Mode-S EHS

Mode S EHS, Mode S Enhanced Surveillance, is an air traffic control transponder protocol whose data downlinks yield aircraft-derived wind and temperature observations for weather models. Aircraft reply to secondary radar interrogation with their air vector and ground vector, from which a wind vector is computed, while temperature follows from true airspeed and Mach number. The European center EMADDC processes these broadcasts; the observing fleet is roughly 15 times larger than the older AMDAR system, giving dense upper-air coverage over Europe. The Met Office assimilates Mode S winds in its hourly 4D-Var, improving 12-hour wind forecasts by up to 6 percent against radiosondes.