Sixty eight ionosonde observatories around the world contributed daytime ionogram images taken on 11 January 2021 to celebrate 90 years of 24/7 ionosonde monitoring of the ionosphere, which started at Slough, UK in 1931.

The majority of ionosondes are streaming their real-time measurements to the Global Ionosphere Radio Observatory (GIRO, http://giro.uml.edu)