Linear Displacement

which the angle is to be measured. This plane is re- ferred to as the reference plane and maybe any plane convenient for use. The horizontal plane is one of the most commonly used reference planes because of the ease of using a spirit level or similar device to deter- mine the plane. In naval combat systems, the refer- ence plane is parallel to the reference element (the director) roller path. Whichever plane is selected for reference, it must be clearly specified before subse- quent measurements are of any significance. When the reference plane is used, a means must be esta- blished to denote the top of the plane to express the angle correctly. Train angles are measured clockwise from the reference direction on the top of the plane. Figure 3-3 shows a typical reference plane. of commission. In such an event, you must be able to use the target data from either of the directors to get a fire-control solution. To interpret data measured in different frames, you must know how the frames are Figure 3-3.—Typical reference plane. The elevation angle is in a plane perpendicular to situated with respect to each other. Figure 3-4 is representation of a complete reference frame. the horizontal reference plane and is measured from the reference plane. The concept of a reference frame is important in the expression of a direction and the problems related to alignment. The reference point is a definite point aboard ship, and the reference plane and the reference direction have a definite orientation in respect to the reference point. In fire control, it is often necessary to operate simultaneously with two or more reference frames. (For instance, these frames might be situated in dif- ferent parts of a ship.) It maybe desirable to measure target data with several directors because of a need for flexibility in controlling different fire-control systems to obtain a wide range of view or if one director is out Figure 3-4.—Representation of a complete reference frame. The difference or displacement between two frames may be of two kinds—linear and angular. Linear Displacement Linear displacement is the distance between two reference frames measured between their reference points. This displacement may be in the vertical direc- tion, the horizontal direction, or both. The corrections made necessary by the linear displacement between the reference points are called parallax corrections and are considered separately from corrections arising from the rotation between the frames. Parallax corrections can be separated into either dynamic parallax corrections or static parallax corrections. DYNAMIC PARALLAX CORRECTIONS.— Position quantities computed for the reference frame 3-3