WMO-GTS is a programme of the World Meteorological Organization (WMO) and is the abbreviation for "Global Telecommunication System".
Its "coordinate the global system of telecommunication facilities and arrangements for the rapid collection, exchange and distribution of observations and processed information within the framework of the Wolrd Weather Watch Programme".
Sonnblick Observatory's employees observe the weather situation every three hours. Such an observation is also called a synoptic observation including the state of clouds, visibility, temperature, pressure and some more meteorological paramter. These weather information are coded based on the so-called FM-12 Code. Each observation is related to a specific number. Hence the whole synoptical observation ends up in a sequence of numbers. The sequence is sended to Vienna to the ZAMG. From there it is forwared into the GTS net. Due to the GTS network weather observation are done at the same time at different places all over the whole using one method. That's why the observation data are available in the internet shortly after the observation and can be used for e.g. weather forecasts.
You want to learn more about GTS? Visit the website of the WMO: https://public.wmo.int/en/programmes