Thanks for the physics - the definitions may be helpful to someone.
However, at any given moment, INSTANTANEOUSLY if you will, the output of a three-axis accelerometer system can be fed to a Heads Up Display (HUD) computer. This INSTANTANEOUS output consists of speed and summed 3 axis direction (velocity, velocity vector) only, until the next speed and direction as a result of any ongoing acceleration is processed.
Can you see your error here? In the first sentence you (correctly) state that the IRS accelerometers measure acceleration. In the second sentence you say that this is
velocity, and it isn't - it is acceleration (as you said).
If the accelerometers measure a change from level flight to a descent (a change in the direction of the aircraft = an acceleration), then a new velocity is
calculated from that
measured acceleration (and that
calculated velocity can be shown on either HUD or PFD). If the accelerometers then measure another change in the velocity, and
calculate from that measured acceleration that the aircraft has levelled out, that can also be shown on the velocity vector - however if there are any errors in any of the
measured accelerations, there will be a consequent error in the displayed vector - the IRS has no way of measuring
directly if the aircraft is actually flying level again
without referring to barometric data. Hence the baro refinement in the vector, and the notes in both the Boeing and Airbus manuals that altimetry errors affect the FPV and VS.
However, there ARE other systems whose IRS position and local level can be updated after the original IRS, INS, or IMS alignment while in-flight.
Yes, all of the IRS platforms I have flown can be updated in flight - but it isn't usually (or automatically) done.
And, there is NO new data input, baro or otherwise, to inertial accelerometers, they are what they are. Mechanical platforms can be torqued in-flight in some systems and existing local level errors refined, however nothing is changed physically in the accelerometers, only their orientation. RLG's don't need this, no moving parts anyway.
I never said this - the barometric refinement isn't to the IRS - it's to the system's calculation of the FPV.
Trust me.