Radar Presentation 05

5 False and Unwanted Radar Responses

Sometimes echoes are displayed on the screen in positions where no genuine targets exists.

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Indirect echoes (reflected echoes)

These can occur when radar energy is deflected in the direction of anobject by some obstructions in the path of the radiated energy, either on board the ship or ashore. The returning energy follows a reciprocal path and so causes an echo to be displayed in the direction of the obstruction

Where echoes are suspected of being false, they should be assumed to be real until proved false beyond all reasonable doubt.

False echoes from bridges

2Multiple echoes ultiple echoes are likely when a target is close and energy bounces back and forth between the hulls of the target and the observing ship, with some of the energy entering the antenna at each return (Figure 3.59). The features of this form of response are that the echoes: a) Lie along a single direction. b) Are consistently spaced.

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Side echoes:

Side echoes are again associated with targets that are at close range and result from the radar beam being surrounded by smaller beams or lobes. Some of the echoes will appear to be separate but all will be at the same range, i.e. as if all were lying on the same range circle. This phenomenon is generally associatedwith smaller antennae and those which are dirty or damaged.

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Radar- to- radar interference

All civil marine radar systems are required to operate within a fairly narrow slot of approximately 200 MHz allocated in the X-band or S-band. When it is considered that the receiver bandwidth of a marine radar system may be as much as 20 MHz, and given the high power and antenna height of a shipboard system, it is obvious that, except in mid-ocean, there is a very high probability of receiving interfering radiation from other vessels in the vicinity which are operating radar equipment. If the radiation received is within the limits of the receiver bandwidth the signals will be amplified in the same way as those reflected from targets and will produce a visible response on the display.

5- Second trace echoes. Under conditions of extra –refraction the radar energy follows closely the surface of the Earth and travels to greater distances than under standart conditions. This means that echoes from distant targets can arrive back at the receiver one trace late. (i.e. on the second trace) or even later, be accepted by the receiver and so be displayed but obviously at an incorrect range.

6- False echoes from power cables It has for some time been recognised that electromagnetic waves can react with the electromagnetic field surrounding a cable carrying a current in such a way that a false echo appears on the radar display. The false echo so produced will appear in the direction of the perpendicular from the vessel to the power cable and at the range of the cable (Figure 3.67). Unfortunately the actual power cable itself does not produce a response and so it can be very difficult to associate the observed echo with the cable and thereby have some indication that the echo may be false. Where

- the cable is at right angles to the channel, the false echo will appear in the channel so that, however the vessel manoeuvres in the channel to avoid it, the false echo will always move into the vessel's path.

Where the cable is angled across the waterway, the false echo may initially appear among the shore echoes and so go unnoticed, but as the vessel approaches the cable, the false echo will appear on the water as if from a vessel on a converging course.

Consider the situations illustrated in Figures 3.67(b) and 3.67(c). As the vessel approaches the cable, a 'vessel' would appear to put out from the starboard bank and proceed on a collision course. Any attempt by the observing vessel to pass under the 'stern' of the false echo would cause the false echo to return toward the bank, i.e. again into the vessel's path. If the observing vessel stopped, the 'target' would also appear to stop.

In Figure 3.67(c), the false echo would put out from the port bank, again on a collision course. Here the logical manoeuvre would be for the observing vessel to move farther over to the starboard side of the channel. The false echo would continue to crowd the observing vessel into the starboard bank. The result of stopping or a port manoeuvre would be as described above. On some waterways, power cables have had radar reflectors fitted to them in order that their line will appear on the radar display. The unusual behaviour of echoes in the vicinity of the cable may thus be associated with the cable and should be treated with due caution.