THE A SCOPE

3-2 to the origin at the antenna. Most radar displays include one or two of these quantities as coordinates of the crt face. The actual range of a target from the radar, whether on the ground, in the water, or in the air is known as SLANT RANGE. The majority of displays use as one coordinate the value of slant range, its horizontal projection (GROUND RANGE), or its vertical projection (ALTITUDE). Since slant range is involved in every radar situation, it inevitably appears in at least one display on every set. Slant range is the coordinate that is duplicated most often when more than one type of display is used. This is partly because displays presenting range have the highest signal-to-noise discrimination and partly for geometrical reasons. Range is displayed by means of a linear time-base sweep, starting from a given point or line at a definite time in each pulse cycle. Thus, distances along this range sweep actually measure slant range. The sweep speed determines the scale factor, which relates the distance on the tube to actual range. The sweep length is the total distance represented. Distances are expressed in miles (statute or nautical) or yards. The origin of the range sweep may be on or off the tube face. The angle at which the antenna is pointing, either in azimuth or elevation, may provide two- dimensional information in the display; that is, range and azimuth, or range and elevation. A radar indicator, sometimes called a radar repeater, acts as the master timing device in analyzing the return of the video in a radar system. It also provides that capability to various other locations physically remote from the radar system. Each indicator should have the ability to select the outputs from any desired radar system aboard the ship. This is usually accomplished by the use of a RADAR DISTRIBUTION SWITCHBOARD. The switchboard contains a switching arrangement that has inputs from each radar system aboard ship and provides outputs to each repeater. The radar desired is selected by means of a selector switch on the repeater. For the repeater to present correct target position data, the indicator must have the following three inputs from the selected radar: 1. Trigger timing pulses. These pulses ensure that the sweep on the repeater starts from its point of origin each time the radar transmits. As discussed earlier, the repeater displays all targets at their actual range from the ship based on the time lapse between the instant of transmission and the instant the target's echo is received. 2. The returning echo. The echo, in rf form, is detected (converted to a video signal) by the radar receiver and applied to the repeater. 3. Antenna information. The angular sweep position of a plan position indicator (ppi) repeater must be synchronized to the angular position of the radar antenna to display target bearing (azimuth) information. The three most common types of displays, called scopes, are the A-scope, the RANGE-HEIGHT INDICATOR (RHI) SCOPE, and the PLAN POSITION INDICATOR (PPI) SCOPE. The primary function of these displays will be discussed in this section. However, detailed descriptions will be limited to the ppi scope, which is the most common display. THE A SCOPE The A-scope display, shown in figure 3-1, presents only the range to the target and the relative strength of the echo. Such a display is normally used in weapons control radar systems. The bearing and elevation angles are presented as dial or digital readouts that correspond to the actual physical position of the antenna.