What are common error sources in radar altitude readouts (Mode C) and how are they mitigated?

• A. Wind shear and microbursts
• B. Antenna sidelobes and ground clutter suppression
• C. Altimeter setting errors, encoding drift, and calibration issues
• D. Receiver saturation and data link outages

Answer: (C)

Mode C altitude readouts come from the aircraft’s pressure-altitude information encoded by the transponder. The most common sources of error are tied to how that altitude value is derived and transmitted: if the altimeter setting used by the system is incorrect, the pressure altitude being encoded will be off; encoding drift in the altitude encoding unit or transponder can cause the transmitted value to wander over time or with temperature; and calibration issues—whether in the altimeter, encoding circuitry, or associated transponder hardware—can produce systematic offsets or improper scaling of the altitude code. Mitigation focuses on keeping the reference altitude accurate and the encoding hardware well maintained: ensure the correct, current altimeter setting (QNH) is used and updated from reliable sources, perform regular calibration and maintenance of the altimeter and the altitude-encoding transponder, and run periodic Mode C checks with verification against independent altitude references (such as GPS or other ground/air reference data). These practices help ensure the altitude readout remains consistent with the aircraft’s true altitude and with ATC expectations.