A GPS locked oblique ionosonde system using IRIS

Extensive use is made of radio sounding systems to probe the ionosphere remotely. One category of remote sensing system is known as the ionosonde, which is essentially a radar emitting pulses, chirp or other waveforms to measure the group delay of the return signal bounced back from the ionosphere. In the case of oblique ionosondes the transmitter and receiver may be separated by distances of up to several thousand kilometres. Oblique chirp sounders sweep a phase-continuous signal over the frequency band of interest at rates of up to several hundred kilohertz per second. Due to the method of receiver operation a comparatively narrow receiver bandwidth, of the order of a few hundred Hertz, is used and so a relatively low transmitter power (10-100 W) is required. One of the main disadvantages of chirp sounders is the relatively complex analysis that must be carried out to determine the ionospheric propagating modes. The advent of specialised digital signal processors, together with other technologies, has simplified this task and enabled more extensive development of chirp sounder receivers to be undertaken. This paper describes a new chirp ionosonde which is time locked, at both transmitter and receiver, to provide accurate absolute measurements of the group delay rather than the more usual relative measurements.