Ionospheric sounding - The G3PLX Chirp Project

Passive Ionospheric Sounding and Ranging

Background

In previous experiments, Peter G3PLX has used Digital Signal Processing (DSP) and doppler techniques to measure small differences in carrier frequency that result from movement in the radio propagation path. While interesting for meteor scatter, aircraft and satellite reflections, and more gross ionospheric effects, in using this technique it is not possible to infer information about a relatively static propagation medium. Peter realised that what was required was a time domain - rather than frequency domain - technique, for example measuring the propagation of pulses. It was soon established that a wideband technique, rather than a carrier based technique would be necessary.

In searching for suitable pulse transmissions to use, preferably transmissions available from all over the world on a 24 hour basis, Peter stumbled across a family a pulse generators that are used as swept frequency ionospheric sounders. In their normal application, research, professional and military groups use these low power devices to probe the ionosphere to measure propagation.

The signal consists of a single long "chirp", sweeping up in frequency at a constant rate. These transmissions are tracked by a companion receiver which is zero beat with the transmitter, and so ionospheric reflections that are returned with short delays are heard as lower sideband audio beats of a few hundred Hz. The equipment then builds an "ionogram" or two dimensional graphical representation of the ionosphere's reflection height or delay against frequency. The adjacent picture illustrates a typical commercial 50W FM/CW (chirped) ionospheric sounding transmitter.

The first step was to discover how these chirped signals could be used in a passive manner, i.e. without reference to the transmitter oscillators or timing reference. To do this, Peter developed a very clever chirped filter, which not only sweeps in frequency at 100 kHz/second, but has properties not possible in a conventional filter - a bandwidth of only 66 Hz, but a pulse resolution of 0.66ms. This filter and matching detection software formed the basis of the adventure to follow. 49169

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