Explanation: For accurate operation, a Variable Range Marker (VRM) on a radar display needs calibration against a known standard. Radar systems inherently generate highly accurate, fixed range rings derived from their internal timing circuits. These fixed rings serve as the reference points. To ensure the VRM provides accurate readings across its entire operating range, it is typically adjusted by aligning it with at least two widely separated fixed range rings: one near the minimum and another near the maximum of its operational limits. By matching the VRM's readout to these corresponding fixed rings, any potential non-linearity or scaling errors in the VRM circuit can be corrected, guaranteeing accurate range measurement throughout its functional span. Calibrating at only one point (like the median) wouldn't catch linearity issues. Adjusting the VRM oscillator frequency directly isn't the standard method for display accuracy; rather, the VRM's internal scaling or display generation is adjusted to align with the fixed radar-generated range rings.