A radiosonde or weather balloon is an instrument with a series of sensors to measure the atmospheric parameters from ground level up to an altitude of 35,000 meters (100,000 feet).
The balloon carrying the radiosonde is made of latex, neoprene, or polyethylene and inflated with hydrogen or helium to have an ascension speed of 5 m/s. Depending on the rating, the balloon bursts when it reaches an altitude between 10 to 35 km. The radiosonde transmits information on the atmospheric conditions using a small radio transmitter. Weather balloons are launched from over 1000 locations worldwide two to four times a day. Weather agencies use the information collected by these radiosondes for wind and weather forecasting.
The International Telecommunication Union (ITU) has allocated the following frequencies for radiosonde, dropsondes1, or rocketsondes2: 400.15-406 MHz and 1668.4 1700 MHz.
The data transmitted by the radiosonde are:
- The sonde serial number
- Altitude in meter
- Temperature at 1/10°C
- Dew point temperature
- Atmospheric pressure in hPa
- Relative humidity
- GPS position (lat/lon)
- Horizontal velocity
- Vertical velocity
The weather services are interested in the data collected during the ascension of the balloon. All the data sent by the radiosonde after the balloon bursts is discarded. Hunting is possible because the radiosonde continues to transmit data for a few hours after it lands on the ground.
Some ham radio operators hunt and retrieve these weather balloons to work on and improve their radio direction finding (RDF) skills. The sensors used in these radiosondes are of high quality. Some operators reprogram the firmware and use the radiosondes they retrieve to build their beacons, weather stations, or amateur weather balloons.
In some areas, competition is fierce and some trackers can be at the landing site before the radiosonde touches the ground.
I have installed a permanent radiosonde receive station in my garage. This station uses a Raspberry Pi with a cheap RTLSDR USB dongle to listen 24/7 to the UHF band (400-405MHz). The software I use to track and decode the data sent by the radiosonde is radiosonde_auto_rx. The decoded data is sent online on sondehub as well as radiosondy. Radiosonde hunters will be able to use this information to recover burst balloons.
I have built a ground plane antenna resonant at 404MHz, which is the frequency of the radiosondes sent from the Oakland airport. The antenna is a chassis Type N connector where I soldered five branches made of household electrical copper wires. (see picture above)
I have also written a short program that reads the logs from radiosonde_auto_rx and draws the weather balloon's path on Google Earth. This program is called sonde2kml and is publicly available on my github account. The screenshot below shows the path of one weather balloon launched from Oakland Airport in California.