What is an Angle of Attack Sensor?

Angle of attack (AOA, α, Greek letter alpha) is a term used in fluid dynamics to describe the angle between a reference line on a lifting body (often the chord line of an airfoil) and the vector representing the relative motion between the lifting body and the fluid through which it is moving. In flight dynamics it is given the shorthand notation α (alpha) and is the angle between the lifting body's reference line and the oncoming flow.

In Aerodynamics, angle of attack is used to describe the angle between the chord line of the wing of a fixed-wing aircraft and the vector representing the relative motion between the aircraft and the atmosphere. Since a wing can have twist, a chord line of the whole wing may not be definable, so an alternate reference line is simply defined. Often, the chord line of the root of the wing is chosen as the reference line. Another alternative is to use a horizontal line on the fuselage as the reference line (and also as the longitudinal axis).   Some literature adopt the so called absolute angle of attack: zero angle of attack corresponds to zero coefficient of lift.

Angle of sideslip (AOS, β, Greek letter Beta), is a term used in fluid dynamics and aerodynamics and aviation. It relates to the rotation of the aircraft centerline from the relative wind. In flight dynamics it is given the shorthand notation β (beta) and is usually assigned to be "positive" when the relative wind is coming from the right of the nose of the airplane. The sideslip angle is essentially the directional angle of attack of the airplane. It is the primary parameter in directional stability considerations.

Useful References:
Military Specification MIL-P-26292 (USAF)
"Pilot and Static Pressure Systems, Installation and Inspection Of"
NASA Reference Publication 1046
"Measurement of Aircraft Speed and Altitude," Wm. Gracey, May 1980 #N80-24296