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Catalogue of spare parts POL 2016

Catalogue of spare parts POL 2016

NORTHROP GRUMMAN SPERRY MARINE

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24.12.2013

RADIOLOCATION part 2

X-band and S-band 

When the weather is clear there is no  significant difference between the X-and S-bands. However, in  heavy rain radar operating in the S-band gives a slightly better  detection than radar operating in X-band.  

Radar resolution 

There are two important parameters that affect the overall resolution  radar capability: the resolution in azimuth and range resolution. 

Azimuth discrimination 

Azimuth discrimination - is the ability to show the radar  screen as separate tags echoes from two nearby targets  located at the same distance from the radar. It is proportional  to the antenna length and inversely proportional to the wavelength. Length radiate- ator antenna must be capable of resolving azimuth better than 2.5 ?  (resolution requirements IMO). For X-band usually this condition  is satisfied for the length of the radiator at least 1.2 meters (4 feet). For radars  operating in the S-range, the required length of the radiator is not  less than 3.6 m (12 feet). 

Range resolution 

Range resolution - the ability of radar  display on the screen as separate tags echoes from two nearby  targets located at the same relative bearing of radar. It is  defined only by the pulse duration. For example, if  the pulse duration of 0.08 ms resolution higher than 40 m . 
 
The test purpose, which are used to determine the resolution  capacity in azimuth and range, are  radar reflectors with reflection area of 10 m2. 

Accuracy bearing 

One of the most important characteristics of the radar is a precision  bearing determination on the target. Accuracy bearing mainly  depends on the width of a radar beam. However, bearing usually  measured relative to the direction of motion of the vessel, so that an important  factor in ensuring the accuracy of the azimuth alignment accuracy is  heading line during installation. To minimize the error  in determining the bearing to the target, set the echo from the target to the extreme  position on the screen by selecting the appropriate distance range. 

Range measurement 

Measuring the distance to the target is also an important function of the radar. In  general, there are two means of distance from the fixed  range rings and the movable ring marker (VRM). Fixed  range rings are displayed on the screen at a specified interval and  provide a rough estimate of the distance to the target. Diameter PKD  need to change so that it touched the inside edge of the goal, allowing  the operator to get a more accurate measurement of the distance. 

Minimum distance range 

Minimum distance range determined by the shortest distance,  which (using the scale range of 1.5 or 0.75 nautical  miles) with the aim of reflecting area of 10 m2 is not discharged to the point  the location of the antenna. 
Minimum distance range depends on the length of the pulse height of  the antenna and signal processing method (eg,  suppression of the main pulse and digital quantization). Use  smaller range scale, as they give better resolution and  clarity. Resolution MSC.192 (79) of the International Maritime  Organization requires a minimum distance range was not  more than 40 m radar this series satisfy this requirement. 

Maximum distance range 

 Maximum detection range, Rmax, varies greatly  depending on the height of the antenna above the target height  of the sea, the size, shape and material goals, as well as atmospheric conditions.  Under normal atmospheric conditions, the maximum range  distance equal to or slightly less than the radar horizon.  Radar horizon on Optical approximately 6%  due to the diffraction phenomenon of the radar signal. Rmax can be found  from the following formula. 
 
Rmax = 2.2 x (? h1 + ? h2)  
where, Rmax: radar horizon (nautical miles): 
   h1: antenna height (m) 
   h2: target height (m) 

 

For example, if the height of the antenna above the sea level of 9 m, and the target height of 16 m,  then the maximum range is the range: 
Rmax = 2.2 x (? 9 + ? 16) = 2.2 x (4 + 3) = 15.4 nm  
The detection range is reduced because of the influence of precipitation (which  absorbs radar signal). 

False echoes 

Sometimes echoes appear on the screen in places where there are no goals, or,  conversely, disappear, even if there is a purpose. These cases can be recognized,  if you understand their causes. Typical false  echoes are shown below. 

Multiply reflected echoes 

Multiply reflected echoes occur when the  transmitted pulse is reflected from an extended object, such as  a large ship, bridge, or breakwater.Echoes reflected two, three or  more times can be observed on the screen at a distance  greater than the actual distance to the target in two, three or more times,  see the figure below. Multiple reflection echoes can be reduced  or completely removed, reducing the gain setting (sensitivity) or properly adjusting function suppression from the sea. 

Echoes sidelobe 

Each time the transfer pulse of the radiation outgoing from the side  of the main beam is called 'sidelobe' diagram  of the antenna. If the target is positioned so that from  it reflected pulses as the main beam and sidelobe  antenna, we obtain more echo signals from the target  are located on the sides of a true echo in the same range.  side echoes usually appear at short ranges  range and from stable targets. They can be lessened by a smooth  gain reduction or adjust the noise reduction function from  the sea. 
 

Virtual image 

A relatively large target close your vessel can be displayed on  the screen in two positions. One image is a true  echo reflected directly from the target, and the second is  a false echo, which arose as a result of the effect  of specular reflection from a large object on his vessel or near (see Fig. below). If the vessel is too close, for example, a  metal bridge-like false echo may temporarily be  visible on the screen. 
 
 

The informal sector 

Chimneys, flues, masts and derricks  in the path of the radiation of the antenna unit, prevent  radar radiation. If the antenna is greater than the angle of shading  a few degrees, it may form a 'blind' sector. Within this  sector, the goal can not be detected. 
 
 

Interference from adjacent radar 

If another radar operating in the same frequency range, is close to his ship, the screen may show some noise. This interference is composed of a series of dots 
Various kinds of host. In many cases, these terms do not appear in the same regions of the screen, and can easily be recognized.
If closely spaced radars same model, the pulse repetition frequency which is practically the same. Therefore, noise may appear in the form of concentric circles. Suppression effect can be enhanced by selecting different pulse repetition frequencies of closely spaced radar.  

 


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