Thomas Krahn KT5TK has launched five Mylar balloons each 80cm diameter, that carried a Pecan 144.390 MHz APRS transmitter KT5TH-11 weighing just 55 grams. It transmitted every 5 minutes to save battery power.
Balloons such as these do not go to extremes of altitude but can float at between 3,500 and 6,000 metres for an extended period.
In the United Kingdom radio amateurs including foundation license holders have been involved in developing Pico payloads. One such is PICO which launched from Suffolk on Saturday, October 20, transmitting RTTY on 434.300 MHz and was last reported approaching Denmark.
PICO doesn't have an amateur radio callsign since balloon flights are outside the scope of the UK amateur license so they operate under license exempt regulations.in the 433.050 - 434.790 MHz band.
You can enter a callsign (e.g. KT5TK-11) or ID into this site to get an APRS track - http://aprs.fi/
You can see also tracks of current launches by checking this page - http://spacenear.us/tracker/
PicoAtlas - http://ukhas.org.uk/projects:picoatlas