Koninklijk Nederlands Meteorologisch Instituut; Ministerie van Verkeer en Waterstaat

Mode-S EHS

Mode-S Enhanced Surveillance
Within European designated EHS airspace:
  • All fixed wing aircraft, having a maximum take-off mass greater that 5,700 kg or a maximum cruising true airspeed in excess of 250 kts, intending to fly IFR as GAT, must be Mode-S EHS compliant.
  • Functionality: Aircraft compliant with Mode-S EHS provide basic functionality features plus the following eight downlinked aircraft parameters (DAPs):

Downlinked aircraft parameters
  • DAPs: not a broadcast but interrogation,
  • Most aircraft in Europe are EHS equipped,
  • ELS and EHS radar systems are identical,
  • LVNL uses ELS operationally,
  • LVNL uses EHS (TAR 1) non operational,
Derivation of Meteorological Parameters
Wind measurements

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.

Necessary Corrections
  • Magnetic correction applied to the heading
  • Heading correction applied to the heading
  • Airspeed correction applied to the airspeed
  • Flight phase dependent corrections for temperature (uncorrected airspeed)