The radiosonde is a small, expendable instrument package that is suspended below a 1.8 metre wide balloon filled with hydrogen or helium. As the radiosonde rises at about 300 meters/minute, sensors on the radiosonde measure profiles of pressure, temperature, and relative humidity. These sensors are linked to a battery powered transmitter that sends the measurements to a ground receiver. By tracking the position of the radiosonde in flight, information on wind speed and direction aloft is also obtained. Observations where winds aloft are also obtained are called “rawinsonde” observations.
The radiosonde flight can last in excess of two hours, and during this time the radiosonde can ascend to over 35,000 meters and drift more than 200 km from the release point. During the flight, the radiosonde is exposed to temperatures as cold as -92C and an air pressure only a few thousandths of what is found on the Earth’s surface.
At liftoff, the balloon is around 1.5 metres tall and 1.2 metres wide. Because the pressure outside the balloon lessens as it goes higher in the atmosphere, the balloon expands. By the time the balloon reaches 30,500 metres it has grown to the height of a two story building. When the balloon has expanded beyond its elastic limit and bursts, a small parachute slows the descent of the radiosonde, minimizing the danger to lives and property.
The balloon launched from the weather bureau site at the western side of the
Mt. Gambier Airport at 9.45 a.m. on Monday morning and Tom, VK5EE, and I
Started tracking it as soon as we had acquired it’s signal.
The projected track would see the balloon and it’s payload landing somewhere
To the North East of Dartmoor.
The potential for finding it seemed to be promising at this stage.
Tom and I waited patiently for the balloon to land.
At this point neither of us have the capacity to track the balloons path from a car and we must, therefore, wait for it to land and then go to the nearest point that we can from the actual touch down and find the Sonde by using directional antennas and attenuators.
So far this has proved to be an easy and, almost, foolproof method but, this time, it really challenged us because of the nature of the scrub that it came down in.
Switching now from the Map view to the Satellite view it becomes obvious that this chase is going to be a challenge.
From the slide above it is easy to see the last know position of the balloon but at this point it is still at an altitude of 355 metres and heading East.
It is here that we realised that we were in for a challenge.
Tom broke out the gear which comprised his Icom RC_10 hand held receiver, a three element Yagi for 70 cm and a step attenuator.
Although we had clear paddocks on one side it was obvious that the balloon had drifted over them and into the scrub.
On getting out of the car and connecting the antenna to the radio, (with the attenuator in line), we could hear the balloon quite well but not on our Baoffeng hand helds, so it was obvious that we were going to have to go into the scrub and expend a bit of effort.
You can see the paddocks on the left, scrub on the right. What about the snakes you ask.
No problem – we are both fearless. (Ahem?)
Without a second thought we plunged into the unknown. As you can see the scrub was quite thick.
Fortunately life was made a little bit easier by being able to, mostly, navigate along very narrow animal tracks, Kangaroos probably, but occasionally we had to bush bash. The signals waxed and waned and we had to change direction a few times as the signal got a bit confused. The three element beam showed it’s shortcomings a bit because of the lack of focus. Without the attenuator we would have been wasting our time.
After about half, or was it three quarters of an hour, Tom reckoned we must be standing right on top of the Sonde as he had full attenuation switched in on the beam and we could hear the Sonde loud and clear on the Baoffengs with the antenna removed.
After looking around for a few seconds I raised my eyes to the heavens and spotted the tether line arcing through the trees.
After recovering the Sonde we walked back looking for the Balloon. It was hanging in a tree about 12 metres away and about 10 metres in the air. It is probably still there.
After congratulating each other on a very good outcome we set off to retrace our steps.
As that was nearly impossible we headed in the general direction and bush bashed back out to the road,
Emerging on the fire break between the paddocks and the scrub about 200 metres South of the car.
After getting back to the car and packing away the radio gear and the Sonde we guzzled a bit of nice cold water and set off for home.
The last photo shows the Shopping Trolley about to turn out of Spencers Track onto the Old Dartmoor Casterton Road
Another adventure done and dusted.
Thanks to Tom for the good company and his excellent skills in DF-ing the thing.