Abstract
Vertical ionospheric soundings have been performed at almost all ionospheric observatories with little attention to measuring the attenuation of the signal between transmission and reception. When the absorption has been determined, this has been achieved by comparing the received power after the first and second reflections, but this method has some limitations due to the unknown reflection coefficient of the ground and the non-continuous presence of the second reflection. This paper deals with a different method based on precise calibration of the sounding system, allowing determination of absolute signal attenuation after a single reflection. This approach is affected by a systematic error due to imperfect calibration of the antennas, but when the focus of interest is to measure a trend over a specified period, it is very accurate. The article describes how calibration was implemented, the measurement output formats, and finally it presents some results from a meaningful set of measurements in order to demonstrate what this method can accomplish.
Highlights
One of the most widely used methods to study the ionosphere is vertical sounding, in particular to determine the electron density profile as a function of height
The special type of radar used for this kind of measurement is called an ionosonde and it does not need to measure the intensity of the reflected echo, but only detect the presence of an echo and measure the delay relative to the transmitted signal [1]
In order to overcome these problems a comparative method had to be applied: the relative amplitudes of the signals received after a first and second reflection were measured and a relatively simple elaboration of the result allowed determination of the actual absorption [21]. This method has the advantage of eliminating the problems mentioned above regarding the determination of system parameters. It subject to three possible critical issues: (a) it is assumed that the reflection off the ground between the first and the second reflections off the ionosphere takes place without loss; (b) the second reflection has to take place and be detected, which does not always occur because often absorption is so great that the signal is no longer detectable after the first reflection; (c) in systems that use linear polarisation antennas, the polarisation plane rotates every times a signal undergoes an ionospheric reflection; this causes a gain which is extremely difficult to predict and after a second reflection becomes varied in an unpredictable way
Summary
One of the most widely used methods to study the ionosphere is vertical sounding, in particular to determine the electron density profile as a function of height. It subject to three possible critical issues: (a) it is assumed that the reflection off the ground between the first and the second reflections off the ionosphere takes place without loss (i.e., all the incident energy is reflected back towards the ionosphere); (b) the second reflection has to take place and be detected, which does not always occur because often absorption is so great that the signal is no longer detectable after the first reflection; (c) in systems that use linear polarisation antennas, the polarisation plane rotates every times a signal undergoes an ionospheric reflection; this causes a gain (or loss, depending on the antenna system) which is extremely difficult to predict and after a second reflection becomes varied in an unpredictable way The need resolve these critical issues induce the authors to try and determine absorption using a direct method, which involves calculating or measuring all the parameters characterising an ionosonde by calibrating the receiver, transmitter, and antennas in a particular way. Various ways of presenting the measurement data obtained from the calibration will be presented, together with some actual measurements
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