This page is mainly for my own reference, but others may find it useful as well.
We are currently using iMet-1-ABxn 403 MHz radiosondes. These report temperature, humidity, pressure, and GPS position. They cost roughly $200 per unit, so we limit our launches to once or twice per year for teaching purposes.
The ground station is the highly portable iMet-3150 , which is essentially an off-the-shelf IC-R20 multi-band receiver together with an audio->modem->USB signal chain. The receiver must be configured with these settings to operate correctly with the newer 6 kHz bandwidth radiosondes.
The receiver provides an audio signal which is decoded by the modem and passed to a virtual COM3 port by way of the USB port on a Windows laptop. Alternatively, the audio signal can be plugged directly into the 1/8″ audio port on the laptop and decoded directly by software with that capability (e.g. Sky Sonde).
The standard software provided with the iMet-3150 system is iMetOS II.
From the vendor: “To download the software, you need to register at http://intermetsystems.com/ to get access to the Customer Resources tab. This is a simple process which also gives access to manuals and
other helpful documents.”
Here is a set of tutorial videos for an older version of the software. Part 3 is important! :
- iMetOS software installation
- Driver installation
- Disabling serial mouse detection
- Part 4?
- Setup of receiver and sonde
- Radiosonde flight
The iMetOS software assumes that the balloon is making a conventional ascent, and it requires a lot of ancillary information as part of the preparation. In addition, there have been times when the software has refused to permit the launch to proceed for reasons that are still unclear.
For the latter reason in particular, it is recommended that you use the record function of the IC-R20 receiver to capture the raw audio signal. You can subsequently play the audio back through the modem to capture the sounding in software, and if the software fails, you can repeat playback until the software is debugged.
The IC-R20 has three recording quality settings. “Fine” is only good for one hour. “Normal” is good for two hours and is probably sufficient both in quality and length, though I haven’t tried it yet.
Radiosonde and balloon prep
To save cost (and helium) we are currently utilizing 100 gm balloons rather than the more conventional 250 gm balloons. One must ensure that they have sufficient free lift to ensure a good ascent rate. An inflated diameter of 42″ turns out to be barely sufficient. In the future we will use a spring scale to directly measure the free lift and will experiment with different values to get the right ascent rate.
In the future, we will also consider using hydrogen instead of helium, as the latter is both costly and a non-renewable resource. Hydrogen has some safety issues which are easily manageable if understood. However, there is widespread anxiety about the “explosive” properties of hydrogen, so we will have to investigate policies for use first.
Since we are on the edge of Class B airspace for the Madison/Dane County airport, it is advisable to call air traffic control at 608-244-5691 a few minutes prior to launch. State that you are 5 miles SW of the airport and indicate the direction the balloon is expected to drift before reaching a higher altitude. Unless it is expected to drift in the path of approaching or departing traffic, they will readily give the all-clear.