The weather service of the Federal Armed Forces at Idar Oberstein had launched volley after volley of DFM09s on June 2nd. Rather unusual and seldom, there were also launches from the Institute of Technology in Karlsruhe. Most radiosondes missed Stuttgart by far. Only 2 probes made it to the vicinity of Stuttgart. As one was quickly recovered in the morning of June 3rd by “Astronomin”, there was only one left. It had landed in the forest near Leonberg. Gisela and myself looking for a (home-office-due-to-Corona-relieve-)walk in an unfamiliar terrain gave it a go.
We arrived in the vicinity of the landing site. After one day I did not expect the probe to have any juice, so I had left all radios at home. The probe had made a steep descent with around 10m/s while only moving slowly forward (about 10km/h). The last transmission came from about 90 meters above the ground which gave it about 10 seconds more flight time. I hence expected the probe to have landed close to the point where radiosondy.info had picked up its last transmission. We quickly spotted it standing on the ground. As the DFM09 is relatively heavy and it had no parachute attached that can entangle itself in the tree tops, it went right through the foliage. Pulling on the string, we were able to completely recover probe, unwinder, and the rest of the balloon. From its last reported position at 48.7862N 8.97691E, the probe had traveled around 15 more meters horizontally before settling on the ground at 48.78610N 8.976807E.
Dismantling the probe at home, I realized that the probe was no longer transmitting but still had energy. The power LED was still illuminated after around 30 hours of operation. Remarkable.
The cord joining the unwinder and the balloon, and the thread connecting the probe the with unwinder must have somehow entangled during flight. There are abrasion on the thread visible where the thread rubbed on the unwinder. I wonder how that is possible. As the thread is on a spool at takeoff, it can have entangled only in flight. But then gravity ensures that the balloon, the cord, the unwinder, the thread and the probe are forming a (long) line. And I would not think that it happened after landing or during the recovery as the abrasion would not be as heavy and localized to the points where the thread is in contact with the unwinder. Maybe there were some gusts on the way releasing the tension from the thread and allowing for an entanglement with a knot in cord.
And here a link that shows the assembly of DFM09’s: https://sondehunt.de/language/en/archive/1189. I have received from a weather sonde group on telegram — thanks Lobelt.
