History of Vertical Ionospheric Soundings
Sounding of the Ionosphere began in 1925. By 1947, an instrument known as the ionosonde was routinely used in the field to measure automatically the characteristics of the Ionosphere. During the International Geophysical Year (IGY) in 1957-58, an international cooperative effort created a worldwide network of ionosondes to record vertical sounding measurements during the 1957-59 period of maximum solar activity.
The IGY ionosonde was a relatively simple, robust, inexpensive piece of equipment by today's standards. Several countries built IGY ionosondes with few differences among them and thus provided worldwide similar photographic records called ionograms.
Since the IGY, a loosely coordinated worldwide network of vertical sounding ionosondes, which has varied between 100 and 200 sites, has operated continuously.