The term radar is an acronym made from the words radio, detection, and ranging. It refers to electronic equipment that uses reflected electromagnetic energy to determine the direction to, height of, and distance of detected objects. Electromagnetic energy of the frequency used for radar is unaffected by darkness. However, it can be affected by weather to some degree, depending on its frequency. It permits radar systems to determine the positions of ships, planes, and land masses that are invisible to the naked eye because of distance, darkness, or weather. Radar systems provide only a limited field of view and require reference coordinate systems to define the positions of detected objects. Radar surface angular measurements are normally made in a clockwise direction from true north, as shown in figure 1-1, or from the heading line of the ship or aircraft. The radar is located at the center of this coordinate system.

Table 1-1 defines the basic terms used in figure 1-1. You must know these terms to understand the coordinate system.

Figure 1-1.-Radar surface angular measurements.

Table 1-1.-Radar Reference Coordinate Terms