Ionosonde

An ionosonde, or chirpsounder, is a special radar for the examination of the ionosphere. The basic ionosonde technology was invented in 1925 by Gregory Breit and Merle A. Tuve and further developed in the late 1920s by a number of prominent physicists, including Edward Victor Appleton. The term "ionosphere" and hence, the etymology of its derivatives, was proposed by Robert Watson-Watt.
Components.
An ionosonde consists of:
The transmitter sweeps all or part of the HF frequency range, transmitting short pulses. These pulses are reflected at various layers of the ionosphere, at heights of 100–400 km, and their echos are received by the receiver and analyzed by the control system. The result is displayed in the form of an ionogram, a graph of reflection height (actually time between transmission and reception of pulse) versus carrier frequency.
An ionosonde is used for finding the optimum operation frequencies for broadcasts or two-way communications in the high frequency range.
Ionogram.
An ionogram is a display of the data produced by an ionosonde. It is a graph of the virtual height of the ionosphere plotted against frequency. Ionograms are often converted into electron density profiles. Data from ionograms may be used to measure changes in the Earth's ionosphere due to space weather events.
Chirp transmitter.
A chirp transmitter is a shortwave radio transmitter that sweeps the HF radio spectrum on a regular schedule. If one is monitoring a specific frequency, then a "chirp" is heard (in CW or SSB mode) when the signal passes through. In addition to their use in probing ionospheric properties, these transmitters are also used for over-the-horizon radar systems.
An analysis of current transmitters has been done using SDR technology. For better identification of chirp transmitters the following notation is used: <repetition rate (s)>:<chirp offset (s)>, where the repetition rate is the time between two sweeps in seconds and the chirp offset is the time of the first sweep from 0 MHz after a full hour in seconds. If the initial frequency is greater than 0 MHz, the offset time can be linearly extrapolated to 0 MHz.