Radar Presentation 04

RADAR OPERATING CONTROLS -04-

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-Brillance control -Receiver gain control -Tuning control -Proper radar tuning -Sea and rain clutter -Factors affecting maximum range -Factors affecting minimum range -Factors affcting range resolution -Factors affecting bearing resolution 2

Brillance control Also called intensity, brightness -Determines the overall brightness of the PPI display -BC is adjısted to make the trace of the rotating sweep visible but not too bright -then it is adjusted so that the trace just fades -with too little brillance The PPI display is difficult to see, echoes may be missed -with excesive brillance the display is unfocused, discrimination and the accuracy of range and bearing measuremnt can be reduced 3

Receiver gain control -Sometimes referred to as the sensivity control -Adjusts the amounts by which all received echoes and receiver noise are amplified. -is adjusted until a speckled back graound just appear on the PPI -if the gain is too weakweak echoes may not be dedected -with excessive gain strong echoes may not be dedected because of the poor contrast between echoes and the background of the PPI display

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--This control is adjusted in accordance with the range scale being used. --When dedection at the maximum possible range is the primary objective , the receiver gain control should be adjusted so that a speckled background is just visible on the PPI

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Tuning control -the function of the Tuning control is to adjust the frequency of the receiver -if the receiver is only slightly mistuned, the extent of the bandwidth may still allow stronger echoes to be displayed -As the correct frequecy is approached, the brightness of the of the displayed echo response will increase, -it will peak at the correcr setting and decay after the setting has been passed. 7

FAST TIME CONSTANT (FTC) Also called DIFFERENTIATOR. -reduces the clutter on the PPI which might be cause by rain, snow or hail. -Reduces the receiver sensivity and the strenghts of the echoes as seen on the display

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Proper radar tuning 1- turn the radar antenna on 2- Turn the brightness control off 3- turn the sea clutter to zero –off 4- turn the rain clutter (FTC) to zero–off 5- set the gyro repeater,if fitted,to the same heading as the master gyro 6-set the brightness control so that a sweep becomes barely visible 13

7-Set the range ring intensity 8- set the headflash intensity-if adjustable (for 7and 8 they are not bighter than the dimmest target)

9- select either the three or six mile range 10-turn the gain control towards its most sensitive position. if you obtain a picture before reaching the end of rotation you may stop at that point. 11- adjust the tuning control for maximum target and noise display 12- reduce the gain setting untill only a small amount of background speckle remains 14

13- repeat steps 11-12 to insure that the tuning is peaked and that no further improvement ispossible. 14- adjust the STC control to permit viewing any targets which might be obscured by sea clutter. 15- adjust the FTC control to view any targets which might be obscured by rain

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Factors affecting detection, display and measurement of radar targets

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Factors affecting maximum range 1- frequency: *higher frequency, greater attenuation (loss in power) *Lower frequency, longer dedection range 2- Peak power: *Range capabilities of the radar increase with peak power(useful power) 3- Pulse length: Longer pulse length , greater range 4- pulse repetion rate: PRR determines the max measurable range of the radar. 17 Ample time must be allowed between pulses

5- beam width: the more concentrated the beam, the greater is the dedection range of the radar

6-target characteristics. large targets can be seen on the scope at greater ranges. Conducting materials(steel hull i.e.)return strong echoes. Nonconducting materials (wooden hull) return much weaker echoes 7-receiver sensivity: The more sensitive receivers providegreater dedection ranges but are more subject to jamming 18

8- antenna rotation rate the more slowly the antenna rotates, the greater is the dedection range of the radar.

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Factors affecting minimum range 1- pulse length: *the min range capability of a radar is determined primarily by the pulse length. *it is equal to the half the pulse length of the radar. * for the min dedection range, select shortest pulse length

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Ex:Calculate the theoetical min dedection range for a radar which has pulse length 0.1 microsec. PL/2=0.1/2 Dmin= (0.1/2)x300m= 15 m (calculate also for 1.0 and 0.5microsec.) Note: IMO per standarts: min range should be 50 m

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2- Sea return: sea return or echoes received from waves may clutter the indicator 3- Side lobe echoes: when operating near land or large targets, side lobe echoes may clutter the indicator and prevent dedection of close targets

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4- Vertical beam width: Small surface targets may escape the lower edge of the vertical beam when close.

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5- Ship structure shadowing

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Factors affcting range resolution Range resolution is a measure of the capability of a radar to display as seperate pips the echoes received from two targets which are on the same bearing and are close together. 1- Pulse length Two targets on the same bearing, close together cannot be seen as two distinct pips on the PPI unless they are seperated by a distance greater than one –half the pulse length.For high degree of RR short pulses must be used 25

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Example : A radar display has a screen diameter of 250 mm and a spot size of 0.5 mm. On the 12 mile and 1.5 mile range scales the pulse lengths are 1.0 and 0.05 microseconds respectively. Calculate the range discrimination on each of the given range scales.

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On the 12 mile range scale The range represented by one spot diameter is given by Rs = range scale in metres metres

= 12 x 1852/250 = 88.9

spot diameters per radius

The range represented by half pulse length is given by R P =pulse length in microseconds X 300

= 1.0/2x300= 150 metres

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Range discrimination = Rs + Rp

= 88.9 + 150 m = 238.9 metres 28

On the 1.5 mile range scale Rs =1.5 x (1852/250) =11.11 metres Rp=(0.05/2) x 300m = 7.5 metres Range discrimination = 11.11 + 7.5= 18.61 metres The best range discrimination can be obtained by selection of the shortest available pulse length

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2- receiver gain: Low receiver gain must be used for better range resolution

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3- CRT spot size The increase in echo image length and width varies wih the size of the CRT and the range in use

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Factors affecting bearing resolution Bearing resolution is a measure of the capability of a radar to display as separete pips the echoes received from two targets which are at the same range and are close together 1- horizontal beam width for better BR beam width must be narrow 2- range of targets 3- CRT Spot size

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IMO performance standarts : The equipment must be capable of displying the echoes seperately when the targets are seperated by not more than 2.5º in azimuth.

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In theory the radar can only discriminate between targets which are seperated by at least one horizontal beamwidth. In practice longation is further increased by the spot size. Influence of spot size will be less on larger screens and on shorter range scales. If the use of an off centered picture will allow the target to be displayed on a shorter range scale, the influence of the spot size can be reduced.

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Example: A radar display has a screen of diameter 250 mm and spot size 0.5 mm. If the horizontal beamwidth Is 1.5º, calculate the bearing discrimination at a range of 10 miles when the 12 mile range scale is selected and 1 mile when the 1.5 mile range scale is selected.

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At 10 miles: The lenght of the arc of the beamwitdh at 10 nm is given by

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Weather Reduces the ranges which targets can be dedected and produce unwanted echoes. Shorter wavelengths are More subject to the effect of weather then radars transmitting at longer wavelengths Wind: Sea return is nornmally greater in the direction from which the wind and seas are coming

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Precipitation: Masks targets by creating cluttered areas. Mask targets within the storm, heavy rain may absorb some strength of the pulse and decrease max.

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