Koninklijk Nederlands Meteorologisch Instituut; Ministerie van Verkeer en Waterstaat

Mode-S EHS research
Upper air atmospheric wind and temperature information is crucial for numerical weather prediction (NWP) and nowcasting. A novel method to measure wind and temperature is related to tracking and ranging by an enhanced surveillance (EHS) air traffic control (ATC) radar.

A KLM Airbus A330-200 lands at Amsterdam Airport Schiphol in The Netherlands
Modern aircraft carry sensors to measure the Mach number (using pitotstatic probe) and the total air temperature (T). An EHS radar interrogates all aircraft in sight in a selective mode (Mode-S), on which the aircraft replies with a message containing, for example, magnetic heading, airspeed and Mach number. From this information wind and temperature can be extracted.

Wind measurements
An aircraft can stay airborne when it has a sufficient speed relative to the air, the so-called airspeed. The atmospheric wind alters the flight track of the aircraft and thus by calculating the difference between the expected flight path and the actual (ground) track, an estimate of the wind can be obtained.

Schematic representation of wind derivation from aircraft flight information. The wind vector (black) is deduced from the difference between the ground track vector (red), the orientation (heading) and speed of the aircraft relative to the air (dark blue). The ground track vector is constructed from ground speed and true track angle. Note that both heading and ground track angle are defined with respect to true north
Temperature measurements
The aircraft measures total air temperature (T) and Mach-number. The airspeed can be derived from the measured Mach-number and temperature, because the Mach-number is the quotient between the airspeed and the speed of sound. The latter is dependent on the temperature. We use the following equation to determine the temperature (T) from the Mach-number (M) and airspeed (Vair),

T = Constant * (Vair/M)2.