Passive Ionospheric Sounding and Ranging

Receiving Chirps

Up to this point, the ionosonde signal has been described as though it was a simple pulse. Thinking of it in this way makes understanding the process easier. However, it is difficult to transmit a narrow enough pulse to provide good time resolution, and at the same time provide sufficient energy in the pulse for good sensitivity. This problem is shared with radar systems, and the solutions are similar. In addition, the sounders require to measure the ionosphere throughout the HF spectrum, which is again not so easy to achieve with a pulse.

The ionosonde transmitter in fact sends a continuous carrier, but with smoothly changing frequency, at a fixed but accurate rate (in the case of most chirp sounders we use, with increasing frequency at 100 kHz/second). Knowing the chirp rate of the transmission, and what frequency the chirp is being received on, one can work out the nominal "chirp time" at which the received signal started at zero - by simply counting back at 100 kHz/second.

In the plot, we see that an example frequency of 18.5 MHz, corresponds to a 'chirptime' of 185 seconds (18.5/0.1), meaning that at 18.5 MHz, the chirp will be heard 185s later than the characteristic time for that station.

Peter's design uses the special chirped filter previously described, with properties not attainable with a conventional filter, and so is able to detect the transmissions with 0.66 ms time resolution, and with very narrow bandwidth that provides high sensitivity.

There are two useful advantages of this chirped filter technique:

- The receiver will have narrow bandwidth, so will work with low power sounder transmissions.

- The pulse response of the filter provides high time resolution.

The graph corresponding to the audio of the chirp in the text

In the case of conventional chirped ionosondes, the receiver is more conventional, but follows (tracks) its matching transmitter throughout the HF spectrum. In this passive sounding project, the receiver tracks many different transmitters using the chirped filter, but only over the width of an SSB receiver bandpass - about 2.4 kHz - since the receiver frequency is fixed. This approach is more than sufficient for sensitive single frequency measurements. You simply set the receiver frequency to suit the band you wish to know about. Download and listen to a typical chirp received in a 2.4 kHz bandwidth.