Every 6 hours (0,6,12,18 UTC), the Bureau of Meteorology (the BOM), launch a weather balloon. Where they land depends on mainly on atmospheric wind conditions at the time of launch. Some days, I'm out chasing them.
There are generally 3 types of payloads attached to these balloons:
Radar Reflector Only
These balloons are used for wind-finding - measuring the wind speeds in the upper atmosphere. They consist of the balloon and a radar corner reflector. They are tracked using a ground based radar, and are usually launched daily at 6Z and 18Z.
Radar Reflector + Analog Radiosonde
On these balloons, the radar reflector is used for wind-finding, and the analog radiosonde measures temperature, pressure, and humidity data and transmits it as a sequence of tones. They are called analog sondes as the is it not binary data that is transmitted. Rather, a sequence of 6 tones is transmitted, consisting of high and low calibration tones, then other tones for the weather parameters measured. To obtain meaningful data from the tones, calibration data (calculated just before launch) is required. The analog radiosondes transmit WFM modulated tones between 400.3 and 403MHz.
In most cases, the BOM will launch an analog sonde around 0Z and 12Z each day, to collect temperature traces, along with wind-finding data.
Digital Radiosonde (+ Radar Reflector)
Digital radiosondes have the usual array of weather sensors, along with a GPS receiver which allows wind-finding without a radar. Data from a digital sonde is transmitted as 2400 baud GMSK (Gaussian multiple-shift keying), and can be decoded using software such as SondeMonitor. The digital sondes also transmit in the 400.3-403MHz band, but use narrowband FM.
Digital sondes are reasonably expensive (~$200/sonde) and are used instead of an analog sonde in locations where a radar is not available. They are also often used when the local wind-finding radar is down for maintenance.
The Vaisala radiosondes are very interesting from an amateur radio standpoint, as they are are a full-featured sensor package in quite a small size. Potential also exists to re-use the sondes, though they would need to be modified to operate in the 70cm amateur band (420-450MHz).
To learn more about the radiosondes, we need samples to investigate. This means collecting radiosondes, which means tracking and chasing them.
To get a rough idea of where the balloon will land, we make use of prediction software written by students at the Cambridge University. The predictor is available online at http://habhub.org/predict/, and can predict the landing site of a weather balloon with reasonable accuracy (within 20km or so).
Once a balloon is launched, how it is tracked depends on whether it's an analog or digital radiosonde. Analog sondes have to be found using traditional direction-finding methods, such as with a doppler array, or a yagi antenna that is pointed at the target.
Digital sondes are much easier to track, as the transmit positioning data from their inbuilt GPS module. SondeMonitor can plot the positions of digital sondes on an internal map, and also makes the sondes position information available via a COM/OLE Automation interface.
Since Sondemonitor's internal mapping system can't handle large map files (i.e. files that cover very large areas), I wrote some code to read data from SondeMonitor and plot it in OziExplorer, a commonly used GPS mapping program. Flight-path predictions are also run every 30 seconds to help us navigate to the landing site for quick retrieval.
Once we get near the landing site, we switch to traditional direction-finding methods, as the sonde usually loses GPS lock once on the ground. Our standard method involves driving around the landing area, collecting (using a yagi antenna) and plotting bearings to the sonde on a map. The bearings will cross each other (intercept points), indicating where the sonde is located. We then either drive or walk to the sonde, using bearings from the yagi antennas to lead the way.