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  • GPS Errors

    I have noticed that a number of planes appear to approach LHR way off the expected track. Am I correct in thinking that they are correctly on the intended path but their GPS is feeding an error to their ADS-B transmitter?
    The planes mostly seem to be DAL coded. All are at the end of a long distance flight so could the error be GPS drift?
    What does this mean if the same data is being used for Collision Avoidance purposes?
    More Collisions?

  • #2
    Hello,
    These shifted trajectories are in fact transponders broadcasting IRS positions. Due to the drift of IRS, these positions are shifted after several hours of flight.
    The aircraft associated to this bad positionning should be "old" planes such as 1st A320 or A310,....
    They are on the right path don't worry :-)

    GPS does not drift in the time.

    And don't worry about avoidance purposes, this function is ensured by TCAS only, ADS-B data does not contribute to this safety purpose. So no more collisions :-)

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    • #3
      Hi,
      Shouldn't IRS be re-aligned with a help of VORs once an aircraft goes from Atlantic into British airspace?

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      • #4
        I think the problem is caused by the Inertial Navigation Systems used in older aircraft and which suffer from "drift":

        "All inertial navigation systems suffer from integration drift: small errors in the measurement of acceleration and angular velocity are integrated into progressively larger errors in velocity, which are compounded into still greater errors in position. Since the new position is calculated from the previous calculated position and the measured acceleration and angular velocity, these errors are cumulative and increase at a rate roughly proportional to the time since the initial position was input. Therefore the position must be periodically corrected by input from some other type of navigation system. The inaccuracy of a good-quality navigational system is normally less than 0.6 nautical miles per hour in position and on the order of tenths of a degree per hour in orientation."
        Mike


        www.radarspotting.com

        Radarspotting since 2005

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