Chapiter 3

Pulsed radar system & phase processing MTIChapter 3

Pulsed radar systemThe time-delay between the transmission of each pulse and the reception of the echo of the same pulse is proportional to the target range.The phase of the echo also depends on the target range. And can be used to extract information from it, if the phase of the transmitted signal is stable from pulse to pulse.

Pulsed radar systemCoherenceA pulsed radar system where the transmitted signal is phase-stable from pulse to pulse. The word coherent means in-phase or maintaining a definite phase relationship with a certain reference waveform. The phase of a coherent signal at any point, relative to the reference signal is completely predictable.

Pulsed radar systemNon-coherenceThe phases of the transmitted signal are random from pulse to pulse. The phases of their echoes cannot be used to predict the range of the target.Conclusion:Coherent radar system: can use the round-trip time or the echoes phases to detect the target range (the speed of the target). No-coherent radar system: use only the round-trip time to detect the target range.

Non-coherent radarThe pulsed transmitted is usually high-power RF oscillator, which is keyed on and off by a pulse modulator.The phase relationship between successive pulses in the transmitted signal is completely random.The received RF signal is mixing with a local oscillator to shift the RF signal down to the intermediate frequency IF.

Non-coherent radar

Non-coherent radar

The IF signal is amplified and filtered by the IF amplifier.The envelope detector produces an output signal whose level corresponds to the envelope of the IF signal (linear detector, square law detector, or logarithmic detector)All frequency and phase information is LOST.

Non-coherent radarEquation

Coherent pulsed radarSuperheterodyne receiverHomodyne receiver

Coherent pulsed radar- Superheterodyne receiver

Coherent pulsed radar- Superheterodyne receiverPresence of a reference signal which is coherent with the transmitted signal, and by the fact that the transmitted RF signal itself is coherent (phase stable from pulse to pulse).The coherence signal is generated by a stable oscillator called COHO (coherent oscillator). The frequency of the COHO is equal to the IF frequency used in the receiver. The local oscillator is also a stable oscillator called STALO. Its frequency is usually near the transmitted frequency.

Coherent pulsed radar- Superheterodyne receiverThe phase detector output is a bipolar pulse signal whose amplitude depends on the phase of the phase detector input signal relative to the coherent reference signal.(moving target change in phase )

Coherent pulsed radar- Homodyne receiverSuperheterodyne are commonly used in a radar because it is usually more convenient to perform phase detection at a relatively low IF frequency than at the RF frequency.If large bandwidth of the IF amplifier, requires the using of higher intermediate frequency: Impractical, circuit losses and instability.

Coherent pulsed radar- Homodyne receiverEquations

Coherent pulsed radar- Homodyne receiver

Fixed target

Moving target

Limitations Blind phases.Blind speeds.

Phase processing MTIThe Doppler frequency shift can be used to allow the radar to reject fixed targets (clutter) and detect only moving targets. This type of signal processing is called moving-target indication (MTI).One way of accomplishing moving-target indication is to use the pulse-to-pulse change in amplitude at the output of the phase detector to differentiate between fixed and moving targets. This is called phase-processing MTI and is implemented using a delay-line canceller.

Phase-processing MTI

Phase-processing MTIThe delay line canceller consists of a delay line and a summer. The delay line delays the video signal by a time equal to the pulse-repetition interval (sometimes called the PRF period) T=1/fp. The delay line operates by sampling the video input signal at a high rate and storing each sample for the required time. The delayed video signal is subtracted from the undelayed video signal by the summer.The output of the delay-line canceller consists of the differences between successive pulses in the video signal. If the target is not moving, the amplitude of each pulse is the same as that of the previous pulse. The result of the subtraction is zero. If the target is moving, however, the pulse amplitude changes from pulse to pulse. The result of the subtraction is not zero; a residue remains in the form of a bipolar pulse train. The bipolar pulse train is usually full-wave rectified before it is sent to a PPI display. Since only moving targets produce a residue at the canceller output, only these targets will be visible on the radar display.

Phase-processing MTIFrequency response

Phase-processing MTI

Phase-processing MTI

Blind phasesBlind phases can cause nulls in the unipolar video signal,This results in reduced sensitivity of the radar. Even if the phases are such that no pulses are completely cancelled, the fact that the amplitude of the unipolar video signal drops periodically causes a loss of sensitivity. The problem of reduced sensitivity caused by blind phases can be eliminated by using a quadrature phase detector

Quadrature phase detector

Vector MTI processing