Definition of ground speed
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PBL - I say you were right about 'muppets'. Anyone got a link to the NASA muppet forum?
LH2 - Let me know when the tower is forecast to fall in to the Seine on your reference systems - then I'll take notice, I promise. Until then....................dear oh dear!
On the other hand, the fact that I didn't need to step out of the way of the leg of the tower as it rushed past MIGHT have simply meant I am in the same reference system. I had not thought of that. Tisk tisk.
LH2 - Let me know when the tower is forecast to fall in to the Seine on your reference systems - then I'll take notice, I promise. Until then....................dear oh dear!
On the other hand, the fact that I didn't need to step out of the way of the leg of the tower as it rushed past MIGHT have simply meant I am in the same reference system. I had not thought of that. Tisk tisk.
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Not so "pure and simple" I'm afraid. You will find that what we call ground speed is in effect the first integral of acceleration in a given reference frame which may or may not be physically referenced[*]. What I think you might be thinking of would be the first derivative of change of position over time, which could be a perfectly valid definition (not without its own set of complications, mind you) but unfortunately is not what is used.[*] Consider the following practical example: If you were measuring the ground speed of the Eiffel tower you will find that your answer will be either about zero, or ~3cm/yr, depending on whether you were using the ETRS89 or ITRF2008 reference systems (due to different velocity fields). In other words, even defining what "the ground" is presents a major problem, past a certain threshold of accuracy.
no such animal as a 'first integral'...double, triple etc integrals are for each axis with the other mutual axes held constant....but I know what you meant
it all depends on local wind velocity and the displacement between nav fixes
I was only jokingly referring to those folks as muppets
this has got to be one of the most complicated discussions I've seen, but really for no reason
Edit: Nature laughs at mathematical complexity
it all depends on local wind velocity and the displacement between nav fixes
I was only jokingly referring to those folks as muppets
this has got to be one of the most complicated discussions I've seen, but really for no reason
Edit: Nature laughs at mathematical complexity
Last edited by Pugilistic Animus; 28th Jun 2010 at 22:39.
Psychophysiological entity
And just when you thought you were flying level.
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I'd say that ground speed should be defined as the speed of the aircraft over level ground, as defined by the rate of change of the position of the aircraft as projected vertically onto the same level ground.
Then again, "vertically" is not constant and usually not "directly towards the center of the earth", nor is "level" usually "perpendicular to a vector through the center of the earth". The geoid is rather uneven, meaning the speed of movement of the vertical projection on the surface is not simply a function of the radii of the geoid surface, the radii of the path of the aircraft and velocity. Constant velocity, constant radii of travel => varying ground speed, as defined in the first paragraph.
I thought I'd throw that in, as the discussion so far was oversimplifying the subject somewhat.
(Edit: Darn. Loose Rivets beat me to it!)
Then again, "vertically" is not constant and usually not "directly towards the center of the earth", nor is "level" usually "perpendicular to a vector through the center of the earth". The geoid is rather uneven, meaning the speed of movement of the vertical projection on the surface is not simply a function of the radii of the geoid surface, the radii of the path of the aircraft and velocity. Constant velocity, constant radii of travel => varying ground speed, as defined in the first paragraph.
I thought I'd throw that in, as the discussion so far was oversimplifying the subject somewhat.
(Edit: Darn. Loose Rivets beat me to it!)
Last edited by ft; 1st Jul 2010 at 07:04.
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I'd say that ground speed should be defined as the speed of the aircraft over level ground, as defined by the rate of change of the position of the aircraft as projected vertically onto the same level ground.
Then again, "vertically" is not constant and usually not "directly towards the center of the earth", nor is "level" usually "perpendicular to a vector through the center of the earth". The geoid is rather uneven, meaning the speed of movement of the vertical projection on the surface is not simply a function of the radii of the geoid surface, the radii of the path of the aircraft and velocity. Constant velocity, constant radii of travel => varying ground speed, as defined in the first paragraph.
I thought I'd throw that in, as the discussion so far was oversimplifying the subject somewhat.
Then again, "vertically" is not constant and usually not "directly towards the center of the earth", nor is "level" usually "perpendicular to a vector through the center of the earth". The geoid is rather uneven, meaning the speed of movement of the vertical projection on the surface is not simply a function of the radii of the geoid surface, the radii of the path of the aircraft and velocity. Constant velocity, constant radii of travel => varying ground speed, as defined in the first paragraph.
I thought I'd throw that in, as the discussion so far was oversimplifying the subject somewhat.
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ft,
I pondered weighing in again on this, since there appears to be a certain amount of noise on the channel which I don't wish to amplify. Your proposal raises a question.
Yes, but it isn't so defined by the nav community, as we have seen. They take the angular velocity as definitive and multiply by the radius of the flight above the nominal centre of the earth (radius of earth + altitude).
Definitions are usually given for a purpose. If you say the definition "should" be as you say, what is the purpose you have in mind?
Indeed so. But these difficulties apply to all reasonable attempts to define it.
PBL
I pondered weighing in again on this, since there appears to be a certain amount of noise on the channel which I don't wish to amplify. Your proposal raises a question.
Originally Posted by ft
I'd say that ground speed should be defined as the speed of the aircraft over level ground, as defined by the rate of change of the position of the aircraft as projected vertically onto the same level ground.
Definitions are usually given for a purpose. If you say the definition "should" be as you say, what is the purpose you have in mind?
Originally Posted by ft
Then again, "vertically" is not constant and usually not "directly towards the center of the earth", nor is "level" usually "perpendicular to a vector through the center of the earth".
PBL
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italia,
if you try to make definitions simplistic, you are often in for it. Definitions are, by definition (), to be exact rather than simple. As has been made clearly apparent, there's a lot of room for different interpretations as to what ground speed is.
PBL,
looking at Kayton and Fried, I see an figure from an avionics text book (which I am grateful for the reference to!), illustrating varius concepts and terms used, rather than a strict definition. In other words, I'd say they made a for most purposes perfectly reasonable simplification, but a definition it is not.
Going for definitions, The United States Air Force Dictionary (Heflin) defines ground speed as:
"The speed of an aircraft, esp. an airborne aircraft, measured by the distance it travels over the ground either by reference to check points on the ground, or by taking wind velocity into account."
The U.S. Navy Hydrographic Office (1955) define ground speed as:
"the rate of motion of the aircraft relative to the earth's surface. It is the speed of the aircraft over the ground; hence the name."
Clearly these definitions do not agree with Kayton and Fried (with the possible exception, and slight ambiguity, introduced after the last comma by Heflin), as can be illustrated by taking it to the extreme of very high altitude flight. If we look at the primary purpose of ground speed, namely navigation, the definitions presented here remain useful while errors would creep into the navigation solution using the IMO simplified explanation in the figure in Kayton and Fried - which is intended for a different audience than navigators.
The definitions listed above still leave it up to the reader to decide what being above a place on the surface of the earth actually means, but it is to the bet of my knowledge widely accepted in geodesy that being above something is referenced to the geoid and not the ellipsoid. In other words, you are above a place on the surface of the earth when a dropped penny will hit that place. Using the ellipsoid as the level gives interesting effects such as streams flowing uphill in places, so it is best avoided.
I think the hobgoblin of this discussion is that for all normal practical purposes, a simplified definition by far exceeds the required precision and hence, many more or less authorative publications will go with the simplification - without explicitly stating so.
References
Heflin, W. A. (Ed.). Publication year unknown, but probably 1950s or 1960s. The United States Air Force Dictionary. D. Van Nostrand Company, Inc., Princeton, NJ, U.S.A.
The U.S. Navy Hydrographic Office, H.O. Pub. No. 216. (1955). Air Navigation. U.S. Navy Hydrographic Office, United States Government Printing Office, Washington, U.S.A.
if you try to make definitions simplistic, you are often in for it. Definitions are, by definition (
), to be exact rather than simple. As has been made clearly apparent, there's a lot of room for different interpretations as to what ground speed is.PBL,
looking at Kayton and Fried, I see an figure from an avionics text book (which I am grateful for the reference to!), illustrating varius concepts and terms used, rather than a strict definition. In other words, I'd say they made a for most purposes perfectly reasonable simplification, but a definition it is not.
Going for definitions, The United States Air Force Dictionary (Heflin) defines ground speed as:
"The speed of an aircraft, esp. an airborne aircraft, measured by the distance it travels over the ground either by reference to check points on the ground, or by taking wind velocity into account."
The U.S. Navy Hydrographic Office (1955) define ground speed as:
"the rate of motion of the aircraft relative to the earth's surface. It is the speed of the aircraft over the ground; hence the name."
Clearly these definitions do not agree with Kayton and Fried (with the possible exception, and slight ambiguity, introduced after the last comma by Heflin), as can be illustrated by taking it to the extreme of very high altitude flight. If we look at the primary purpose of ground speed, namely navigation, the definitions presented here remain useful while errors would creep into the navigation solution using the IMO simplified explanation in the figure in Kayton and Fried - which is intended for a different audience than navigators.
The definitions listed above still leave it up to the reader to decide what being above a place on the surface of the earth actually means, but it is to the bet of my knowledge widely accepted in geodesy that being above something is referenced to the geoid and not the ellipsoid. In other words, you are above a place on the surface of the earth when a dropped penny will hit that place. Using the ellipsoid as the level gives interesting effects such as streams flowing uphill in places, so it is best avoided.
I think the hobgoblin of this discussion is that for all normal practical purposes, a simplified definition by far exceeds the required precision and hence, many more or less authorative publications will go with the simplification - without explicitly stating so.
References
Heflin, W. A. (Ed.). Publication year unknown, but probably 1950s or 1960s. The United States Air Force Dictionary. D. Van Nostrand Company, Inc., Princeton, NJ, U.S.A.
The U.S. Navy Hydrographic Office, H.O. Pub. No. 216. (1955). Air Navigation. U.S. Navy Hydrographic Office, United States Government Printing Office, Washington, U.S.A.
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Originally Posted by ft
looking at Kayton and Fried, I see an figure from an avionics text book.... illustrating varius concepts and terms used, rather than a strict definition.
I am generally wary of quoting definitions from engineering sources and taking them as "definitive", for in my experience many engineering definitions are messy and ultimately inappropriate.
See, for one of the most fundamental examples in safety-critical systems engineering, Problems Calculating Risk via Hazard. See this essay on the International Functional Safety Standard IEC 61508 for some (notorious) problems with the definitions there, and Definitions For Safety Engineering for my suggestion of how to do it right. (Full disclosure: I wrote all of these.)
I read Heflin as wisely leaving it open which definition to use. For many purposes, they will agree, especially when the margins of error due to non-uniformities of the geoid are taken into account.
You make the point that in geodesy, one references to what Kayton calles the "astronomic latitude", the direction of apparent gravity (where the force vector points where you are). That may be (I wouldn't know). But the WGS84 system is referenced to an ellipsoid. Kayton says that national ellipsoids may differ, and there are conversion tables. The differences may amount to hundreds of feet. He also points out (p23) that any ellipsoid is appropriate for "the navigator" as long as there is a one-to-one mapping between its surface and the earth's surface (also p23).
I think you are right about the hobgoblin!
PBL
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PBL,
yes, and they do indeed go on using a ground speed as given by a doppler radar to derive Vnorth and Veast. Ground speed given by a doppler radar would in fact (more or less... subject to inherent errors and inaccuracies in the system) be the rate of change of position of the aircraft as projected orthogonally onto the geoid, as per my preferred definition above.
This also agrees with the initial use of ground speed for a ground-bound vehicle.
For all flights which are not very local in nature, I suspect that we can happily ignore the geoid vs. ellipsoid debacle as the errors introduced on the momentary ground speed will even out.
Interesting, this!
yes, and they do indeed go on using a ground speed as given by a doppler radar to derive Vnorth and Veast. Ground speed given by a doppler radar would in fact (more or less... subject to inherent errors and inaccuracies in the system) be the rate of change of position of the aircraft as projected orthogonally onto the geoid, as per my preferred definition above.
This also agrees with the initial use of ground speed for a ground-bound vehicle.
For all flights which are not very local in nature, I suspect that we can happily ignore the geoid vs. ellipsoid debacle as the errors introduced on the momentary ground speed will even out.
Interesting, this!
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Definitions are, by definition (), to be exact rather than simple.
So yes definitions are exact, but can represent something that is not exact at all!
The reason I'm saying not to overcomplicate it is because if you're talking about groundspeed of an aircraft, it's that damn simple! It's kind of stupid to bring it up on a forum like this because all the little nerdy/geniuses of this board LOVE to pick apart the simplest definition and make it the most complicated, and then in the end they find they've gotten nowhere! Believe me I like picking apart stuff too (partly the reason I joined), but I think groundspeed should be left alone!
The NASA "Muppets" use the simple definition....Of course I'm in awe of all the complicated equations
GS is a function of TAS, Wind vector, and nav fix displacement...easy...that's why we can't stop the leak
So much crap
did you know that dG = [dU+pdV+VdP] - [SdT+TdS]
it's true; but as written it's a little pedantic, as I have included all possible contributions
..of course it can be expanded a little too
Lester
GS is a function of TAS, Wind vector, and nav fix displacement...easy...that's why we can't stop the leak
So much crap
did you know that dG = [dU+pdV+VdP] - [SdT+TdS]
it's true; but as written it's a little pedantic, as I have included all possible contributions
..of course it can be expanded a little tooLester
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Folks, please excuse this slight digression.
italia, I for one have registered many times that you prefer one definition of ground speed. I don't know what it contributes to a discussion to have it repeated many times. What would contribute, to my mind, would be to have new, good, reasons for preferring it over another.
I don't think it is at all stupid. Indeed, I enjoy it! Being one of the "little nerdy/geniuses... [who] LOVE to pick apart the simplest[-seeming] definition", indeed having made much of a career doing that, I don't see anything wrong with it. Especially since I have seen the effects of what I do improve safety for some and derive appropriate compensation for not-so-lucky others.
Consider, the people whose views on "ground speed" we are discussing actually built the equipment you use on a daily basis in your flying. Some people think flying isn't half as much fun and full of expertise as it used to be, but the statistics say that it's noticeably safer!
PBL
Originally Posted by italia458
The reason I'm saying not to overcomplicate it is because if you're talking about groundspeed of an aircraft, it's that damn simple!
Originally Posted by italia458
It's kind of stupid to bring it up on a forum like this because all the little nerdy/geniuses of this board LOVE to pick apart the simplest definition and make it the most complicated, and then in the end they find they've gotten nowhere!
Consider, the people whose views on "ground speed" we are discussing actually built the equipment you use on a daily basis in your flying. Some people think flying isn't half as much fun and full of expertise as it used to be, but the statistics say that it's noticeably safer!
PBL
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I don't think it is at all stupid. Indeed, I enjoy it! Being one of the "little nerdy/geniuses... [who] LOVE to pick apart the simplest[-seeming] definition", indeed having made much of a career doing that, I don't see anything wrong with it. Especially since I have seen the effects of what I do improve safety for some and derive appropriate compensation for not-so-lucky others.
Consider, the people whose views on "ground speed" we are discussing actually built the equipment you use on a daily basis in your flying. Some people think flying isn't half as much fun and full of expertise as it used to be, but the statistics say that it's noticeably safer!
Consider, the people whose views on "ground speed" we are discussing actually built the equipment you use on a daily basis in your flying. Some people think flying isn't half as much fun and full of expertise as it used to be, but the statistics say that it's noticeably safer!
Oh yes I'm very sure these random ppl on here built my Garmin GPS and avionics suite from Rockwell Collins....
EDIT: Like I said before, I'm one of those nerdy/geniuses too and I enjoy topics that provoke thought, however, I still do retain a sense of reality and the relationship of concepts. Some of the greatest minds in the world have said the simplest things and those quotes are used all the time because of the truth they contain! Not everything has to be complicated PBL!
the thermodynamic sound velocity 'c' or 'a' is the slope of the isoentrope passing through -rho^2*c^2 and that is equivalent to [dp/dv]s
so c =[1/rho^2* [dp/dv]s}^1/2
but for flight this is more useful
http://www.faa.gov/library/manuals/a...apter%2015.pdf
so c =[1/rho^2* [dp/dv]s}^1/2
but for flight this is more useful
http://www.faa.gov/library/manuals/a...apter%2015.pdf
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Originally Posted by italia458
What is your career/job?
Originally Posted by italia458
Oh yes I'm very sure these random ppl on here built my Garmin GPS and avionics suite from Rockwell Collins....
I think there is somewhat of a disconnect here. There are people on PPRuNe who know as much about or more about what they write than any others in the world. A wise PPRuNe participant learns to distinguish such people from those who sit at home and participate in so-called flight-simulation games and pretend they are pilots. We have all types here. My tip is to try to distinguish who is who and not be rude to the wrong people
PBL