Radar Systems: Detection and Ranging

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Questions and Answers

A radar system uses an antenna with a gain of 250. Compared to an isotropic antenna radiating the same total power, what is the power density at a target located a distance R away?

250 times smaller.

The relationship depends on R.

Equal.

250 times greater. (correct)

A radar with transmitted power $P_t$ detects a target with a radar cross section (RCS) of $\sigma$ at a distance R. If the transmitted power is doubled, what happens to the power density of the echo signal at the radar?

It doubles. (correct)

It remains unchanged.

It quadruples.

It is halved.

A radar system, operating with a directive antenna of gain G, detects a target at a distance R. If the radar cross-section (RCS) of the target is significantly reduced due to stealth technology, how is the power density of the echo signal at the radar affected?

It decreases proportionally to the RCS reduction. (correct)

It increases proportionally to the square of the RCS reduction.

It decreases proportionally to the square of the RCS reduction.

It remains the same, as RCS only affects the probability of detection, not the signal strength.

Consider a scenario where the distance, R, from the radar to the target doubles. How is the power density of the echo signal at the radar affected, assuming all other parameters remain constant?

It is reduced to one-sixteenth. (D) Signup and view all the answers

A radar system transmits with power $P_t$ and uses an antenna with gain G. If atmospheric attenuation is considered, and it reduces the power density by a factor of A (where A > 1) at the target, how does this attenuation affect the power density of the echo signal received back at the radar?

The echo signal power density is reduced by a factor of $A^2$. (A) Signup and view all the answers

What is the primary difference between how a sound system and a radar system determine the location of an object, according to the text?

Sound systems rely on acoustic waves, while radar systems use high-frequency radio waves. (A) Signup and view all the answers

How does the size of a target relative to the wavelength of a radar signal affect its reflectivity?

Targets much larger than the wavelength reflect signals more effectively. (A) Signup and view all the answers

What is the purpose of the duplexer in the radar system block diagram shown in Figure 7-2?

To switch the antenna between the transmitter and the receiver, preventing the high-power transmission from damaging the receiver. (C) Signup and view all the answers

Considering the principles of radar operation, how would increasing the frequency of the radio signal generally affect the radar's performance?

It would improve detection of small objects but reduce the radar's range due to increased atmospheric attenuation. (B) Signup and view all the answers

In the context of radar technology, what information does the shift in the carrier frequency of the reflected wave provide?

The relative velocity of the target in relation to the radar. (B) Signup and view all the answers

A radar system detects a shift in the carrier frequency of the reflected wave. What phenomenon is responsible for this shift, and what does it indicate about the target?

Doppler effect; indicating that the target is in motion relative to the radar. (A) Signup and view all the answers

If a radar system uses a very long wavelength signal, what type of targets would it be most effective at detecting, and why?

Large objects, because they reflect long wavelengths more effectively. (D) Signup and view all the answers

In a radar system, after the reflected energy is received by the antenna, what is the next critical step in processing the signal to determine the target's presence and characteristics?

Processing and display of the signal to extract distance and position information. (C) Signup and view all the answers

What critical function does the pulse modulator perform in a radar system?

It controls the duration for which the transmitter is active by responding to timing pulses. (A) Signup and view all the answers

In the context of a radar system's duplexer, what is the primary purpose of the Anti-TR (ATR) switch?

To protect the receiver by presenting a short circuit to the transmitter after the transmission pulse. (C) Signup and view all the answers

How does a duplexer prevent the transmitter and receiver from being connected to each other in a radar system?

By employing TR and ATR switches arranged to alternately connect the antenna to either the transmitter or receiver. (B) Signup and view all the answers

What is the immediate effect on the TR and ATR switches when the transmitter generates an RF impulse?

Both switches become short-circuited, preventing any effect on the transmission path. (B) Signup and view all the answers

Following the termination of a transmitted pulse, what action does the ATR switch perform to protect the transmitter?

It throws a short circuit across the waveguide leading to the transmitter to block incoming signals. (B) Signup and view all the answers

Consider a scenario where the TR switch guide becomes continuous and correctly matched. What is the effect on the signal path?

The signal from the antenna can directly reach the receiver. (A) Signup and view all the answers

If the quarter-wave section connected to the ATR switch reflects an open circuit across the main waveguide, what is the immediate consequence during the transmission pulse?

The presence of the open circuit ensures that the ATR switch does not affect the transmission. (C) Signup and view all the answers

A radar system uses a pulse-forming network (PFN). What is the primary role of the PFN?

To control the shape and duration of the pulses sent to the transmitter. (A) Signup and view all the answers

How does increasing the Pulse Repetition Frequency (PRF) generally affect the maximum unambiguous range of a radar system, assuming sufficient power?

It decreases the maximum unambiguous range because less time is available for echoes to return before the next pulse is transmitted. (C) Signup and view all the answers

A radar system transmits pulses at a rate of 200 pulses per second, with a pulse width of 0.5μs. Considering a 1μs/meter radar system, what is the maximum unambiguous range and the minimum range?

Maximum range: 5 km, Minimum range: 75 meters. (B) Signup and view all the answers

If a radar system requires a minimum unambiguous range of 15 kilometers, what is the highest Pulse Repetition Frequency (PRF) that can be used, assuming a propagation velocity of 1μs/meter?

Approximately 33.3 Hz. (D) Signup and view all the answers

A radar system emits pulses with a width of 0.25μs. What is the theoretical minimum range that this radar can reliably detect targets at?

37.5 meters. (A) Signup and view all the answers

Consider two radar systems: System A has a PRF of 500 Hz and System B has a PRF of 1000 Hz. Both systems have the same peak power. In what scenario would System A be preferred over System B?

When detecting targets at very long ranges, requiring a larger unambiguous range. (C) Signup and view all the answers

How does the pulse width of a radar signal primarily affect its ability to resolve between two closely spaced targets at the same range?

A narrower pulse width improves the ability to distinguish between closely spaced targets due to reduced overlap of returning echoes. (C) Signup and view all the answers

A radar system designer needs to improve the radar's minimum range without altering the system's peak power or PRF. What adjustment should the designer make?

Decrease the pulse width. (C) Signup and view all the answers

What is the primary method used in modern radar systems to discern targets?

Pulse Modulation (PM) detection, which transmits short pulses and measures the time delay of the returning signals. (C) Signup and view all the answers

A radar system transmits pulses with a certain pulse width (τ). If the energy per pulse is kept constant, how is the peak power of the pulse related to τ?

Peak power is inversely proportional to τ. (D) Signup and view all the answers

In radar technology, what is the significance of measuring the time ($t_d$) it takes for a pulse to travel to a target and return?

It is used to calculate the target's range, utilizing the known speed of electromagnetic waves. (C) Signup and view all the answers

Why is the factor of 2 included in the denominator of the radar range equation $R = \frac{c \cdot t_d}{2}$?

To represent the two-way travel of the radar signal from the source to the target and back. (C) Signup and view all the answers

A radar system emits pulses with a duration ranging from 0.1s to 50s. What is the primary reason for using such short pulses?

To allow the receiver to differentiate between the transmitted and reflected pulses clearly. (A) Signup and view all the answers

What is the result of a radar transmitter being switched off before the reflected power returns from an object?

The receiver can differentiate between the transmitted and received pulses. (C) Signup and view all the answers

A radar system detects a return pulse 25s after transmitting a signal. Considering the speed of light, what is the calculated range to the target?

3,750 meters (C) Signup and view all the answers

In pulse modulation radar, what critical role does the indicator play after the reflected signal is received?

It measures the time interval between the transmitted and received pulses. (D) Signup and view all the answers

A radar system is designed to detect moving targets amidst permanent structures. Which aspect of the returning signal is most crucial for differentiating between these?

The frequency shift (Doppler effect) of the returning signal, which indicates the object's motion. (B) Signup and view all the answers

A radar system has a pulse width of 2μs and a pulse repetition frequency of 200 Hz. What is its duty cycle?

0.04% (A) Signup and view all the answers

What adjustment to radar peak power and average power would allow for smaller, more compact transmitter tubes?

High peak power and low average power (B) Signup and view all the answers

A radar system transmits a pulse with a peak power of 200 kW for a duration of 0.5 μs. If the pulse repetition period is 5 ms, what is the average power?

20 W (B) Signup and view all the answers

In radar pulse characteristics, what does 'sag' refer to?

A condition where the maximum amplitude of the pulse decreases slowly over time. (A) Signup and view all the answers

What is the primary purpose of using pulse modulation in radar transmitters?

To produce radio frequency (RF) pulses by switching the carrier wave on and off for short durations. (C) Signup and view all the answers

Which of the following changes to pulse characteristics would increase the likelihood of accurately detecting fast-moving targets?

Decreasing both rise time and fall time (D) Signup and view all the answers

In a radar system, ringing is observed after the initial amplitude rise. What does this indicate?

The initial amplitude rise exceeded the correct value, leading to overshoot. (D) Signup and view all the answers

A radar pulse has a rise time of 0.2 μs and a fall time of 0.15 μs. If the pulse width is 1.5 μs, what percentage of the total pulse duration is spent transitioning (rising and falling)?

Approximately 23.33% (D) Signup and view all the answers

Flashcards

Power Density from Isotropic Antenna

Power density at distance R from radar using isotropic antenna: Pt / (4πR²).

Directive Antenna Gain

Gain (G) measures power increase in targeted direction compared to isotropic antenna.