A Nichrome wire is tightly pulled by two supports. This arrangement is mounted on the outside of the airplane such that airflow is at right angles to the axis of the wire.
At speed, the wire is pulled along by the airflow and exerts force on the supports. This force is measured and correlated to the airspeed as measured by Pitot tube or other airspeed sensors.
The turbulence on the wire vibrates it as well as pulling it along and this is monitored by a microphone in one of the supports. The microphone might be a moving coil dynamic type (very rugged). Spectrum analysis of the turbulence is done by DSP to monitor the condition of the wire and whether or not something is caught on it, such as a tree leaf. If so, the spectrum changes and the measurement can be disqualified.
The wire is heated by means of an electric current so that its average temperature is at the boiling point of water. This can be done with a fairly low voltage (less than 12V), which is interrupted at regular intervals to measure the resistance of the wire to determine the temperature. The heated wire prevents ice from forming.
The wire supports must hold the wire above the worst-case icing surface height, so that airflow around the wire remains unobstructed in bad conditions.
A practical implementation might consist of three of these sensors so that a majority-rule consensus output might be maintained. Accuracy is not required to be greater than about +/-5% since this sensor augments standard Pitot tube sensors.
The nichrome wires must be easily and quickly replaceable by ground maintenance, as they are easily broken by bird strikes, and they will wear out from metal fatigue.