Tromsø real-time ionogram
Ny-Ålesund real-time ionogram
Ionospheric soundings are performed by so called ionosondes. Ionosondes are radar systems that sweep their transmitted frequency from 1 MHz to about 20 MHz. This is the same frequency band as the resonant frequency of electrons in the overhead ionosphere. These electrons will reflect the signal from the ionosonde. Since the resonant frequency is proportional to the electron density and the density varies as function of height, the ionosonde signal is reflected from different altitudes, enabling the possibility to determine the electron density profile (electron density as function of height).
The information from the ionosonde aids users of HF communication systems to choose the appropriate frequency for their operations under the current conditions. Modern ionosondes have additional capabilities such as measuring the location of the reflection in the ionosphere (angle of arrival) and the drift velocity of the ionosphere. The data from an ionosonde is normally represented as a chart where the received signal is placed according to their frequency and apparent altitude of reflection, the angle of arrival is indicated by a color code.
Tromsø Geophysical Observatory operates a Lowel Digisonde outside Tromsø and a Canadian Advanced Digital Ionosonde (CADI) in Ny-Ålesund on Svalbard.