4.2.12. Scale factor and along-track bias

Along-track errors in the measured velocity may be small, but when the ship’s speed is added, the resulting ocean velocity may show large errors aligned with the ship’s direction of motion when the ship is underway. These errors could result in ocean velocities that are always in the same direction the ship is travelling or in velocities that are opposite to the ship’s heading.

When the ship is underway and ocean velocities point in the direction of the ship’s motion, arises when the measured velocities are biased towards zero.

Underway bias can be due to a bad soundspeed correction. This can happen if (for example) the transducer well is filled with glycol. The ocean velocities will show a bias in the along-track direction, either ahead or behind the ship.