Ozonesonde RS92 Hunt 2018-01-15

The “normal” radiosondes are launched twice a day from capital city airports at the international standard of 45 minutes before 00Z and 12Z (midnight and noon UTC time).

However, on occasion we’ve received some interesting signals from sondes that appear to have been launched from the general area of a location in Broadmeadows - which a bit of judicious Google Street View stalking reveals to be BoM’s training facility.

These appeared to be a mixture of the old fashioned RS92s and the new, much more desirable RS41s. While RS41s are quite nice, further decoding of the RS92 signal revealed something even more interesting - it had an auxillary payload to measure ozone.

Noticing one launched early Monday afternoon (a surprise because we’d previously only seen launches from that facility on Wednesdays and Fridays), Michael Wheeler and I decided to attempt a retrieval mission. Due to work commitments, we couldn’t just drop everything and run, so we decided to head out after work and hope it was still sitting where it landed.

The signal was received down to about 1500m altitude - an encouragingly good result leaving us with a search radius of about 1km. The plan was to drive out there then drive around until we receive the signal, get GPS coordinates then find it.

We drove two hours out there, we received the signal but then there was one small problem.

The sonde had no GPS lock. Mark Jessop had always said this was a possibility after landing if the antenna position is not pointing the right way… but we hadn’t encountered it yet. First time for everything.

Irritatingly, we could pick up the signal on the rooftop omni antenna, but our direction finding yagi couldn’t. The DF antenna normally suffices despite being made of measuring tape and watering system pipe because when it’s previously been in use, we’ve been quite close to the sonde, just unable to see it. Additionally, with the sonde possibly on the ground, wire fences between it and us don’t help.

We knew it had to be somewhere local as we could receive it - so we drove around looking at the signal strength. When the signal was the strongest, we could finally pick it up with the yagi. This gave us a direction for the field it was probably in. A little further down the road, we came across the gate into the field, wide open.

We attempted to find someone to ask for permission to enter the field, but being unable to find any house on the block of land, we decided that it would be safe to enter given that it was daytime, the gates were left wide open (not just unlocked), no signage was present advising people not to enter and the field was not currently cropped, nor containing livestock.

The direction finding worked a little better once inside the wire fence, and we had a direction. In that direction was something that looked like a large plastic bag blowing around - and if you didn’t spot it in the above photo:

The direction finding gear indicated it was in the direction of the mystery object, so we walked about 1km in until we reached it. The package had clearly hit the ground with a large amount of force - the sonde itself had detached from the ozone payload and was found a metre away. The quadrifilar GPS antenna was bent out of shape.

Gathering up the sonde, the sensor, the parachute and all the scraps of balloon, we exited the field and went back to the car, successful.

Normal sondes don’t have a parachute (if you don’t count the cardboard radar reflector) but due to the size, weight and re-usability of this payload, one was fitted.

The ozone sensor payload will be going back to BoM as unlike the RS41 / RS92 units, it can be used again.


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