Also called SEA CLUTTER CONTROL, ANTI-CLUTTER CONTROL, SWEPT GAIN, SUPPRESSOR.

Normally, the STC should be placed at the minimum setting in calm seas. This control is used with a circuit which is designed to suppress sea clutter out to a limited distance from the ship. Its purpose is to enable the detection of close targets which otherwise might be obscured by sea clutter. This control must be used judiciously in conjunction with the receiver gain control. Generally, one should not attempt to eliminate all sea clutter with this control. Otherwise, echoes from small close targets may be suppressed also.

Figures 11-24, 11-25, and 11-26 illustrate STC settings which are too low, correct, and too high, respectively.

Reproduced by Courtesy of Decca Radar Limited, London.

Figure 11-24.-STC setting too low.

Figure 11-26.-STC setting too high.

The performance monitor provides a check of the performance of the transmitter and receiver. Being limited to a check of the operation of the equipment, the performance monitor does not provide any indication of performance as it might be affected by the propagation of the radar waves through the atmosphere. Thus, a good check on the performance monitor does not necessarily indicate that targets will be detected.

When the performance monitor is used, a plume extends from the center of the PPI (see figure 11-27). The length of the plume, which is dependent upon the strength of the echo received from the echo box in the vicinity of the antenna, is an indication of the performance of the transmitter and the receiver. The length of this plume is compared with its length when the radar is known to be operating at high performance.

Any reduction of over 20 percent of the range to which the plume extends when the radar set is operating at its highest performance is indicative of the need for tuning adjustment. If tuning adjustment does not produce a plume length within specified limits, the need for equipment maintenance is indicated.

With malfunctioning of the performance monitor, the plume appears as illustrated in figure II-28.

inspecting their effects on the plume produced by the echo from the echo box.

Pulse Lengths and Pulse Repetition Rate Controls

On some radar sets the pulse length and pulse repetition rate (PRR) are changed automatically in accordance with the range scale setting. At the higher range scale settings the radar operation is shifted to longer pulse lengths and lower pulse repetition rates. The greater energy in the longer pulse is required for detection at longer ranges. The lower pulse repetition rate is required in order that an echo can return to the receiver prior to the transmission of the next pulse. At the shorter range scale settings, the shorter pulse length provides better range resolution and shorter minimum ranges, the higher power of the longer pulse not being required. Also, the higher pulse repetition rates at the shorter range scale settings provide more frequent repaintings of the pips and, thus, sharper pips on the PPI desirable for short range observation.

On other radar sets the pulse length and PRR must be changed by manual operation of controls. On some of these sets pulse length and PRR can be changed independently. The pulse lengths and PRR's of radar sets installed aboard merchant ships usually are changed automatically with the range scale settings.

The brightness of the heading flash is adjusted by control, labeled FLASHER INTENSITY CONTROL in figure 11-29. The brightness should be kept at a low level to avoid masking a small pip on the PPI. The heading flash should be turned off periodically for the same reason.

Electronic Bearing Cursor

The brightness of the electronic bearing cursor is adjusted by a control for this purpose. The indicator illustrated in figure 11-29 does not have the electronic bearing cursor feature. Unless the electronic bearing cursor appears as a dashed or dotted line, the brightness levels of the electronic bearing cursor and the heading flash should be different to serve as an aid to their identification.

Fixed Range Markers

The brightness of the fixed range markers is adjusted by a control, labeled FIXED RANGE MARK INTENSITY CONTROL in figure 11-29. The fixed range markers should be turned off periodically to avoid the possibility of their masking a small pip on the PPI.

Variable Range Marker

The brightness of the variable range marker is adjusted by the control labeled VARIABLE RANGE MARK INTENSITY CONTROL in figure II-29. This control is adjusted so that the ring described by the VRM is sharp and clear but not too bright.

Usually, ranges are measured by means of the variable range marker (VRM). In figure 11-29 the control for varying the setting of the VRM is labeled VARIABLE RANGE MARKER CONTROL, whereas in figure 11-30 the control is labeled RING SET. In figure 11-30 the notation indicates that the VRM on this radar set indicator can be used to measure ranges up to only 20 miles although the maximum range scale setting is 40 miles. For distances greater than 20 miles, the fixed range rings must be used with this indicator.

The radar indicators designed for merchant ship installation have range counter readings in miles and tenths of miles. According to the range calibration, the readings may be either statute or nautical miles. The range counter has three digits, the last or third digit indicating the range in tenths of a mile. As the VRM setting is adjusted, the range is read in steps of tenths of a mile. The VRM control may have coarse and fine settings. The coarse setting permits rapid changes in the range setting of the VRM. The fine setting permits the operator to make small adjustments of the VRM more readily. For accurate range measurements, the circle described by the VRM should be adjusted so that it just touches the inside edge of the pip.