Radiosondes are devices who are launched across the globe two times a day (sometimes more often) with the help of wheather ballons by meteorological organisations. Vaisala is one of the leading manufacturer and even the inventor of modern radiosondes. Check out sondehunt.de or radiosonde.eu for more details.

The RS41 to be thorn down is of the newer variety with an EPS (Styropor) housing, weighing only 84 g, which helps saving in helium and reduces the amount of waste in the landscape if no sonde hunter comes picking it up - pretty cool.

The -SGP suffix means the sonde is equipped with a barometric pressure sensor. The more common -SG version calculates the pressure based on GPS data. There is also an -SGM version for military applications featuring a 'radio silence' mode activating the radio transmitter at a certain altitude so no enemy can determine the launch site.