RADAR Radio Detection and Ranging ships


Radar( Radio Detection and Ranging ) is a bridge equipment which is perfectly synchronised and gives the bearing and range of objects in vicinity. Radar transmits electromagnetic waves and these waves hit objects in range and come back to the radar. Then Radar filters and processes the recieved signal and make them useful information. 

Radar helps Mariners to fix position in coastal waters , determining collision avoidance action, Maintaining lookout in restricted visibility and at night , detecting storms at sea and so on. 

A Marine Radar consists of 4 units:
. Transmitter unit 
. Scanner unit
. Reciever unit
. Display unit

Working Principle of Radar :
The transmitter unit generates electromagnetic waves and send them out as powerful bursts. These are called pulses. These pulses are then sent to the scanner unit . Scanner unit sends these pulses out a specific number of times per second. This frequency is called the pulse repetition frequency (PRF) . Pulses which are sent out travels at a speed of light ( 3*10^8 m/s ). The pulses then hits any object and gets reflected back towards the  scanner as echoes. The reciever unit processes each of the echoes and shows the position of the object as a blip on the display .

Range = C*T/2
          Where,
          C= 3*10^8 m/s
          T/2- Time taken for the pulse to one               side ( Total time taken divided by                   two)
        
Parts of Radar :
. Power source : AC input is required. It should be from the ship's mains or convert to AC by a transformer if mains are DC.

. Delay line : AC power source received is stored by the delay line.

. Trigger unit : This unit sends spike wave signals to the modulator , Time base unit and trace blanking unit. Spike waves are interrupted direct current waves whose pulse length is very small. Current goes from maximum to minimum very quickly in the spike waves. These waves sent by trigger unit in the same frequency as the pulse repetition frequency (PRF).

. Modulator : This part switches on and switches off Magnetron as required. Each spike waves from trigger line makes the modulator to release one powerful DC pulse from the delay line to the magnetron.

. Magnetron : It is high power radio frequency oscillator which switches on switches off in desired PRF, by the pulses from the modulator. Electro magnetic waves are created by the magnetron in the desired PRF and are sent through the scanner.

. Scanner : Scanner sends the pulses out and recieves echoes at the same time. It rotates at a constant speed and scans the area around regularly. Marine scanners are slotted waveguide type.

. Local oscillator : It oscillates at a low power radio frequency of about 30 to 60 HZ below or above the magnetron frequency .This frequency is called the intermediate frequency.

.  Mixer : It mixes the echoes with the local oscillations and the echoes are reduced from radio frequency to intermediate frequency. It is then sent to amplifier.

. IF and Video amplifier : IF amplifier amplifies the echoes of intermediate frequency several million times and passes to the video amplifier. Video amplifier then controls the amplification of signals and is fed to the CRT.

. CRT : Visual display of all the targets in range is given by the CRT. It is given like a bird's eye view. Hence it called a plan position indicator. 

. Time base unit : This unit helps in showing , as the scanner is rotating on the Plan position indicator (PPI). It is timed and matched the Pulse repetition frequency. Hence each time scanner is rotating, at the same rate it is shown as rotating in the PPI.