1. Field of the Invention
The present invention relates to a method of sounding a channel such as a radio channel, or an acoustic channel, in a general manner.
2. Description of the Prior Art
A method of this kind has applications in the ionosphere sounding techniques, for example. Being made up of ionized layers, the ionosphere behaves as a medium of which the dielectric constant varies according to the ionization and the square of the frequency of the received wave leading accordingly to reflection of this received wave.
The person skilled in the art will nevertheless understand that the invention is equally applicable to the sounding of transmission channels on wires, to the detection of targets in radar or sonar applications, or to the radiography of organs in medical applications or of structures in industrial applications.
All the applications mentioned above are based on the emission by an emitter of a signal that is reflected or transmitted in the channel and received as a noise signal added to a sounding signal characterizing the medium in which the emitted signal was propagated, for example indicative of the presence of targets, ionized layers, . . . etc. The sounding signal received as a result of reflection or transmission has characteristics correlated to the characteristics of the emitted signal but with a highly attenuated and therefore very low amplitude. The very low amplitude of the sounding signal received as the result of reflection or transmission is not a problem in itself since it is sufficient to amplify the received signal before analyzing it.
However, this attenuation of the sounding signal received as the result of reflection or transmission is always accompanied by background noise of internal or external origin, such as atmospheric noise. This means that errors can arise on analyzing the received signal through confusing a high amplitude noise peak with the sounding signal (false alarm) and conversely through confusing a low amplitude received sounding signal with noise.
All prior art solutions partially solve this problem by calculating the correlation between the emitted signal and the received signal or by filtering the received signal, for example. They all require very high power signal emitters, for example a power in the order of several hundred kilowatts, for a sounding range of 1000 km, and are therefore very costly to implement.