[Merad]

Unless you limit yourself to flying very near the ground and very near sea level, the speed of an aircraft is more complex than a single number. In fact four different speed numbers are commonly used: indicated airspeed, calibrated airspeed, true airspeed, and ground speed.

* IAS is the raw airspeed reading from the pitot tube.

* CAS is IAS corrected for instrument errors, e.g. if the plane is at an angle that disrupts air flow around the pitot tube.

* TAS is basically CAS adjusted for altitude and air pressure. It’s the aircraft’s speed relative to the air around it.

* Ground speed (or speed over the ground) is TAS adjusted for the wind. This is the number that GPS is going to give you.

IAS and CAS are particularly important for describing performance characteristics - if an aircraft stalls at 100 knots CAS, then it always stalls at that CAS. If you try to describe the stall speed in terms of TAS you go from a single data point to a graph of speed and altitude.