What is calibrated airspeed (CAS) corrected for?

Calibrated airspeed (CAS) is an important parameter in aviation that reflects the true airspeed of an aircraft, taking into account specific errors that can affect the readings of an airspeed indicator. The correct answer identifies that CAS is corrected for both instrument errors and position errors.

Instrument errors occur when the airspeed measurement system itself is not measuring accurately due to calibration issues or quirks in the system. Position errors arise from the aircraft's location in relation to the airstream; for example, the airflow around the aircraft can cause discrepancies in what the airspeed indicator shows compared to the actual airspeed. By correcting for both of these factors, CAS ensures a more accurate depiction of the aircraft's true speed through the air, which is crucial for safe operations and flight planning.

While other options mention environmental conditions or variations in the speed of sound, CAS specifically focuses on internal measurement corrections rather than external atmospheric conditions or physical properties of air. Thus, recognizing that CAS encompasses both instrument and position errors makes the correct answer a comprehensive view of what is involved in achieving this essential measure of speed in aviation.