Wind measurement - Observing earth boundary layer while landing

 With XCVario, several speeds are measured :

• True AirSpeed (TAS) can be reconstructed from dynamic pressure & temperature measurements.
• GPS measurement (from FLARM at this stage) gives direct access to Ground Speed (GS)

 --> Difference between both TAS & GS is then an estimate of instantaneous wind in direction of flight.

As an illustration for this, the difference between TAS & GS is plotted during final landing phase, representing earth boundary layer above the airfield.

A few observations can be made

• On the 8th of may :
  
• Both landings show a reclined boundary layer profile, with weaker wind at landing altitude than @ 20m above ground
• On the 2nd landing, wind is stronger & deeper present on the ground
  
• Gust content is rather low for both landings
  
• On the 13th of september in St Auban :
• Wind is present almost down to landing altitude, with little reduction of wind component before the last 5m of altitude
  
• @t~10-12sec, a sensible gust can be observed ~70m above landing altitude, with a "gap" of TAS not visible on GS, and roll activity.
• Another gust is visible @t~30sec, 10m above ground
• A further gust is visible during ground roll -that has been filtered out of wind speed vertical profile
  

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Landing on the 8th of may 2021 against "vent d'Autan" with no lift

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Landing again on the 8th of may 2021 against "vent d'Autan" after a flight in weak distorded lift

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Landing on the 13th of september 2021 in St Auban