# What are these acceleration parameters produced by the Inertial Reference System?

Following acceleration parameters are transmitted from Inertial Reference System (IRS) to Flight Control System (FCS):

• Flight Path Acceleration
• Along Track Acceleration
• Cross Track Acceleration
• Vertical Acceleration
• Unbiased Normal Acceleration

I only know acceleration based on the aircraft axis i.e lateral, longitudinal & Normal acceleration but what these acceleration parameters signifies?

• Do you have specific IRS and FCS in mind? Or a link from where these names come? I suspect there are alternate names for each of them. Mar 6, 2014 at 17:38
• "track" is the direction the plane is flying, "heading" is direction it is facing (they differ due to wind and side-slip). The track ones would certainly be in horizontal plane, but I am not sure whether the heading ones would be in horizontal plane or aircraft plane. The ones in aircraft plane seem to be more useful, but those would be better called longitudinal and lateral. I would guess unbiased normal means normal with gravity subtracted. But I have no idea what the flight path one might be; my only idea was vertical, but that is a separate item. Mar 6, 2014 at 19:39
• Looks like they came from here. Mar 7, 2014 at 5:32
• @Jan Hudec: Yes Specific FCS & IRS but cannot name the project. Longitudinal, lateral & Normal accelerations are transmitted by IRS along with these all accelerations. I think these accelerations may be somewhere computed from GPS in IRS and transmitted to FCS Mar 7, 2014 at 13:54
• @ToUsIf: The primary source of acceleration in IRS is the accelerometers and gyroscopes. GPS is only used to correct drift of the position calculated by dead reckoning, that is integrating the velocity calculated by integrating the accelerations. This is much more reliable as it can't be affected by basically any outside factors and has better temporal resolution. GPS only gives 1 position a second, which is good enough for navigation, but not AP or FBW. Mar 7, 2014 at 14:41

I can't be sure without seeing actual documentation of the thing, but I would expect:

• Flight path is direction of the total velocity vector, i.e. the direction the plane is flying, in Earth-relative frame of reference. This differs from direction of longitudinal axis by alpha (angle-of-attack), beta (side-slip) and wind.
• Track is projection of flight path to horizontal plane.
• Heading is projection of the longitudinal axis to horizontal plane.

Then:

• flight path acceleration would be simply acceleration along the flight path
• along track and across track are, well, along and across track as defined above.
• vertical is vertical; should not include gravity, so when maintaining vertical speed it should be 0.
• unbiased normal acceleration is most probably what is also called inertial normal acceleration, that is acceleration without gravity.
• along and across heading are, well, along and across heading as defined above.
• This is almost correct, except for two points. Wind has no bearing on the difference between the longitudinal axis and the flight path; only alpha and beta do. "Flight path" means the path of flight through the air. Also, "vertical acceleration" probably means acceleration along the local gravitational vector ("up"). "Normal" means perpendicular to the flight path. These two are only the same if the aircraft is at zero pitch and roll. So an airplane doing a loop at constant airspeed, at the moment of 90° pitch, has positive normal acceleration but zero vertical acceleration. Mar 7, 2014 at 19:22
• @dvnrrs: At least in Airbus there is a "flight path angle" mode in autopilot and this definitely means flight path angle relative to Earth, not wind. Mar 7, 2014 at 19:31
• Further comment: the reason for all these different definitions/measurements is ease of consumption by the flight guidance system. The designers of the "control laws" for autopilots use these values in various ways as feedback, as a way to detect and reject spurious commands due to turbulence, etc. Each control law and each situation requires a slightly different parameter. The "norm" is for the data producer (AHRS, IRS, whatever) to do all the number crunching so that all the FGC has to do is select and use whatever value's appropriate for the situation. Mar 7, 2014 at 19:35
• @dvnrrs: And the more important argument, IRS has no clue about anything relative to wind. IRS is inertial reference system, which can measure position, velocity and acceleration to inertial reference frames, which Earth is to sufficient approximation. It is calibrated relative to Earth during start up and thus all the values are relative to Earth - or the plane, but never the wind. Mar 7, 2014 at 19:36
• @dvnrrs: Would you then be so kind and formulate full answer? You seem to be more knowledgeable than me. Mar 7, 2014 at 19:43