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    Re: gyroscopic compasses
    From: Nicol�s de Hilster
    Date: 2007 Jun 24, 11:39 +0200

    George Huxtable wrote:
    > ... the plane of
    > the Earth's surface, at the position of the navigator, is changing, all the
    > time, for two reasons; because of the rotation of the Earth, and because of
    > the motion of the observer with respect to the surface of the Earth. So, any
    > such laser "gyro" would need to have some sort of knowledge of these
    > quantities. In a similar way, a ship's gyroscopic compass needs to have some
    > sensor of the local horizontal, and have correction factors applied which
    > allow for the vessel's motion.
    > I imagine that in the case of a laser "gyro", this information would be
    > provided by an interface with some form of GPS receiver, with a sensor of
    > the direction of gravity.
    The Octans not only measures the heading, pitch and roll, or rather the
    rotations of the X, Y and Z axis, but also the acceleration along these
    same axis. This makes the apparatus ideal for inertial navigation. In
    the past I did tests with the Octans to find out the stability of the
    pitch and roll measurements. Most motion sensors have problems when
    exposed to lateral accelerations. These accelerations would normally
    induce an amount of pitch and/or roll. The Octans however also measures
    the rotations of the pitch and roll axis and therefore knows the
    difference between the earth gravity and applied acceleration. The test
    was quite simple: the Octans was laid on a flat surface (a table) and
    then swung fore and backwards very quickly, as fast as one could move
    these 4.8kgs by human power. There was no change in pitch and roll at
    all, very impressive (the representative of the manufacturer turned
    quite pale when I did that ;-)).
    For the speed of the Octans one could rely on inertial calculation, but
    that is not the way the Octans handles it. If not corrected for the
    speed will induce an heading error equal to V/5PI x secants(latitude),
    where V is the North component of the speed in knots. There are two
    options: either a fixed value for the speed is entered into the on-board
    software, or the speed, calculated by a GPS-receiver, is sent to the
    device. The same applies for the latitude. The Octans also needs
    latitude for optimum performance (The heading error varies with 0.045
    degrees at 0 degrees latitude to -0.02 degrees at 90 degrees latitude).
    This value can also be entered manually or by sending a NMEA message
    from a GPS-receiver to the device.
    As far as I know the Octans is a downgraded device that originates from
    military use, so I wonder how well the original device performs...
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