Which of the following is NOT a type of radar?

The reason "Echo radar" is the correct answer is that it is not a standardized or recognized type of radar system in military or technical literature. Radar systems are typically categorized into specific types based on their operating principles and the way they process electromagnetic waves to detect objects.

Pulse radar, for instance, is known for sending out short bursts (pulses) of radio waves and measuring the time it takes for the returned signal, allowing it to calculate distances and locate targets. Continuous wave radar operates differently; it continuously transmits a signal and analyzes the frequency shifts in the return signal to detect targets and determine their velocity. Frequency-modulated radar employs variations in frequency to improve target detection and resolution, making it effective for certain applications.

In contrast, "echo radar" does not fit within these established categories and does not represent a distinct radar technology used in military applications. Thus, identifying it as not a type of radar is accurate.