Simple Pulse Radar
The problem associated with CW Radar devices regarding the de-coupling from transmission path to reception path are solved by pulse Radar through the temporal separation of transmission and reception. For the suppression of very strongly cross-talking transmission pulses into the receiving system, the receiver input is short-circuited or decoupled by a circulator. Figure 1 shows a typical, simple pulse Radar system. To produce the pulses either magnetrons or switched amplifiers are employed. There is no reference from the transmission oscillator to the receiver, therefore non-coherent pulse Radar cannot measure Doppler and thus only the distance and not the speed of a target can be determined.

Figure 1: Simple, non-coherent Radar.
Coherent Pulse Radar/Pulse Doppler radar
Coherent pulse Radar is in the position to deliver information regarding the range as well as Doppler information, i.e. the velocity and/or speed of an object. Here a coherent oscillator (COHO) is introduced on the transmission side, which also delivers the phase reference for the receiving signal. The transmitting oscillator (LO for Local Oscillator) is added on the transmitting end and is again taken out at the receiving end. In comparison to the Doppler frequency, which can be measured, both oscillators must be sufficiently stable for the entire time. In Figure 2, an example of coherent pulse Radar, a quadrature modulator is employed to additionally determine the direction of the target movement.

Figure 2: Coherent pulse Radar.

Two various fundamental methods of Doppler analysis are known and will be subsequently and shortly introduced. The transmitting signal for pulse Radar is:
Us=Assinω0t
The reference signal:
Uref=Arefsinω0t+φref
φref= constant, φref=0. The receiving signal from a target at range R becomes:
UE=AEsinω0+ωdt-2ω0Rc
After combining one obtains:
Udiff=Ud=Adsin2πfdt-4πf0Rc
By demonstrating a video signal at the indicator it is possible to distinguish two cases: fd > 1τ fd< 1τ
From which the illustrations according to Figure 3 result

Figure 3 a) Transmitting signal b) Video signal fd > 1τ c) Video signal fd< 1τ
The pulsing is identical to a sample of the Doppler oscillation. For uniqueness the sampling theorem must be fulfilled, meaning at minimum each half period of the Doppler oscillation must be sampled.
PRF=4.vmaxλ0
The resolution is a result of the observation period, meaning of the number of the used sample values NFFT of the Fourier transform.
∆v=λ0.PRF2NFFT
One speaks about blind frequencies for the phase differences between the transmitting and receiving signal of ±n2π. For blind frequencies, fd=n.1T=n.fblind result in a non-detectable Doppler frequency.

Blind Speed
At the comparison of the echoes between two or more pulse periods the fall can appear that the airplane flies with exactly this one radial speed, some a phase shifting of correct 360° causes. In accordance with the periodicity of the sine function this fall can appear even at all integral multiples of ±n · 360°. The value of phase shifting is zero in these falls too. Well, the target isn't recognized as a moving target therefore. It flies with a so called blind speed and the MTI system won't report it like a ground clutter.
The blind speed is dependent on the transmitted frequency and on the pulse repetition frequency of the radar unit.

vblind=λ2.Ts
Where,
vblind=one of the blind speeds λ=wavelength of the transmitted pulse
Ts=pulse repetition time (PRT)
With reference to the lower diagram on the figure 1:
If the Doppler frequency produced by a moving target is exactly the same as the PRF (fd=PRF) then “sampling” occurs at the same point on each Doppler cycle. As far as the signal processor is concerned, it is as if the target were stationary. The same effect occurs if fd is an integer multiple of PRF. Hence targets with certain radial velocities tend to be invisible to MTI pulse radar.
Example given:
A radar unit works with the tx-frequency of 2.8 GHz and a pulse repetition time of 1.5 ms. Under these conditions the first blind speed has got the value: vblind=λ2.Ts=c02.f.Ts=3×1082×2.8×109×1.5×10-3=35.72m/s This speed of converted about 130 km/h and all integral multiples of this also well cause that the target isn't visible in the range of the effectiveness of the MTI system.

Tracking Radar
4.1 INTRODUCTION
A radar which detects a target, determines its location and trajectory in future is called tracking radar.
4.1.1 Block Diagram of Tracking Radar
The block diagram of simple tracking radar is shown in Fig. 4.1.

4.2 Operation of Tracking Radar
Its tracking operation usually depends on angular information. The antenna beam is very narrow and it tracks one target at a time. This is achieved by range gating and Doppler filtering. The timing control is used for range tracking and Doppler gating is used for Doppler tracking. The angle error signal is fed to servo control system. This servo system steers the antenna to track the target.
4.3 Types of Tracking Radars
These are of following types:

* Single Target Tracking (SZT) radar * Automatic Detection and Tracking (ADT) radar * Track While Scan (TWS) radar * Phased array tracking radar * Angle tracking radar * Mono pulse tracking radar

4.3.1 Single Target Tracking (STT) Radar
It tracks a single target continuously at a reasonably high data rate. It is often used to track the missiles. The typical scan rate is 10 observations per second. It performs the above job by using antenna beam in following a target, an angle-error signal and a closed loop system.
4.3.2 Automatic Detection and Tracking (ADT) Radar
The automatic detection and tracking radar is used in air surveillance and air traffic control. It is achieved by the rotation of the antenna. The rotation of antenna is about eight times per minute. It can track a large number of targets simultaneously.
4.3.3 Track While Scan (TWS) Radar
The TWS radar scans at a moderate data rate. It covers a reasonably good angular sector in free space. It is used for air defense, aircraft landing, aircraft intercepting and to track multiple targets.
4.3.4 Phased Array Tracking Radar
In this type of tracking radar, antenna is not rotated but antenna beam is rotated electronically by excitation phase control. The scan rate is very fast and is a few microseconds.
4.3.5 Angle Tracking Radar
The angle tracking is done by the pencil beams from the radar. Fig. 4.2 has two squinted beams with squint angle of ±θs from boresight direction. These beams are simultaneously generated or only one beam is quickly scanned between two angular positions. The crossover of the two beams gives boresight direction.
In this tracking, boresight is maintained in the direction of target.

Fig 4.2: Angle Tracking

Four beam positions are required to obtain angle tracking in an orthogonal plane.
4.3.6 Monopulse Tracking Radar
Monopulse tracking radar is radar in which the information about angle error is obtained on a single pulse. This is also known as simultaneous lobing.
In this radar the angular location of a target is obtained by comparing the received signals in two or more simultaneous lobes. It is useful for angle measurement with high precision. It is used in tracking radars to obtain an angle error signal. It is obtained in two orthogonal coordinates. The tracking antenna keeps its boresight position in the direction of the moving target.

4.4 AMPLITUDE COMPARISON MONOPULSE TRACKING RADAR

The block diagram of amplitude comparison Monopulse tracking radar is shown m Fig. 4.3. It is used for the measurement of single angular coordinate of the target.

Fig 4.3: Block diagram of amplitude comparison monopulse tracking radar for a single angle coordinate measurement.
4.4.1 Operation of Amplitude Comparison Monopulse Tracking Radar
The hybrid junction is a four port device and consists of two ports- Two signals from the two squinted beams are given to the Input ports of the hybrid junction. The sum and difference of the two signals are obtained at the output ports of the hybrid junction. The sum of two signals and local oscillator signal are given to the mixer 1 and its output is given to the IF amplifier 1. The output of ‘IF amplifier 1’ is given to the amplitude detector whose output is a range signal. The range signal appears in the display unit.
Similarly, the difference signal output signal of the hybrid junction and the signal from local oscillator are given to mixer 2. Its output is given to IF amplifier 2. The signals from IF amplifier 1 and 2 are given to the phase sensitive detector device. The phase sensitive detector device is basically a non-linear device which compares these two signals of the same frequency. The output of this device is an angle error signal which appears in the display unit. The magnitude of the error signal is proportional to θt-θc. Here, θt is target angle and θc is crossover angle or boresight angle. The sign of the phase detector output signal gives the direction of the angle error relative to the boresight.
The sum and difference patterns are characterized by the same phase and amplitude characteristics. The squinted beam, sum pattern, difference pattern and error signal are shown in Fig. 4.4.

Fig. 4.4 Squinted beam, sum pattern, difference pattern and error signal

The sum pattern is used for transmission. The sum and difference patterns are used in reception. The received signal with the difference pattern provides range measurements. The same signal is used as reference to obtain the sign of the error signal. The received signals from the sum and the difference patterns are amplified sequentially and they are combined in phase detector. The detector produces error signals.
4.5 Phase Comparison Monopulse Radar
The phase comparison monopulse radar is also called Interferometer radar. In this radar, two antenna beams targeting in the same direction are used. Here, the amplitudes of the signals are the same with different phases. The phase difference in the two signals received by the two antennas is given by ∆ψ=2πdλsinθ. Here, λ is the wavelength, d is the spacing between the two antennas, θ is the direction of received signal with respect to normal to the baseline. The pulse comparison method used in one angle coordinate is shown in Fig. 4.6. It consists of two antennas producing identical beams.

4.5.1 Advantage of Phase Comparison Monopulse Radar
The scanning of radiation beams and beam shaping are very fast.
4.5.2 Disadvantages of Phase Comparison Monopulse Radar * It is less efficient than the amplitude comparison method. * It has the effect of grating lobes due to spacing of the two antennas. * It is a less popular method. * Only one-fourth of the available antenna area is used for transmitting and only one-half the area is used while receiving, to obtain each angle coordinate. * When the spacing between the antennas is greater than the antenna diameter, then side lobes in the radiation patterns are high and EMI is produced.
4.6 Sequential Lobing Radar
In sequential lobing radar, only one beam is switched between two squinted sequential angular positions for target-angle measurement. This method is called sequential lobing. It is also called sequential switching or lobe switching. Here, time sharing is done using single antenna beam. This method is simple and requires less equipment and is cost effective. But it is not very accurate.

4.6.1 Principle of Operation of Sequential Lobing Radar
An antenna and its lobe which is switched sequentially between polar coordinates X and Y are shown in Fig. 4.7.

In this, a pulse is transmitted and received when the beam is squinted right, up, left and down. The echo signal amplitude corresponding to the two positions of the beam are measured and the difference gives the error signal. This error signal is nothing but the angular displacement of the target from the boresight. Fig. 4.7 shows that the target is not located in the boresight direction. The boresight direction is also called switching axis. When the echo signals for the two beam positions are identical, the target is said to be located in the switching axis.
The angle measurement in the orthogonal coordinate is done by two additional switching positions. In two-dimensional sequential lobing, radar consists of four horn feeds for a single reflector antenna. This radar has right, left, up and down antenna positions. Sometimes, central feed (fifth feed) is used for transmission and the remaining four feeds are used for reception sequentially.
4.6.2 Advantages of Sequential Lobing Radar * It requires only one antenna. * Its operation is simple. * It requires less equipment. * It is cost affective.
4.6.3 Disadvantages of Sequential Lobing Radar
It is not very accurate.