radar presentation 05

8/14/2019 Radar Presentation 05

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5

False and UnwantedRadar Responses


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Sometimes echoes are displayed on the screen in

positions where no genuine targets exists.


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1- 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


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Where echoes are suspected of being false,

they should be assumed to be real until

proved false beyond all reasonable doubt.


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False echoes from bridges


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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 theenergy 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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1- 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 operatewithin 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 andantenna 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.


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5- Second trace echoes.

Under conditions of extra refraction the radar energy

follows closely the surface of the Earth and travels togreater 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 butobviously at an incorrect range.


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6- False echoes from power cablesIt has for some time been recognised that electromagnetic

waves can react with the electromagnetic field surroundinga 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


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the cable is at right angles to the channel, the false echowill 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.


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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 aconverging course.


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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 ona 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.


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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 portmanoeuvre 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 inthe vicinity of the cable may thus be associated with the

cable and should be treated with due caution.


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