Before discussing on CAS, IAS, TAS, GS and Mach Number it is important to consider the measurement of total (pitot) and static pressure.
Total Pressure – To measure total pressure airflow is stopped by means of a forward facing tube called as pitot tube. This tube measures the impact pressure which is sum of static and dynamic pressure.
Static Pressure – Static Pressure is measured by series of symmetric static probes which are placed perpendicular to airflow. By keeping them perpendicular impact pressure can be avoided and only static (ambient) pressure can be captured.
Dynamic Pressure – This is the difference of Total Pressure and Static Pressure.
Calibrated Air Speed (CAS)
CAS is the measurement of dynamic pressure acting on aircraft. It is obtained from the difference between total pressure and the static pressure.
Indicated Air Speed (IAS)
This is the speed indicated by airspeed indicator irrespective of flight conditions. IAS is equal to CAS when pressure measurement is accurate (hypothetical situation). Some factors like aircraft angle of attack, configuration, winds, ground proximity etc always introduce some measurement errors – mainly on static pressure. This small difference between IAS and CAS is called instrument correction.
IAS = CAS + Instrument Error
True Air Speed (TAS)
Since the medium (air) in which aircraft flies itself moves as wind aircraft now has two physical speeds – TAS and GS (explained later). TAS is the physical speed of aircraft with respect to air.
Ground Speed (GS)
With reference to para above, GS is the physical speed of aircraft with respect to ground (earth surface).
Ground Speed = True Air Speed + Wind Component
Mach Number (M)
Mach number is defined as the comparison between TAS and the speed of sound. As the aircraft speed increases the effect of pressure waves become more and more dominant. Since these pressure waves are nothing but sound waves it is important to compare the speed of aircraft with speed of sound.
where LSS local is speed of sound
LASS = 39 SQRT(SAT-K)
SAT-K : Static Air Temperature in Kelvin