In this lecture, we are going to learn about the basic Pulse Radar System, block diagram, and components of the Pulse Radar System, and in the last, we will cover the parameters of the Pulse Radar System. So let’s start with the basic block diagram of the pulse radar system.
Pulse Radar System
- A pulse radar system is a type of radar system that uses short pulses of radio frequency (RF) energy to determine the range, velocity, and/or angle of objects.
- The system emits a short burst of RF energy and listens for the reflected signal to return.
- By measuring the time delay between the transmitted pulse and the received reflection, the system can calculate the range to the target.
- Pulse radar systems are widely used for applications such as air traffic control, weather monitoring, and naval navigation.
- Pulse radar systems can also measure the velocity of a target by analyzing the frequency shift (Doppler effect) of the reflected signal. This information can be used to identify and track moving targets, such as aircraft or ships.
- Additionally, pulse radar systems can determine the angle of the target by using multiple antenna elements in an array configuration. This information can be used for target tracking, fire control, and other advanced radar applications.
- Pulse radar systems typically have a limited range and accuracy compared to continuous-wave radar systems, but they have the advantage of being able to transmit high-power pulses, which are useful for detecting small or low-reflectivity targets.
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Block Diagram of Pulse Radar System
A detailed block diagram of the high-power pulse radar system is shown in the figure below.
We will now discuss each component of a block diagram of the pulse radar system mentioned above in the figure.
1. Duplexer:
- This is a switch, which alternatively connects the transmitter or receiver to the antenna. its purpose is to protect the receiver from the high power output of the transmitter.
- During the transmission of an outgoing pulse, the duplexer will be aligned to the transmitter for the duration of the pulse, PW. After the pulse has been sent, the duplexer will align the antenna to the receiver. When the next pulse is sent, the duplexer will shift back to the transmitter.
- A duplexer is not required if the transmitted power is low.
2. Antenna:
- The antenna takes the radar pulse from the transmitter and puts it into the air. Furthermore, the antenna must focus the energy into a well-defined beam which increases the power and permits a determination of the direction of the target. The antenna must keep track of its own orientation, which can be accomplished by a synchro transmitter. There is also an antenna system, which does not physically move but is steered electronically.
3. Transmitter (Output Tube):
- The transmitter creates the radio wave to be sent and modulates it to form the pulse train. The transmitter must also amplify the signal to a high power level to provide adequate range. The source of the carrier wave could be Klystron, Traveling wave tube (TWT), or magnetron. Each has its own characteristics and limitations.
4. Radar Modulator:
- The radar modulator is a device, which provides high power to the transmitter’s tube to transmit during the transmission period. It makes the transmitting tube ON and OFF to generate the desired waveform. Modulator allows strong energy in a capacitor bank during the rest time.
- The stored energy then can b put into the pulse when transmitted. It provides rectangular voltage pulses which act as the supply voltage to the output tube such as a magnetron, thus switching it On and OFF as required. Commonly a thyristor valve is used as a switching device, which is a gas-filled valve.
5. Trigger Source (Synchronizer):
- The synchronizer coordinates the timing for range determination. It regulates the rate at which pulses are sent (i.e. sets PRF) and resets the timing clock for range determination for each pulse. Signals from the synchronizer are sent simultaneously to the transmitter, which sends a new pulse, and to the display, which resets the return sweep.
6. Receiver:
- The receiver is usually of the superheterodyne type whose function is to detect the desired echo signal in the presence of Noise, interference, and clutter.
- The receiver in pulsed radar consists of the Low Noise RF amplifier, mixer, local oscillator IF amplifier, detector, video amplifier, and radar display.
7. Low Noise RF Amplifier:
- Low Noise RF amplifier is the first stage of the receiver. it is a low-noise transistor amplifier or a parametric amplifier or a TW amplifier.
- Silicon bipolar transistor is used at lower radar frequencies (below L-band 1215 to 1400 Mhz) and GaAs FET is preferred at higher frequencies.
- It amplifies the received weak echo signal.
8. mixer and Local Oscillator:
- These convert RF signal output from the RF amplifier to comparatively lower frequency levels called Intermediate Frequency (IF). The typical value for pulse radar is 30 Mhz or 60 Mhz.
9. IF amplifier:
- It consists of a cascade of tuned amplifiers, these can be synchronous, that is all tuned to the same frequency, and have identical bandpass characteristics. If a really large bandwidth is needed, the individual IF amplifiers may be stagger-tuned. The typical value for pulse radar is 30 Mhz or 60 Mhz.
10. Detector:
- The detector is often a Schottky barrier diode that extracts the pulse modulation from the IF amplifier output. The detector output is then amplified by the video amplifier to a level where it can be properly displayed usually on CRT (Cathode Ray Tubes) directly or via digital signal processors. Synchronizing pulses are applied by the trigger source to the display unit.
11. Display Unit:
- The received video signal is displayed on the CRT (Cathode Ray Tube) for further observations and actions. Different types of display systems are used in radar which we already discussed in the previous lecture.
- To read more about the Radar displays please go to the below link:
Radar Display | List of displays used in Radar system
Radar Range Performance
- The simple form of the radar range equation is not adequately predicting the range performance of actual radars. There are some drawbacks of a simple form of radar range equation appended below:
- The fluctuation and uncertainties in target radar cross-section.
- The nature of the minimum detectable signal is affected by the receiver noise.
- The losses experienced in a radar system.
- Propagation effect by earth’s surface and atmosphere.
- The range of radar therefore will be a function of the probability of detection (Pd) and the probability of false alarm (Pfa).
- Note: We have derived the simple form of the radar range equation in the previous lectures. You can visit that lecture by clicking the below link. First, learn the Radar range equation and then come back to see the parameter of the radar which affect the performance of that radar system.
Radar Range Equation | Radar Range Equation Derivation
Parameters of Pulse Radar System
All the Parameters of the basic pulse radar system will affect the performance of the radar system in some way. here we find some major parameters that are affected the most.
1. Transmitter Power in Radar System
- The power Pt in the radar range equation is called by the radar engineer, the Peak Power. The peal power as used in the radar equation is not the instantaneous peak power of a sine wave.
- It is defined as the power averaged over that carrier frequency cycle which occurs at the maximum of the pulse power. Peak power is usually equal to one-half of the maximum instantaneous power.
- The average power Pav is defined as the average transmitter power over the pulse repetition period. If the transmitted waveform is a train of the rectangular pulses of width (T) and PRP (Tr).
\mathbf{PRP = 1/PRF}
\mathbf{P_{av} (Average\; Power) = P_t \times T/T_r = P_t \times T \times PRF}
(Average Power = peak Power x Pulse width/PRP = Peak Power x Pulse width x PRF)
- The ratio of average power to the peak power or pulse width to the PRP or pulse width x PRF is called the Duty cycle of the radar.
- The fraction of time the radar transmitter (Tx) is generating pulse power is termed the duty cycle. Thus, in any one second, a radar producing 1 \mu sec pulses at a rate of 1000 per sec in ‘ON’ for (1 x 10-6) x (1000) = 0.001 s
- The duty cycle is then 0.001 and the average power is given by the product of the duty cycle and peak power; the if the peak power is 500kw, the average power should be 500w.
- Typical values of radar power lie within the range of 100 to 500Kw.
- Writing the equation in terms of the average power rather than the peak power, we get
\boxed{\mathbf{ R^4_{max} = \frac{P_{av}.G.\sigma.A_e}{(4\pi)^2S_{min}.T.f_r}}}
- If the radar range is to be doubled, we have to increase the transmitter power 16 times since R_{max} \propto (Peak\; Power)^{1/4} .
2. Pulse Width in Pulse Radar System
- The duration of the pulse and the length of the target along the radial direction determine the duration of the returned pulse. In most cases, the length f the return is usually very similar to the transmitted pulse.
- In the display unit, the pulse (in time) will be converted into a pulse distance. The range of the values from the leading edge to the trailing edge will create some uncertainty in the range to the target. Taken at face value, the ability to accurately measure range is determined by the pulse width.
- If we designate the uncertainty in the measured range as the ranges resolution, RRES, then it must be equal to the range equivalent of the pulse width, namely:
\boxed{\mathbf{R_{RES}= c PW/2}}
- Now you may wonder why not just take the leading edge of the pulse as the range which can be determined with much finer accuracy. The problem is that it is virtually impossible to create the perfect landing edge. In practice, the ideal pulse will really appear like this:
- Creating a perfectly formed pulse with a vertical leading edge would require an infinite bandwidth. In fact, you may equate the bandwidth, \beta, of the transmitter to the minimum pulse width, PW by:
\boxed{\mathbf{PW= 1/ \beta}}
- Given this insight, it is quite reasonable to say that the range can be determined no more accurately than cPW/2 or equivalently
\boxed{\mathbf{R_{RES}= c/ 2 \beta}}
- In fact, high-resolution radar is offered as wide-band radar, which you now see as equivalent statements. one term is referring to the time domain, and the other the frequency domain. The duration of the pulse also affects the minimum range at which the radar system can detect. The outgoing pulse must physically clear the antenna before the return can be processed. Since the lasts for a time interval equal to the pulse width, PW, the minimum display range is then:
\boxed{\mathbf{R_{min}=c \times \frac{PW}{2}}}
- The minimum range effect can be seen on the PPI display as a saturated or blank area around the origin.
- Increasing the pulse width while maintaining the other parameters in the same well also affects the duty cycle and therefore the average power. For many systems, it is desirable to keep the average power fixed. then the PRF must be simultaneously changed with PW in order to keep the product PW x PRF the same. For example, if the pulse width is reduced by a factor of 1/2 in order to improve the resolution, then the PRF is usually doubled.
3. Pulse Repetition Frequency in Radar
- The frequency of pulse transmission affects the maximum range that can be displayed. Recall that the synchronizer resets the timing clock as each new pulse is transmitted. Returns from distance targets that do not reach the receiver until after the next pulse has been sent will not be displayed as if the range were less than actual. If this were possible, then the range of information would be considered ambiguous. An operator would not know whether the ranges were the actual range or some greater value.
- The maximum actual range that can be detected and displayed without ambiguity, or the maximum unambiguous range, is just the range corresponding to a time interval equal to the pulse repetition times, PRT. Therefore the maximum unambiguous range,
\boxed{\mathbf{R_{UNAMB}=c PRT/2 = c/(2PRF)}}
- The PRF is determined primarily by the maximum range at which targets are expected. If the PRF is made too high, the likelihood of obtaining target echoes from the wrong pulse transmission is increased. Echoes signals received after an interval exceeding the PRP (pulse repetition period) are called multiple times around echoes.
- They can result in erroneous or confusing range measurements. The nature of some multiple-time-around echoes may cause them to be labeled as ghost or angle targets or even flying saucers.
- (a) Three targets A, B & C where A is within Runamb and B & C are multiple time around echoes (targets); (b) appearance of the three targets on A-scope (c) appearance of three targets on the A-scope with a changing PRF.
- To minimize the multiple time-around targets, a staggered PRF can be used.
- When a radar is scanning, it is necessary to control the scan rate so that a sufficient number of pulses will be transmitted in any particular direction in order to guarantee reliable detection. If too few pulses are used, then it will more difficult to distinguish false targets from actual ones. Falae targets may be present in one or two pulses but certainly not in ten or twenty in a row. Therefore to maintain a low false detection rate, the number o pulses transmitted in each direction should be kept high, usually above ten.
- For systems with high pulse repetition rates, the radar beam can be repositioned more rapidly and therefore scan more quickly. Conversely, if the PRF is lowered the scan rate needs to be reduced. For simple scans, it is easy to quantify the number of pulses that will be returned from any particular target. let \tau represent the dwell time, which is the duration that the target remains in the radar,s beam during each scan. The number of pulses, N, that the target will be exposed to during the dwell time is:
\boxed{\mathbf{N = \tau PRF}}
- We may rearrange this equation to make a requirement on the dwell time for a particular scan
\boxed{\mathbf{\tau_{min}=N_{min}/PRF}}
- So it is easy to see that high pulse repetition rates require smaller dwell times. For a continuous circular scan, for example, the dwell time is related to the rotation rate and the beam width.
\boxed{\mathbf{\tau = \theta_B/\theta_s}}
- where \theta_B = beam width [degrees], \theta_s = rotation rate [degrees/sec] which will give the dwell time (\tau) in seconds. These relationships can be combined; giving the following equation from which the maximum scan rate may be determined for a minimum number of pulses per scan:
\boxed{\mathbf{\theta_s = \theta_B \; PRF/N}}
- The scan rate may be calculated by using the following equation,
\boxed{\mathbf{Scan\;rate(\theta_s) = \frac{RPM \times 360^o}{60\;s }}}
4. Radar Operating Frequency
- Finally, the frequency of the radio carrier wave will also have some effect on how the radar beam propagates. At the low-frequency extreme, radar beams will retract in the atmosphere and can be caught in ducts which results in long ranges. At the high extreme, the radar beam will behave much like visible light and travel in very straight lines. Very high-frequency radar beams will suffer high losses and are not suitable for long-range systems.
- The frequency will also affect the beam width. For the same antenna size, the low-frequency radar will have a larger beam width than a high-frequency one. In order to keep the beam-width constant, the low-frequency radar will need a large antenna.
