9 ms·
This is probably a stupid question, but how is "ground speed" measured? Is it how quickly the airplane moves past a fixed point at it's altitude, or along the
by kgermino 8y ago
This is probably a stupid question, but how is "ground speed" measured?
Is it how quickly the airplane moves past a fixed point at it's altitude, or along the earth's surface? I.e. the plane is effectively moving along the surface of a sphere, but if the plane is 30,000 ft up, then the radius of the plane's sphere is 30,000 ft greater than the radius of the "ground" so traveling at the same angular velocity will require a higher speed the higher you go.
If I'm thinking about this right, across their respective "planes," the airplane will be moving faster than it's shadow. So maybe a better way to ask this is: is ground speed the speed of the plane, or the speed of its shadow?
- ozzyman700 8y agohttps://www.grc.nasa.gov/www/k-12/airplane/move.html https://www.grc.nasa.gov/www/k-12/airplane/move.html ground speed is the speed of the plane, relative to an observer on the ground, air speed is how fast the people on the plane are going relative to themselves. Airspeed = Ground Speed - Wind Speed (wind blowing towards front of plan is positive, tailwind is negative)
- BonesJustice 8y agoI’m a bit under the weather, so my comprehension may be lacking, but I think you’ve got your signs reversed (or your head/tail terminology flipped). If tail wind (moving with the plane) were negative, then air speed would exceed ground speed. Intuitively, that doesn’t make sense. Air speed can only exceed ground speed if the wind is blowing in an opposing direction, i.e., the plane must travel faster against the wind to achieve the same speed relative to a ground observer.
- deleted 8y ago[deleted]
- FabHK 8y agoWind is "named" after where it comes from. If you fly north, and the wind is a north wind, you fly slower. (This terminology is in line with what GP said.) Then, ground speed = air speed - wind speed. Thus, GP seems to have either the sign wrong, or ground and air speed mixed up (but not both :-)
- bartread 8y ago> Wind is "named" after where it comes from. This is absolutely true, but has always infuriated me because it seems needlessly contrary as compared to the way we speak about anything else travelling in a direction as read from a compass. Does anybody know why it's the case?
- ahartmetz 8y agoProbably because it matters where the air was before. Stuff like: north wind is could, south wind is warm, west wind is humid, etc
- bartread 8y agoDoesn't that very much depend on where you are though? E.g., with respect to the equator, major bodies of water and landmasses, mountain ranges, prevailing weather conditions. Also perhaps on seasons. I can understand that it might be just an ancient tradition originating from a particular part of the world, that we've turned into a convention, but it loses meaning when you go elsewhere.
- Arnt 8y agoIt rather depends on where you are, but so does language. The words we use were cast locally. If you speak English some words were cast in the Ukraine, some in Germany, some in Rome, some in France, some in Britain, but the people living in those places weren't obliged to make the words globally meaningful. If you go to Cape Town, a "north wind" presumably doesn't mean cold, because the area north of the city isn't colder than the city itself. This didn't matter to the people who first said "north wind" and I don't think it means much nowadays either. Just another little thing to watch out for in intercultural communication.
- yesenadam 8y agoCan you name anything else named after where it's going, not where it comes from? I can't think of anything named like that, but I guess there must be some.
- subhro 8y agoNo you are right. In case of tailwind, it is KIAS + WS whereas headwind is KIAS - WS KIAS: Knots Indicated Airspeed (i.e. the speed at which air is hitting the pitot tube) WS: Wind speed
- aeijdenberg 8y agoSlight, but important (if you don't want to run out of fuel) nit, indicated airspeed (KIAS) is not the same as true airspeed (KTAS). To calculate ground speed (as required for navigation and fuel planning) you need true airspeed (not indicated) as well as wind direction and speed. See some discussion here: https://www.quora.com/In-aviation-what-is-the-difference-between-IAS-CAS-EAS-and-TAS https://www.quora.com/In-aviation-what-is-the-difference-bet...
- TylerE 8y agoNot so slight. At airliner altitudes they can differ by 50% or so. IAS is what the pilots care about while flying because it’s what matters for aerodynamics (ie stall speed)
- ars 8y ago> are going relative to themselves. You mean relative to the air outside the window. Relative your yourself you're always standing still.
- stephenr 8y agoMost people on the plane were probably sitting still. I'll show myself out.
- cmurf 8y agoFor a given altitude, trim, and power, the airspeed is always the same no matter the wind. Wind only affects groundspeed. Ground speed = true airspeed + wind component (where you're computing a vector based on wind direction to determine what amount of it is either tail wind (+) or headwind (-).
- robotresearcher 8y ago> For a given altitude, trim, and power, the airspeed is always the same no matter the wind. If acceleration is zero. (Internet, therefore pedantry).
- Bendingo 8y ago> air speed is how fast the people on the plane are going relative to themselves That makes no sense. How can people be moving relative to themselves?
- jazzyjackson 8y agorelative to their own location Δt ago?
- cxseven 8y agoLocation with respect to what?
- FPGAhacker 8y agoThemselves meaning the ground observer as that was the point of view GP was speaking about.
- foota 8y agoHm, I think the difference is marginal because of the large radius of the Earth, at least for planes.
- dragonwriter 8y agoThe Earth has almost a 4000 mile radius; 30,000 feet is less than 6 miles. The difference between movement over ground and movement with respect to the top of a 30,000 foot pole stuck in the ground is pretty negligible.
- deleted 8y ago[deleted]
- kevmo314 8y ago30k ft is nothing compared to the radius of the Earth which is 21M ft. The measurement instrument has larger error sources, so the answer is it doesn't actually matter.
- frankchn 8y agoThe difference is about 1.1 mph (edit: out of 800 mph) at 30,000 ft compared to ground, so I don't think it matters too much.
- TheLoneAdmin 8y agoThe radius of the earth + 30,000 ft is about 0.14% more than just the radius of the earth. Basically ignore it, unless you are programming guidance control systems.
- ummonk 8y ago0.14% is enough to throw the number off by 1mph so it isn’t irrelevant.
- loco5niner 8y agolol, ok. Make that 800 MPH then.
- dogma1138 8y agoIt’s likely less than the reading error in most speedometers including those which are used for autonomous vehicles unless you are writing a guidance system for bombs or attempting to land on another planet that’s pretty negligible.
- ummonk 8y agoGiven that the ground speed was likely drawn from gps data, which should have velocity readings more accurate than 1mph (particularly for an airborne gps unit), the choice of calculation method for ground speed does make a difference.
- microcolonel 8y agoThat's less error than slopes and oblateness introduce, so I think it is acceptable.
- ummonk 8y agoThat is also an interesting question. If they’re measuring speed as if the airplane were at the surface rather than at 30,000 feet, is it speed along the WSG84 earth model?
- 8y ago
- btian 8y agoIt's measured by repeatedly taking GPS coordinates divided by time.
- snowwrestler 8y agoGPS measurements are comprised of a 2D surface coordinate system (e.g. lat/long) and an altitude above mean sea level. So, "speed" as measured by GPS is speed along the Earth's surface. EDIT: assuming a constant altitude flight
- heavenlyblue 8y ago>> GPS measurements are comprised of a 2D surface coordinate system GPS is inherently 3 dimensional.
- snowwrestler 8y ago> and an altitude above mean sea level
- heavenlyblue 8y agoNah, you're clearly misunderstanding how GPS works. GPS allows you to directly measure displacement of any 2 points in 3 dimensions. You're speaking of the altitude + lat/lon instead, which is in polar coordinates.
- kens 8y agoThere's some interesting stuff going on behind the scenes. GPS inherently gives you a 3-dimensional coordinate, which is useless for most applications. So the coordinate gets converted to a 2D surface coordinate (latitude/longitude) plus an altitude, which is what you generally want. However, since the Earth isn't a perfect sphere, this conversion is not well-defined. The solution is a datum, a model of the Earth's surface that is used for the conversion. WGS84 is the datum used by GPS, and approximates the Earth as a specified ellipsoid. However, maps can use use different datums (data?), so there can be a discrepancy of many meters between what GPS says the location is, and what the map says the location is, and they can both be right relative to their datum.
- JoblessWonder 8y agoGround speed is what most people would think of when they think of a speed. How fast is it moving from Point A to Point B. Airspeed takes into account how fast you are going you are going AS WELL as how fast and in what direction the air around you is going. Another example would be a person walking on a motorized walkway (like at an airport.) Person A is going WITH the direction of the walkway like someone normally would. Person B is going AGAINST the direction of the walkway but still making progress forward. How fast the person is going when observed from off the walkway is ground speed. To maintain the same ground speed, Person A needs to only walk slowly (less effort/lower airspeed) while Person B needs to be sprinting all out (more effort/high airspeed.) * Here is an interactive example from NASA: https://www.grc.nasa.gov/www/k-12/airplane/Animation/airrel/anrela.html https://www.grc.nasa.gov/www/k-12/airplane/Animation/airrel/...
- jessriedel 8y agokgermino is asking about the difference between (1) the actual speed (relative to the Earth) at the altitude where the plane is flying and (2) the speed of the path of the airplane when it is projected onto the ground. These are extremely close but not the same (~0.15%) because the Earth is a sphere and the airplane is at a slight further distance from the center of that sphere than it's projected point on the ground. You are explaining the difference between airspeed and ground speed, which can be much more substantial (e.g., ~10%, or ~%100 in extraordinary situations).
- deleted 8y ago[deleted]
- lisper 8y agoPrivate pilot here. As others have pointed out, the difference is mostly negligible, but ground speed is the speed of the vertical projection of the aircraft's location on the earth's surface.
- mc32 8y agoSo essentially the speed at which the plane's shadow on earth travels while projecting on level ground.
- lgierth 8y agoMh no, the shadow's position depends not just on the planes movement, but also on earth's rotation and trajectory.
- adamson 8y agoIs that not also true of the plane's projection onto the Earth's surface?
- jessaustin 8y agoI think "trajectory" refers to the position of earth relative to sun, which affects the shadow but of course has no effect on the projection.
- glass_of_water 8y agoI don't think so, since projection would be like taking the line between the center of the earth and the plane and finding where that intersects the surface of the earth. That doesn't depend on the trajectory of the earth through space.
- adamson 8y agoWhoops, I totally ignored the trajectory aspect and was just thinking both would be affected by the rate of rotation of the earth
- cmurf 8y agoIn the old days, you'd take the time between two "fixes". A fix could be a visual reference, or a NAVAID. That'd result in your shadow's speed. Then came DME, distance measuring equipment. I don't know if DME ever took altitude into account or if the error resulting from altitude was just ignored, but analog DME radios would just tell distance at first and then digital versions had a groundspeed feature. At least instrument rated pilots were aware of the increasing error the closer to the NAVAID. At some point in the ATC radar system, a controller's screen could show groundspeed. Pilots can ask ARTCC what ground speed is, although I would never ask approach or departure control such a question. This would have the same error as DME. Today it's a value reported by GPS. I don't know if this is based on the plane's movement, or its shadow, or if there's a regulation that dictates it for aviation certified GPS. From a pilot perspective we don't distinguish between kinds of ground speed, whereas we do distinguish between kinds of airspeed: indicated, calibrated, and true. Mainly groundspeed is important navigationally to know if there's an unexpected headwind that'll lengthen enroute time and thus impact fuel management. And while there can be big enough error between estimated winds aloft and actual that this computation will affect enroute planning, I tend to only care about tens of minutes or hours, not seconds.
- tjohns 8y agoDME equipment explicitly does not take altitude into account. It's colloquially called "slant range" for this reason, and if you care about actual ground distance you have to use trigonometry.
- glitchc 8y agoGround speed is measured by GPS on modern aircraft.
- dzhiurgis 8y agoWhich is probably measured in it's trajectory and then recalculated as ground speed, which is sea level? Wonder if it accounts for sea levers being higher at certain parts of planet?