Explanation: To eliminate stationary objects like trees and buildings from a PPI (Plan Position Indicator) radar display, Moving Target Indication (MTI) systems are used, which rely on the **Doppler effect**. Moving targets cause a frequency shift in the reflected radar signal, while stationary objects reflect signals with no significant frequency shift relative to the transmitted pulse. A **discriminator** is a circuit that produces an output voltage proportional to the *difference* in frequency from a reference. When a discriminator is used as a second detector in an MTI radar system, it processes the Intermediate Frequency (IF) signal. It identifies and emphasizes signals with a Doppler shift (from moving targets) while suppressing signals with no shift (from stationary clutter). This allows only the echoes from moving targets to be displayed on the PPI, effectively "eliminating" the stationary objects. The other options are incorrect: * **A) Remove the discriminator:** This would remove the ability to distinguish moving from stationary targets if the system is designed for MTI. * **C) Calibrate the IF circuit:** Calibrating the IF circuit ensures proper signal amplification and filtering but does not provide the mechanism for Doppler-based clutter rejection. * **D) Calibrate the local oscillator:** Calibrating the local oscillator ensures accurate frequency conversion, essential for general reception but not specifically for motion detection.