From AMS Glossary
(Also called ionosonde.) A radio device for determining the distribution of virtual height with frequency, and the critical frequencies of the various "layers" of the ionosphere.
A pulse at a certain frequency is transmitted vertically, and the time for its return is recorded on an oscilloscope; another pulse at a different frequency is then transmitted and timed. The process is repeated until the entire frequency range of interest, usually from about 1 to 25 MHz, has been explored.