intend to cause trouble and everybody has pounced on me since I posted my first message, I am a freal pilot and I would like to enjoy my time on here without some of you getting at me