Range resolution
Range resolution is the ability to display as separate pips the echoes received from two targets which are on the same bearing and close to each other. This is determined by pulselength only. Practically, a 0.08 microsec- ond pulse offers the discrimination better than 35 m as do so with all FURUNO radars.
Test targets for determining the range and bearing resolution are radar reflectors hav- ing an echoing area of 10 m2.
Bearing accuracy
One of the most important features of the ra- dar is how accurately the bearing of a target can be measured. The accuracy of bearing measurement basically depends on the nar- rowness of the radar beam. However, the bearing is usually taken relative to the ship’s heading, and thus, proper adjustment of the heading line at installation is an important fac- tor in ensuring bearing accuracy. To minimize error when measuring the bearing of a tar- get, put the target echo at the extreme posi- tion on the screen by selecting a suitable range.
Range measurement
Measurement of the range to a target is also a very important function of the radar. Gen- erally, there are two means of measuring range: the fixed range rings and the variable range marker (VRM). The fixed range rings appear on the screen with a predetermined interval and provide a rough estimate of the range to a target. The variable range marker’s diameter is increased or decreased so that the marker touches the inner edge of the tar- get, allowing the operator to obtain more ac- curate range measurements.
2.2 False Echoes
Occasionally echo signals appear on the screen at positions where there is no target or disappear even if there are targets. They are, however, recognized if you understand the reason why they are displayed. Typical false echoes are shown below.
Multiple echoes
Multiple echoes occur when a transmitted pulse returns from a solid object like a large ship, bridge, or breakwater. A second, a third or more echoes may be observed on the dis- play at double, triple or other multiples of the actual range of the target as shown below. Multiple reflection echoes can be reduced and often removed by decreasing the gain (sensitivity) or properly adjusting the A/C SEA control.
True echo
Target
Own ship
Multiple echo
Figure 2-2 Multiple echoes
Sidelobe echoes
Every time the radar pulse is transmitted, some radiation escapes on each side of the beam, called “sidelobes.” If a target exists where it can be detected by the side lobes as well as the main lobe, the side echoes may be represented on both sides of the true echo at the same range. Side lobes show usually only on short ranges and from strong targets. They can be reduced through care- ful reduction of the gain or proper adjustment of the A/C SEA control.