Last Friday evening we made the sixth field test of Windsond. We obviously already have some experience with it but this was the first test of the final circuit board and there’s always usability kinks to work out.
We connected the battery to the sond and closed the plastic bubbles that protect the electronics. The computer with connected radio dongle picked up the signal directly. The computer program made a sound every time a data packet was received to let us know the connection was good. (This feature is turned off by default.) We inflated the balloon with helium and attached with a thread to the detachment mechanism on the sond. After a minute or so, the sond made a sound to let us know the GPS fix was good enough.
We let the balloon go after this brief photo shoot. It sailed away to slowly become a dot in the sky, and then invisible to the naked eye. The computer told us it was still ascending. 10 minutes and 40 seconds after start, the balloon reached 1000 meters altitude, the default top altitude. This was at a ground distance of 2651 meters, giving an average ground speed of 4.1 m/s and ascent speed 1.6 m/s. Previously we’ve filled the balloon more for a higher ascent speed. The radio connection was still good but since we’ve previously used the radio link at 5 km distance with no problem, we expected the connection to be strong.
A few seconds later, the sond released from the balloon as planned. It fell at a leisurely 4.7 m/s and continued to drift 647 meters with the wind during the free fall. During the fall, we lost radio contact with the sond. I had tested the sond with lower output power earlier and it might still be configured for lower power. I sent a command to the sond to increase output power again. This did the trick and we had good contact with the sond for the rest of the fall, down to 65 meters above ground. Due to lag in GPS updates, the real altitude was probably lower and the cut-off was caused by tree tops getting in the way. Still, this gave us an accurate GPS position of the landing site.
A mapping application on a smartphone translated the coordinates to a patch of forest about 3 km away. We drove there and I took a hike into the forest to recover the electronics. A trip of 350 meters each way, according to GPS. I brought my netbook along and about 230 meters from the site, I started receiving radio updates from the sond again. This confirmed the coordinates. A smartphone GPS application provided a compass needle that pointed towards the position which took out the guesswork of the hike even though the vegetation was thick. Some 10 or 15 meters from the coordinates, I heard beeping from the sond and could walk right up to it. It was laying next to a spruce. The envelope cushioned the impact well and nothing was damaged.
All that remained was to disconnect the battery, walk back to the car and study the generated graph of the winds up to 1000 meter altitude. In a real situation, we would use the graph as soon as the balloon reached the peak altitude but for this test retrieving the sond was more important. The plan was to make a hot air balloon flight later that evening but the wind picked up so we chose to postpone the flight.The computer software could use some polishing but all in all, Windsond is ready for live action now. After this confirmation, we’re starting to make more of the sonds.