http://www.ultralight-glider.fr/5625m-altitude-wave/

Leaving over the Fort de Dormillouse, the glide was very good, no
strong sink as feared, and surprisingly, the variometer went better and better,
and it eventually started to “sing”, in a perfect smooth air.

I observed the head wind indication of my Compeo+, it increased progressively,
so as the vario; when I got 1m/s I stopped flying NNE, and started a leg to the
right, monitored on the map display of my Oudie 3; it started to decrease after
a short time so I reversed the course, flying exactly the same line on the GPS
map, but on a reverse course.

The drift angle was about 30 to 40° the rate of climb increased again, reaching
+1,6m/s… when later it started to decrease, I reversed the course, and noticed
the maximum headwind indication on my Compeo during the turn: -35km/h.

I was in the wave. Passing 4000m I switched my oxygen on. I continued the out
and returns in the area of best rate of climb, trying to keep the line on my GPS
map. I had to pay attention, as the drift angle increased with the altitude.
Pascal had forgotten to open the valve of his oxygen bottle before take off, so
he had to decide to stop climbing at 4100m. Too bad, but wise decision.

After a turn, at around 5200m, when trying to set the good heading in order to
keep the line, my Oudie showed strange courses: S, then SE, “Hey! The GPS
failed” was my first thinking. I checked the course indicated by my Compeo: it
was the same! “Damned! I’m flying rearward!” I didn’t set big enough a drift
angle, so I turned more the nose of the Swiftlight in the direction of the wind…
the ground speed was 20km/h, but the course went to WSW! I turned too much!

I reversed the turn, slowly, to find the exact direction of the wind, watching
the wind component on the Compeo: I could read -85km/h! “Waooo!” I decided to
accelerate, full head wind, setting my flaps to zero: my ground speed went from
20km/h, to 15! I continued to accelerate, the ground speed continued to
decrease, then increased again! I was flying forward again