Radar Presentation 03

TARGET DEDECTION

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The ability of the radar system to detect and display a given target depends on a large number of factors some of which are constant and others which may vary in quite a complex manner. The radar range equation is an expression which attempts to formalize the relationship between the maximum range at which a target can be detected and the parameters on which that range depends. 2

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Transmitter characteristics As might be expected, the ability to detect distant targets can be improved by using a more powerful transmitter . The maximum detection range varies as the fourth root of transmitter power. The power of transmitters designed for fitting to large vessels varies with manufacturer, but 10kW and 50kW are representative of low and high values. 4

Antenna characteristics The maximum detection range is a function of antenna gain and aperture area. Detection range varies as the square root of antenna gain. Thus aerial gain has a greater influence than transmitter power on long range performance. It may seem strange that wavelength does not appear in the equation. It is in fact implicit in the aerial gain and the target radar crosssection, both of which are functions of wavelength 5

Receiver characteristics Smin, the minimum detectable signal, is a function of receiver sensitivity. The receiver sensitivity, and hence the maximum range at Which targets can be dedected, is thus a function of the pulse lenght selected by the observer

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Target characteristics Energy in the pulse which is intercepted by the target is then available for return towards the antenna and hence to the receiver which is now in a receptive state. The amount of energy which is returned toward the antenna, as opposed to that energy which is absorbed and scattered by the target, is dependent upon the following five prime characteristics of the target.

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1- Aspect Aspect is the angle which the radar rays make with the plane of the mirror and, the response will be good when the aspect is 90° and poor at virtually all other angles.

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2-Surface texture The extent to which reflection is specular is dependent upon the surface texture of the target, i.e. whether the surface is 'rough' or 'smooth'.

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3-Material In general, materials which are good conductors of electricity also return good radar responses. This occurs as a result of absorption and re-radiation of the waves at the same wavelength as those received

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Some bodies absorb radiation but, when they reradiate, the wavelength is different from that at which it was received; still other bodies absorb radiation and re-radiate very little of the energy (this results in the temperature of the body rising, i.e. the received radiation is coverted to heat). Some materials are simply transparent to radar energy. GRP behaves to large extent in this way, steel will return good responses, while wooden boats generally produce poor responses.

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4. Shape

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5- Size The response is related to the area of the target irradiated by the beam (at any instant). This is not necessarily the same as the intrinsic size of the target. Since the radar beam is angularly wider in the vertical plane than in the horizontal, tall targets will in general produce stronger responses (all other factors being equal).

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Consider two targets presenting the same area to the radar; if the linear width of the horizontal beam at the range of the targets is equal to the linear width of target A then, in the case of B, only the small irradiated portion of the target will contribute to echo strength, while, in the case of A, virtually the total area will be irradiated.

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Responses from specific targets

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1 -Ice Large icebergs, such as those which are formed on the east coast of Greenland and drift down toward the North Atlantic shipping routes, have been found to give greatly varying radar responses. Detection ranges as great as 11 nautical miles have been experienced while, on the other hand, quite large icebergs have approached to within 2 n mile without being detected. Even the same iceberg may give greatly differing responses when viewed from different directions. 19

Strength of echoes returned from icebergs are only about 1/60 th of the strength of echoes which would be returned from a steel ship of equivalent size .

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The best use of radar under ice danger is: (a) a dedicated radar watch by one observer; (b) regular searching with the anti-clutter control on the short ranges remembering to check at frequent intervals on the longer ranges for larger targets; (c) use of the long pulse in weak clutter; (d) use of the longer wavelength of the S-band radar. 21

2- Radar-conspicuous targets Targets which are designated radar-conspicuous should be those which are known to provide good radar responses and are readily identifiable.

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3 -Ships The structure of ships is such that there are many natural 'corner reflectors' and hence, when a target vessel is rolling and pitching in a seaway, its echo strength does not vary quite as much as might be expected. Long vessels may appear as two or three individual echoes (each of which when tracked by ARPA might appear to be going in a slightly different direction). They may also be confused for a tug-and-tow or vice versa. Supertankers, because of their low freeboard, may not be dedected inordinately great ranges. 23

Target Enhancement-Passive It is essential that some targets which would normally provide poor radar responses, e.g. buoys, glass fibre and wooden boats etc, are detected at an adequate range by radar.

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1- Corner reflectors A corner reflector was seen as a simple device which would return virtually all of the energy which entered it, i.e , the energy would be returned in the direction from which it had come almost irrespective of the angle at which it had entered the corner

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Target enhancement –Active 1- The racon The ship’s radar pulse triggers the racon transmitter on the navigation mark, which then responds by transmitting a pulse (virtually instantaneously)

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2-The Ramark This is a radar beacon in which the frequency is swept continously at such high speed that the transmission at each frequency is in effect continuous (not triggered)

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Sources of radar beacon information The Admiralty List of Radio Signals Volume 2 contains information ralating to racons working in both the S-band and the X-band as well as ramarks.

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ALRS VOLUME 2

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Racons for survival craft To assist in the detection and identification of survival craft and other small craft in distress, the IMO have specified a Search and Rescue Transponder (SART) which is a coded racon and which will appear on the radar screen as a series of twenty dots. Since the racon flash will always appear as a radial signal beyond the target, it will immediately indicate the course to steer to the target.

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It is intended that the beacons should respond to radar equipment working in the standard marine X-band. A feature of the beacon is an audible or visible signal that lets the survivors know there is a craft in the area with operational radar. The increasing strength of the signal would indicate that the vessel using the radar is approaching and this is seen to provide a valuable psychological stimulus for survival. It is mandatory for certain classes of craft with the implementation of the Global Maritime Distress and Safety Service (GMDSS). 35

The radar horizon At marine radar transmission frequencies (nominally 10000 and 3000 MHz) , the paths followed by the signals nay be considered as 'line of sight', This means that even though the radar is delivering a powerful pulse and the target is capable, if irradiated, of returning a detectable response, the target will not be detected if it is below the radar horizon. This is analogous to the visual observation of objects in the vicinity of the horizon, The effect of the atmosphere on the horizon is a further factor which must be taken into account when assessing the likelihood of detecting a particular target and especially when considering the expected appearance of coastlines, 36

1 -The effect of standard atmospheric conditions Under standard atmospheric conditions, the radar beam tends to bend slightly downward, the distance to the radar horizon being given by the formula dnmile=1.22 √ h ft or dnmile=2.21√ hm where h is the height of the antenna in feet or metres

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The possibility of dedecting targets beyond radar horizon will, in addition to all the ıther factors that will be discussed in this chapter ,depend upon the height of the target.Thus the teorical dedection range based purely on the antenna and target heights is given by the formula. Rd = d+D Rd = 1.22 √ h ft +1.22 √ H ft Rd = 2.21 √ h m+2.21√ H m

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Where h and H are heights of antenna and target respectively in feet or metresIn both cases , Rd is the theorical dedection range in nautical miles. Rd is the theoretical detection range in nautical miles.39

This relationship is of course theoretical since it assumes that: (a) Standard atmospheric conditions prevail. . (b) The radar pulses are sufficiently powerful. (c) The target response characteristics are such as to return detectable responses. (d) The weather conditions, such as precipitation etc., through which the pulses have to travel, will not unduly attenuate the signals.

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Standard atmospheric conditions 'Standard' conditions are precisely defined as: Pressure=1013mb decreasing at 118mb/1000 m of height Temperature = 15°C decreasing at 6.5°C/1000 m of height Relative humidity = 60% and constant with height.

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Sub-refraction 1 The effects of sub-refraction on detection ranges (Figure 3.49) Sub-refraction Occurs when the refractive index of the atmosphere decreases less rapidly with height than under standard conditions. As a result, the radar beam is bent downward slightly less than under standard conditions. This means that, with an other factors constant, the same target will be detected at a slightly reduced range. In practice, this is likely to mean something of the order of 80% of the detection range under standard conditions but will obviously depend on the severity of the conditions prevailing at the time. 42

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Super refraction: Super refraction occurs when the rate of decrease in refractive index with height is greater than under standart conditions.When sub refraction occurs .the radar beam tends to be down slightly more and so targets may be dedected at ranges which are slightly greater than standart.Increase of some 40% are not uncommon. Atmopheric conditions associated with superrefraction are : 45

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Extra super –refraction or ducting Under these conditions , the radar energy is, in effect, trapped in a “duct” formed by the Earth’s surface and a highly refractive layer which may be as litle as 100 ft above the ground.The effect is concentration of energy. This increase energy will follows the Earth surface , thus reducing contraint of the radar horizon and considerably extending the dedection ranges of targets.

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If the rate of change of reflactive index is of the order of 4 times the standart rate, Extra super –refraction occurs. The areas which are normally associated with extra superrefraction are the Red Sea, the Arabian Gulf, the Mediterranean in the summer with the wind from the south, and the area off the west coast of Africa in the vicinity of the Canary Islands. However, extra superrefraction can occur anywhere if the conditions are right.

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The radarflare This is a rocket which is fired from a pistol. At some 400 metres altitude, the rocket ejects a quantity of dipoles which respond strongly to 3 cm radar waves and at the same time the rocket gives out a very bright white light.

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