It won't actually have the same speed through the air, but I know what you're trying to say.
Forgetting instrument error etc., SAT is the temperature of the outside air, disregarding the motion of the aircraft; it is calculated from the TAT.
TAT is the temperature of the outside air, measured directly, ie it takes account of the kinetic heating of the air - easier to measure than SAT.
TAT is of more use, if the TAT(for example) in cloud is below+11c=engine antice on.
Free air temp?no idea!