Radar engineering application: In real-world electronics and communication systems, the “radar principle” (transmit a pulse or continuous wave, receive echoes, and process time, phase, or frequency shifts) is commonly used in which of the following everyday and defense technologies?

Introduction / Context:
Radar, short for Radio Detection and Ranging, works by transmitting electromagnetic energy and analyzing the energy that returns after reflection from objects. By observing time delay, phase, and Doppler frequency shifts, a receiver can estimate range, velocity, and sometimes angle. While many learners associate radar only with military applications, the same underlying physics is widely used in civilian sensors and convenience devices. This question checks whether you recognize the breadth of radar usage across domains.

Given Data / Assumptions:

• The radar principle involves an active transmitter and a receiver that processes echoes.
• Sensing can be pulsed, continuous wave, or frequency-modulated continuous wave.
• Use cases can be long range or short range depending on power, antennas, and waveforms.

Concept / Approach:

All three listed civil and defense examples can rely on radar physics. Large surveillance systems use high power and directive antennas to detect aircraft at long range. Compact motion sensors employ low-power CW or FMCW modules to detect moving humans for security or to automate doors and lighting. Regardless of scale, the measurable quantities are the same: delay for range, and Doppler for motion.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets for 5.8 GHz motion sensors and 24 or 77 GHz automotive modules describe Doppler or FMCW processing identical in concept to defense radars. Aviation references describe primary surveillance radars that compute range from pulse time-of-flight and speed from Doppler, confirming commonality of the principle across scales.

Why Other Options Are Wrong:

• Selecting only one application ignores the shared physics across products.
• Only military devices is incorrect because many consumer and industrial products use the same principle at far lower power and shorter range.

Common Pitfalls:

Learners sometimes confuse radar with passive infrared presence sensors. Passive infrared does not transmit RF energy and therefore cannot measure Doppler frequency shifts. Another pitfall is assuming line of sight is always required; diffraction and multipath often enable detection in non-ideal geometries, although these can complicate interpretation.

Final Answer:

All of the above