Radar systems - MTI Radar
If radar is used to detect the moving target, then the radar should only receive the echo signal due to this moving target. This echo signal is the desired one. However, in practical applications, the radar receives the echo signals due to stationary objects in addition to the echo signal due to this moving target.
Echo signals due to fixed objects (places) such as land and sea are called congestion because they are unwanted signals. Therefore, we have to choose the radar in such a way that it only considers the echo signal due to the moving target but not the congestion.
For this, Radar uses the principle of the Doppler effect to distinguish non-stationary targets from fixed objects. This type of radar is referred to as a moving target indicating radar or simply MTI Radar .
According to Doppler effect , the frequency of the signal received will increase if the target moves in the direction of the radar. Likewise, the frequency of the received signal will decrease if the target moves away from the radar.
Types of MTI radars
We can classify MTI radars into the following two types depending on the type of transmitter used.
- MTI Radar with Power Amplifier Transmitter
- MTI Radar with Power Oscillator Transmitter
Now let's talk about these two MTI radars one by one.
MTI Radar with Power Amplifier Transmitter
MTI Radar uses a single antenna for the transmission and reception of signals using the duplexer. The block diagram of the MTI radar with transmitter power amplifier is shown in the following figure.
The function of each MTI radar block with amplifier transmitter power is below.
- Pulse modulator - Itproduces a pulse modulated signal and is applied to the power amplifier.
- Power Amplifier - It amplifies the power levels of the pulse modulated signal.
- Local oscillator - It produces a signal with a stable frequency $ f_l $. Hence, it is also called a stable local oscillator. The output of Local Oscillator is applied to both Mixer-I and Mixer-II.
- Coherent Oscillator - It produces a signal with an intermediate frequency, $ f_c $. This signal is used as a reference signal. The output of the Coherent Oscillator is applied to both the Mixer-I and the Phase Detector.
- Mixer-I - The mixer can produce the sum or difference of the frequencies applied to it. Signals with frequencies of $ f_l $ and $ f_c $ are applied to Mixer-I. Here the Mixer-I is used to produce the output, which has the frequency $ f_l +f_c $.
- Duplexer - It 'sa microwave switch, which connects the antenna to the transmitter section or to the receiver section in function needs. The antenna transmits the signal having the frequency $ f_l + f_c $ when the duplexer connects the antenna to the power amplifier. Likewise, Antenna receives the signal having a frequency of $ f_l + f_c pm f_d $ when the duplexer connects the antenna to Mixer-II.
- Mixer-II - The mixer can produce the sum or difference of frequencies applied to it. The signals of frequencies $ f_l + f_c pm f_d $ and $ f_l $ are applied to Mixer-II. Here, the Mixer-II is used to produce the output, which has the frequency $ f_c pm f_d $.
- IF Amplifier - The IF amplifier amplifies the intermediate frequency (IF) signal. The amplifier IF shown in the figure amplifies the signal of frequency $ f_c + f_d $. This amplified signal is applied as inputto the phase detector.
Phase Detector - It is used to produce the output signal having the frequency $ f_d $ of the two signals d 'input applied, which have the frequencies of $ f_c + f_d $ and $ f_c $. The output of the phase detector can be connected to the delay line canceller.
MTI Radar with Power Oscillator Transmitter
The schematic diagram of MTI Radar with power oscillator transmitter looks like the schematic diagram of MTI Radar with amplifier transmitter power. The blocks corresponding to the receiver section will be the same in both block diagrams. While the blocks corresponding to the section of the transmitter may differ in the two block diagrams.
The block diagram of the MTI radar with power oscillator transmitter is shown in the following figure.
As shown in the figure, MTI Radar uses the single antenna for transmission and reception.eption of signals using the duplexer. The operation of the MTI radar with power oscillator transmitter is mentioned below.
- The output of the magnetron oscillator and the output of Local The oscillators are applied to Mixer-I. This will further produce an IF signal , the phase of which is directly related to the phase of the transmitted signal.
- The output of Mixer-I is applied to the coherent oscillator. Therefore, the phase of the coherent oscillator output will be locked to the phase of the IF signal. This means that the phase of the Coherent Oscillator output will also be directly related to the phase of the transmitted signal.
- Thus, the output of Coherent Oscillator can be used as a reference signal to compare the received echo signal with the corresponding transmitted signal using the phase detector .
The above tasks will be repeated forr each newly transmitted signal.