Dynamic Adjustment of Moment Arms

Measuring moment arms or extracting them from the ship's drawing can be difficult. The athwartship component of the moment arm is usually the easiest measure and, in most cases, the antenna is centered across the ship. This leaves the vertical and the for/aft moments.

You can check these moments dynamically by observing ground targets in special ship or antenna configurations.

  1. Isolate the vertical component of the moment arm by scanning over the beam of the ship while looking at on-shore clutter targets.

    In this case, the velocity correction is dominated by the roll of the ship and the vertical moment arm. The athwartship position error of the antenna moment arm has the same effect. In this configuration, adjust the vertical moment arm in Antsetup until the observed velocity of ground targets is 0. The ship must be rolling during this test.

  2. Point the antenna fore or aft to observe clutter targets.

    Adjust the fore/aft component of the moment arm to remove any remaining velocity error. The ship must be pitching during this test.