4) Calibrated Airspeed (CAS) Calibrated airspeed is indicated airspeed corrected for instrument and positional errors. At certain airspeeds and with certain flap settings, the installation and instrument errors may total several knots. This error is generally greatest at low airspeeds, with nose high pitch attitudes.
Ground Speed (GS) Ground speed is TAS corrected for tailwind or headwind component. Most of the time, the wind will not be a direct headwind or tailwind. You must figure out what portion of the wind is acting as a tailwind/headwind and then use that number to correct TAS and come up with a groundspeed. Click on this
True airspeed is important because it is used extensively in flight planning and it’s tied to aircraft performance. What is true airspeed? Here are three book definitions. True airspeed is the airspeed of an aircraft relative to undisturbed air. It’s the aircraft speed relative to the airmass in which it’s flying.