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In INS (Inertial navigation system) sensors, is there a distinction between the following?

  • Orientation (heading, pitch and roll)
  • Position (latitude, longitude and altitude)

Is the orientation relative to a specific axis while the position is absolute? If yes, what is the axis?

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Yes there is a distinction, pitch, roll and heading are angles [unit = radian] and latitude, longitude and altitude are linear distances [unit = meter]. Together, these six parameters define the six Degrees of Freedom that define the momentary state of a rigid object with reference to the earth.

All six are defined relative to earth axes. For the linear dimensions this is clear, we just need to define a suitable zero point. For the angles:

  • Pitch is the nose up/down angle relative to the gravity field of the earth.
  • Roll is the wingtip up/down angle relative to the gravity field of the earth.
  • Heading is the angle relative to a suitable earth reference, usually magnetic north or geographical north.

Conversion of aircraft axes into earth axes is a non-trivial matter and the cause of much confusion, for instance in questions comparing lift to weight: lift is always coupled to aircraft axes, weight to earth axes. The axes sets are defined as follows:

Aircraft axes

  • Origin: Centre of Gravity of aircraft
  • X-axis: in the symmetry plane of the aircraft, + = forward to the aircraft nose
  • Y-axis: in the span wise direction of the aircraft, + = to the right wingtip
  • Z-axis: perpendicular to the XOY-plane, + = down into the floor.

Earth axes

  • Origin: Centre of Gravity at the start of the response
  • X-axis: In the horizontal plane (relative to gravity), + = pointing north.
  • Y-axis: Perpendicular to XOZ-plane, + = pointing east.
  • Z-axis: Pointing to the centre of the earth = +

Edit

Yes navigation lat/lon is given in [degrees] - the INS sensor ultimately converts displacement into proper navigational definitions of course. But for measuring 3-D aircraft inertial effects the SI system is used, and displacement is internally treated as a linear motion, units [metres].

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  • $\begingroup$ I think Yaw should be the third point... Along with Pitch and Roll, it forms the 'Big-3' Rotations of an aircraft, or the 3 Degrees of Freedom... : ) $\endgroup$ – Anand S Nov 15 '17 at 7:50
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    $\begingroup$ Yes the aircraft pitches, rolls and yaws - but relative to what? $\endgroup$ – Koyovis Nov 15 '17 at 7:58
  • $\begingroup$ I think you misunderstood - I meant replacing the 'Heading' point with 'Yaw', as it sounds more complete... : ) $\endgroup$ – Anand S Nov 15 '17 at 14:57
  • $\begingroup$ Down is positive Z? That seems backwards. $\endgroup$ – Monty Harder Nov 15 '17 at 18:51
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    $\begingroup$ I hate to be pedantic, but latitude and longitude, are angles not linear distances...just angles relative to the centre of the earth though the Greenwich Meridian. $\endgroup$ – Trevor_G Nov 15 '17 at 18:56
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Position is where you are; orientation is which way you're pointing (including any roll).

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  • $\begingroup$ Changing the word "pointing" to "facing" includes roll (in my mind at least) $\endgroup$ – jdkorv11 Nov 15 '17 at 14:57
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    $\begingroup$ I've added "including roll" because I agree that's not included in "pointing" and I don't think it's implied by "facing", either. $\endgroup$ – David Richerby Nov 15 '17 at 15:02
  • $\begingroup$ If you twist your hand while pointing, you have roll covered as well. $\endgroup$ – Koyovis Nov 17 '17 at 1:23
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"position" is absolute. You can think of it as the x, y, z coordinates of the aircraft in the 3D space.

"orientation" is the state of the aircraft relative to its own axes:

enter image description here

Image from https://en.wikipedia.org/wiki/Aircraft_principal_axes

"position" (coordinates) - https://en.wikipedia.org/wiki/Geographic_coordinate_system

orientation - https://en.wikipedia.org/wiki/Aircraft_principal_axes

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    $\begingroup$ Sorry, but position is never absolute, and orientation is not a state, but a set of angles. $\endgroup$ – bogl Nov 15 '17 at 8:24
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    $\begingroup$ @bogl An object's orientation in a specific point of time definitely describes its state. $\endgroup$ – DeepSpace Nov 15 '17 at 8:28
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    $\begingroup$ A state is a complete description of a system. The position, orientation, speed vector, and angular speed vector together describe the state of a rigid body. Orientation alone is far away from completely describing the state of a plane. $\endgroup$ – bogl Nov 15 '17 at 8:35
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    $\begingroup$ @DeepSpace: No, it's 100% correct, because when the plane moves (pitches, rolls, yaws), its axes move too. $\endgroup$ – psmears Nov 15 '17 at 13:55
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    $\begingroup$ @DeepSpace: I'm not being picky - sorry if it seems that way - but this is an important point. You can express change in the aircraft's orientation relative to its axes, but you can't usefully specify its current orientation that way - you need to relate it to some other, external reference. To clarify: can you give an example of how would you give the current orientation of an aircraft relative to its own axes? $\endgroup$ – psmears Nov 15 '17 at 14:14

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