False echoes

Any large obstruction may reflect the radar beam, causing false echoes. The surface of the obstruction reflects a significant proportion of the transmitted energy at an angle, creating a false echo. Reflected signals from these objects reach the antenna and are presented on the bearing at which the antenna is pointing. The range of the false echo is the same distance (via the reflecting surface) of the object causing the false echo. It is also possible to have multiple false echoes at equal distances.

False echoesFalse targets (echoes) usually occur as a result of reflections originating from large structures such as other ships, a harbour building, storage tanks or wind farms etc.

Ownship structures can also generate similar reflections. These reflections are normally seen as a large arc on the radar screen. Adjusting the radar’s signal processing control functions may reduce or suppress a reflection, but at the expense of lower target detection performance. Radar systems provide techniques to prevent false targets resulting from previous transmissions (second-time-around echoes). Raymarine Pathfinder Radar has superior processing techniques to reduce these reflections.

Radar operators must make themselves aware of the bearings of obstructions which may produce false echoes.

Multiple echoesMultiple echoes

Multiple echoes can occur when another ship or vessel is passing on a parallel course at short range. The radar signal will be reflected back and forth between the actual target and ownship, resulting in multiple echoes being displayed beyond the range of the actual target. Multiple echoes always occur on the same bearing as the actual target and at exact multiples of the actual target’s range.

The false echoes become weaker as the amount of energy reflected diminishes with each return. Multiple echoes can be reduced and often removed by decreasing the Gain (sensitivity) or adjusting the Sea anti clutter control.

  1. Ownship.

  2. Actual target.

  3. Multiple echoes.

Side lobe echoesSide lobe echoes

Side lobe patterns are produced by small amounts of energy from the radar beam which are radiated outside of the narrow main beam. The effects of side lobes are most noticeable with larger targets at short ranges (normally below 3 NM). Side lobe echoes form either arcs on the radar screen, or a series of echoes on either side of the actual target, forming a broken arc. The appearance of side lobe echoes can be reduced using the Gain and / or Sea anti-clutter controls.

  1. Radar antenna.

  2. Side lobes.

  3. Main lobe.

  4. Arc.

  5. Actual target.

  6. Side echoes.

Inconsistent echoesInconsistent echoes

In built up areas and in narrow congested waters the radar beam may be reflected along a number of paths, producing confusing spurious echoes on the radar screen. Inconsistent echoes may not always appear in the same location and may not correlate. Adjusting the Gain control can minimize inconsistent echoes.

  1. Actual object.

  2. Radar antenna.

  3. Funnel.

  4. Inconsistent echo.

Ghost echoesGhost echoes

Ghost echoes can occur when the radar beam is reflected off of an obstruction, like a ship passing between the antenna and the actual target. This can cause a ghost echo of the actual target to appear on the bearing to which the antenna is pointing. The ghost echo will behave in the same manner as the actual target. However, because the antenna is not directed at the actual target, the returns from the ghost target will be weaker than those of the actual target. The range of the ghost echo will be the same as the range of the actual target. The ghost echo will appear on the radar screen at the same radius as the actual target. The VRM facility can be used to confirm this. However, there is no way of determining if the target is a ghost echo or the actual target.

Other types of ghost echoes include echoes of groups of targets (which appear to be real). When in the vicinity of land masses, these may be from large inland objects and may be caused by a combination of atmospheric conditions, unusual propagation conditions, and reflection.

  1. Actual target.

  2. Common radius.

  3. Ghost target.

  4. Passing ship or other obstruction.

Virtual imageVirtual image

A large target close to ownship may be represented at two positions on the screen. One of them is the actual echo directly reflected by the target, and the other is a false echo which is caused by the mirror effect of a large object on or close to ownship. For example, if ownship comes close to a large metal bridge, a false echo may temporarily be seen on the radar screen.