The navigator must be able to recognize various abnormal echoes and effects on the radarscope so as not to be confused by their presence.
INDIRECT (FALSE) ECHOES
Indirect or false echoes are caused by reflection of the main lobe of the radar beam off ship’s structures such as stacks and kingposts. When such reflection does occur, the echo will return from a legitimate radar contact to the antenna by the same indirect path. Consequently, the echo will appear on the PPI at the bearing of the reflecting surface. This indirect echo will appear on the PPI at the same range as the direct echo received , assuming that the additional distance by the indirect path is negligible.
Characteristics by which indirect echoes may be recognized are summarized as follows:
- The indirect echoes will usually occur in shadow sectors.
- They are received on substantially constant bearings although the true bearing of the radar contact may change appreciably.
- They appear at the same ranges as the corresponding direct echoes.
- When plotted, their movements are usually abnormal.
- Their shapes may indicate that they are not direct echoes.
Side-lobe effects are readily recognized in that they produce a series of echoes on each side of the main lobe echo at the same range as the latter.
Semi-circles or even complete circles may be produced. Because of the low energy of the side-lobes, these effects will normally occur only at the shorter Ranges . The effects may be minimized or eliminated through use of the gain and anticlutter controls. Slotted wave guide antennas have largely eliminated the side-lobe problem
Multiple echoes may occur when a strong echo is received from another ship at close range. A second or third or more echoes may be observed on the radarscope at double, triple, or other multiples of the actual range of the radar contact .
SECOND-TRACE (MULTIPLE-TRACE) ECHOES
Second-trace echoes (multiple-trace echoes) are echoes received from a contact at an actual range greater than the radar range setting. If an echo from a distant target is received after the following pulse has been transmitted, the echo will appear on the radarscope at the correct bearing but not at the true range.
Second-trace echoes are unusual except under abnormal atmospheric conditions, or conditions under which super-refraction is present. Second-trace echoes may be recognized through changes in their positions on the radarscope on changing the pulse repetition rate (PRR); their hazy, streaky, or distorted shape; and their erratic movements on plotting.
A target pip is detected on a true bearing of 090° at a distance of 7.5 miles. On changing the PRR from 2000 to 1800 pulses per second, the same target is detected on a bearing of 090° at a distance of 3 miles. The change in the position of the pip indicates that the pip is a second-trace echo. The actual distance of the target is the distance as indicated on the PPI plus half the distance the radar wave travels between pulses.