All questions of RADAR (Radio Detection & Ranging) for Electronics and Communication Engineering (ECE) Exam

Introduction:
In radar systems, the minimum range is the shortest distance at which the radar can detect a target. The minimum range is determined by the pulse width of the transmitted radar signal. To increase the minimum range, the pulse width needs to be increased. However, increasing the pulse width also affects the peak power of the radar signal. In order to maintain a constant signal-to-noise ratio, the peak power must be increased when the pulse width is increased.

Explanation:
When the minimum range is doubled, it means that the radar needs to be able to detect targets at twice the distance compared to its current capability. To achieve this, the pulse width of the radar signal needs to be doubled.

Relationship between pulse width and range:
The pulse width of a radar signal is directly related to the range resolution. Range resolution refers to the ability of the radar to distinguish between two targets that are closely spaced in range. The shorter the pulse width, the better the range resolution. Conversely, a longer pulse width results in a poorer range resolution.

Effect of pulse width on peak power:
When the pulse width is increased, the energy of the radar signal is spread over a longer time duration. This means that the peak power of the radar signal decreases. In order to maintain a constant signal-to-noise ratio, the peak power needs to be increased when the pulse width is increased.

Relationship between pulse width and peak power:
The relationship between pulse width and peak power is inverse. As the pulse width increases, the peak power decreases, and vice versa. Mathematically, this relationship can be expressed as:

Peak Power ∝ 1 / Pulse Width

Doubling the minimum range:
When the minimum range is doubled, the pulse width needs to be doubled. Since the relationship between pulse width and peak power is inverse, the peak power needs to be increased by a factor of 2^2 = 4 in order to maintain a constant signal-to-noise ratio.

Therefore, the correct answer is option 'C' - The peak power has to be increased by a factor of 16.

Explanation:

To understand why the peak transmit power of the radar has to be increased by a factor of 16 when the range is doubled, we need to consider the basic principles of radar operation.

Radar Operation:
Radar works by transmitting a high-frequency electromagnetic wave, known as a radar pulse, and then detecting the echo reflected back from a target. The time taken for the pulse to travel to the target and return is used to calculate the range or distance to the target.

Radar Range Equation:
The range or distance to a target can be calculated using the radar range equation:

R = (c * Δt) / 2

Where:
R = Range to the target
c = Speed of light
Δt = Time taken for the pulse to travel to the target and return

Relationship between Transmit Power and Range:
In the radar range equation, the time taken for the pulse to travel to the target and return (Δt) is directly proportional to the range (R). This means that to increase the range, the time taken for the pulse to travel to the target and return must be increased.

The time taken for the pulse to travel to the target and return is determined by the speed of light (c) and the distance traveled by the pulse. Since the speed of light is constant, the distance traveled by the pulse needs to be increased to increase the time taken.

To increase the distance traveled by the pulse, the transmit power of the radar needs to be increased. This is because the power of the radar pulse decreases with distance due to spreading and attenuation effects.

Relationship between Transmit Power and Range:
The power of a radar pulse decreases with distance according to the inverse square law:

P = P0 / (4πR²)

Where:
P = Power of the radar pulse at a distance R
P0 = Power of the radar pulse at a reference distance

From the above equation, it can be observed that the power of the radar pulse is inversely proportional to the square of the range. Therefore, to double the range, the power of the radar pulse needs to be increased by a factor of 2² = 4.

However, we also need to consider that the transmit power is squared in the radar range equation:

R = (c * Δt) / 2

Therefore, to double the range, the transmit power needs to be increased by a factor of 4² = 16.

Hence, the correct answer is option 'D' - increased by a factor of 16.

Introduction:
RADAR stands for Radio Detection and Ranging. It is a system that uses radio waves to detect and locate objects in its vicinity. LIDAR, on the other hand, stands for Light Detection and Ranging. It uses laser light to detect and locate objects. Both RADAR and LIDAR are remote sensing technologies commonly used in various applications.

Similarities between RADAR and LIDAR:
RADAR and LIDAR are similar in several ways:

1. Principle of Operation: Both RADAR and LIDAR work on the principle of emitting a signal and then measuring the time it takes for the signal to return after reflecting off an object. This time delay is used to calculate the distance to the object.

2. Remote Sensing: Both RADAR and LIDAR are remote sensing technologies that can detect and locate objects without direct physical contact. They are commonly used in applications such as weather monitoring, navigation, and surveillance.

3. Range and Resolution: Both RADAR and LIDAR can provide information about the range (distance) and resolution (detail) of the detected objects. They can differentiate between nearby objects based on the time delay or the intensity of the reflected signal.

4. Multiple Applications: RADAR and LIDAR have numerous applications in various fields. They are used in aviation for navigation and collision avoidance, in meteorology for weather monitoring, in automotive for object detection and autonomous driving, and in military for surveillance and target tracking.

Differences between RADAR and LIDAR:
While RADAR and LIDAR share similarities, they also have distinct differences:

1. Wavelength: RADAR uses radio waves with longer wavelengths, typically in the range of centimeters to meters. LIDAR, on the other hand, uses laser light with much shorter wavelengths, typically in the range of nanometers to micrometers.

2. Penetration and Reflection: RADAR waves can penetrate through certain materials, such as fog or rain, and reflect off objects behind them. LIDAR waves, being optical in nature, are more easily scattered or absorbed by such materials, limiting their ability to penetrate and reflect.

3. Resolution and Accuracy: LIDAR generally provides higher resolution and accuracy compared to RADAR. The shorter wavelengths of LIDAR allow for finer detail in the detected objects, making it suitable for applications that require high precision, such as 3D mapping or autonomous navigation.

4. Cost and Complexity: RADAR systems are generally less expensive and less complex compared to LIDAR systems. LIDAR systems require specialized laser sources, detectors, and optics, making them more costly and complex to design and maintain.

Conclusion:
In summary, LIDAR is a system similar to RADAR. They both use the principle of emitting a signal and measuring the time it takes for the signal to return to detect and locate objects. However, they differ in terms of the type of signal used (radio waves for RADAR and laser light for LIDAR), their ability to penetrate and reflect off objects, the resolution and accuracy they provide, and the cost and complexity of the systems.

Monopulse Tracking System
Monopulse tracking is the best system for accurate tracking when the target cross section is changing. This system provides precise tracking by comparing the received signals from different parts of the antenna pattern.

Explanation:

Monopulse Tracking Principle:
- Monopulse tracking compares the phase differences between the received signals from multiple feed horns or elements on the antenna to determine the direction of the target.
- By analyzing the phase differences, the system can accurately track the target even when its cross section is changing.

Advantages of Monopulse Tracking:
- Monopulse tracking provides high accuracy and resolution in tracking moving targets.
- It compensates for errors caused by target maneuvers and changing cross sections.
- It offers better performance in cluttered environments compared to other tracking systems.

Comparison with Other Tracking Systems:
- Conical scanning tracks targets by rotating the antenna in a cone pattern, which may not be as accurate when the target cross section is changing rapidly.
- Sequential locking tracks targets by sequentially locking onto different parts of the target signal, which may lose accuracy if the target changes rapidly.
- Lobe switching tracks targets by switching between lobes of the antenna pattern, which may not provide as precise tracking as monopulse in dynamic situations.
In conclusion, monopulse tracking is the best system for accurate tracking when the target cross section is changing due to its high accuracy, resolution, and ability to compensate for target variations.

The TWT is sometimes preferred to magnetron as a radar transmitter output tube because it is capable of a larger duty cycle.

The Traveling Wave Tube (TWT) and magnetron are both used as radar transmitter output tubes, but they have different characteristics that make them suitable for different applications. In the case of the TWT, one of its advantages over the magnetron is its capability of handling a larger duty cycle.

Explanation:

Duty Cycle:
The duty cycle of a radar transmitter refers to the ratio of the pulse duration to the pulse repetition interval (PRI). It represents the amount of time the transmitter is actively transmitting radar pulses compared to the total time of operation. A larger duty cycle means that the transmitter is active for a longer period of time, transmitting radar pulses more frequently.

Advantages of a larger duty cycle:
1. Increased average power: A larger duty cycle allows for increased average power output from the transmitter. This is important in radar applications where a higher power output is required for long-range detection or for overcoming losses in the radar system.

2. Improved target detection: With a larger duty cycle, the radar pulses are transmitted more frequently, providing a higher pulse repetition frequency (PRF). This leads to better target detection and tracking capabilities, especially for fast-moving targets or in environments with high clutter.

3. Reduced range ambiguity: A larger duty cycle helps to reduce range ambiguity in radar systems. Range ambiguity occurs when multiple targets are located at different ranges but appear to be at the same range due to the radar pulses being transmitted too infrequently. By increasing the duty cycle, the radar system can transmit pulses more frequently, reducing the chances of range ambiguity.

TWT vs. magnetron:
The magnetron is a commonly used radar transmitter output tube known for its compact size, simplicity, and high peak power output. However, it is typically not suitable for applications that require a larger duty cycle. The TWT, on the other hand, is capable of handling a larger duty cycle due to its inherent design.

The TWT works based on the interaction between an electron beam and a slow-wave structure, which allows for efficient energy transfer and amplification. This design enables the TWT to handle continuous wave (CW) operation or long pulse operation, making it suitable for radar systems that require a larger duty cycle.

In conclusion, the TWT is preferred over the magnetron as a radar transmitter output tube in certain applications because it is capable of handling a larger duty cycle. This allows for increased average power output, improved target detection, and reduced range ambiguity in radar systems.

Problem of ambiguous range measurements encountered in Pulse Doppler Radar only.

Explanation:
The problem of ambiguous range measurements is encountered in Pulse Doppler Radar. This issue arises due to the presence of range ambiguity in the radar system. Range ambiguity occurs when the radar receives multiple echoes from different targets within the same range bin.

Causes of Range Ambiguity:
Range ambiguity can occur in Pulse Doppler Radar due to the following reasons:

1. Pulse Repetition Frequency (PRF): In Pulse Doppler Radar, the PRF is used to determine the maximum unambiguous range. If the PRF is too high, it can lead to range ambiguity because the radar pulses may overlap in time and cause echoes from different targets to be received within the same range bin.

2. Target Velocity: Another factor that contributes to range ambiguity is the target velocity. If the target is moving at a high velocity, the received echoes from different pulse repetitions can overlap in range and result in range ambiguity.

Effects of Range Ambiguity:
Range ambiguity can have several effects on the radar system:

1. False Target Detection: When range ambiguity occurs, the radar system may incorrectly detect multiple targets at the same range. This can lead to false target detections and inaccurate tracking.

2. Range Measurement Errors: The presence of range ambiguity can also introduce errors in the range measurement of targets. The radar system may not be able to accurately determine the true range of a target due to the overlap of echoes from different targets.

Solutions to Range Ambiguity:
To overcome the problem of range ambiguity in Pulse Doppler Radar, the following techniques are commonly used:

1. Pulse Repetition Frequency (PRF) Switching: By using multiple PRFs, the radar system can avoid range ambiguity. The PRFs are switched in a predetermined sequence, and the range measurements are obtained by comparing the received echoes with the corresponding PRFs.

2. Pulse Compression: Pulse compression techniques, such as matched filtering, can be employed to improve the range resolution of the radar system. This can help in resolving the range ambiguity and accurately determining the true range of targets.

Conclusion:
In conclusion, the problem of ambiguous range measurements is encountered in Pulse Doppler Radar only. This issue arises due to the presence of range ambiguity, which can be caused by factors such as PRF and target velocity. It can lead to false target detections and range measurement errors. To overcome this problem, techniques such as PRF switching and pulse compression are used in Pulse Doppler Radar.