Talk:Ground speed

I Changed "over the ground" to "relative to the ground". This makes it obvious that it is the speed of the airplane that is measured against the ground, as opposed to it possibly meaning that the airplane itself is over the ground at the time you measure its speed. 212.138.64.171 16:45, 13 December 2005 (UTC)

merger proposal

Ground speed is not an airspeed; it is a vehicle's speed over the ground rather than through the air (a medium which can itself be moving). They're totally different concepts, and it wouldn't make any sense at all to merge them. ericg ✈ 04:05, 23 October 2006 (UTC)

What about vessels?

I came to this page from a link on the Knots page and was surprised to find that this page only references aircraft. What about boats? Ground speed relates to vessels also because speed on water is not the same as on land. There are factors to take in to consideration such as the speed of the fluid which the vessel is in, wind speed etc. That's really just what I've learnt from the 'Knots' page so if anyone knows anymore about the aquatic side of ground speed, it would be helpful to be included in this page. Or is there another name for the aquatic version of ground speed? - Frank --202.138.10.189 (talk) 09:11, 16 December 2007 (UTC)

basis is at ground or flight level? earth rotates?

To measure ground speed for (atmospheric bound) aircrafts one can take the earth (geoid) surface as the fixed bases, where the velocity (vector) v=0. As aircrafts do not fly (much) higher than 10 km, which gives a position about 1/600 part higher then the radius (6200 km) of earth. This is even more likely valid for u-boats.

ISS "flies" in a Low Earth Orbit of 400 km above ground. This height is a relevant fraction, about 6 % of earth radius. When now the article about ISS speaks about ground speed this could mean

(a) the speed at flight height relative to an imaginary calm atmosphere there up. This atmosphere rotates with the earth surface.

(b) the speed of of the projection line from the flying object the the center of earth where it hits the (rotating) earth surface.

(c) the speed of (a) but assumed the earth would not rotate

(d) the speed of (b) again assuming a non-rotating earth.

For GPS- or survey-satellites or intercontinental rockets the geographical position is relevant, but the rotation of earth (1/day around the N-S-axis) does (after the start and the few minutes of ascend through the lower - let us say 30 km - atmosphere) not interfere with the orbit or flight path of the object. So it seems it needs further definition "which groundspeed" one looks at particularly. --Helium4 (talk) 11:55, 26 October 2011 (UTC)

The primary purpose of ground speed is to be able to determine the time taken to pass two points on the Earth's surface, so I think your option (b) is the relevant one in the case of an object in orbit. Dolphin (t) 21:25, 26 October 2011 (UTC)