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Block diagram of Marine Radar

A radar system operates in a way that it radiates electromagnetic energy into space and detects various aspects related to objects by analyzing the echo generated when the radiated energy gets re-radiated by the object. The figure below shows the basic principle of radar:  

Let us now discuss how radar operates:

Transmitter Section: The transmitter section is composed of the following units:
1. Waveform Generator: The waveform generator (usually a magnetron) generates a radar signal at low power which is to be transmitted into space.
2. Transmitter: The signal generated by the waveform generator is fed to the transmitter. The transmitter section can be a magnetron, traveling wave tube, or transistor amplifier. In the case of pulse radar, magnetrons are widely used as transmitters but whenever there exists a need for high average power then amplifiers are used.
3. Pulse modulator: A pulse modulator is used to build synchronization between the waveform generator and transmitter. The pulse modulator causes the turning on and off of the power amplifier according to the input pulses generated by the waveform generator.
4. Duplexer: A duplexer is used to form isolation between the transmitter and receiver section. A duplexer allows the use of a single antenna for both transmission and reception purposes. However, both sections operate at different power levels, therefore, a duplexer is used to isolate the two sections. Thus the signal from the transmitter is provided to the antenna through the duplexer. As the duplexer short circuits the input of the receiver section. Also, the re-radiated signal received by the common antenna is fed to the receiver section using a duplexer.

Receiver Section
: The following components are present inside the receiver section:
5. Low noise RF amplifier: The receiver must be superheterodyne. The unit acts as the input stage for the receiver section. The RF amplifier generates an RF pulse which is proportional to the echo of the transmitted signal.
6. Mixer and Local Oscillator: The RF pulse received from the low-noise RF amplifier is converted into an IF pulse. Usually, the RF amplifier acts at the input stage of the receiver section but sometimes the mixer acts at the input stage by eliminating the RF amplifier. But this leads to a less sensitive receiving section due to the high noise figure of the mixer.
7. IF amplifier: The IF pulse generated by the mixer circuit is amplified by the IF amplifier. It acts as a matched filter and increases the SNR of the received signal. Also, it enhances the echo-detecting ability of the receiver section by reducing the effects of unwanted signals. The receiver’s bandwidth is associated with the bandwidth of the IF stage.

8. 2nd Detector or Demodulator: This unit is nothing but a crystal diode that performs demodulation of the signal by separating the transmitted signal from the carrier.

9. Video Amplifier: This unit amplifies the received signal to a level that can be displayed on the screen.

10. Threshold decision: This unit decides the existence of the target in space. It has some threshold limit set which is compared with the magnitude of the received signal. If the threshold value is surpassed by the output signal, then this shows the presence of the target. Otherwise, it is assumed that only the noise component is present in the space.
11. Display: The display unit shows the final output of the receiver section. 

PPI i.e., plan position indication is typically used as the radar display unit. It presents the range and location of the object by mapping it in polar coordinates. PPI is implemented with CRT. The output signal modulates the electron beam of the cathode ray tube to permit the electron beam to sweep from the center in the outward direction of the tube. And this sweep shows rotation in synchronization with the pointing of the antenna.

Radar (1)

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