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Ionosonde Probes HF Propagation

ionosonde probes

Ionosonde probes provide you with  accurate, real-time measurements of Maximum Useful Frequency for shortwave radio propagation.

Tucked away in the back of January 2022 QST, you will find a great article by Eric KL7AJ. “Using an Ionosonde to Understand the Ionosphere” is brilliant and long overdue. Here’s the how and why.

An ionosonde is HF radar which sends signals straight up and measures reflections from various layers. So, an ionosonde probes to find out at what frequencies signals start and stop being reflected by ionized free electrons way up there. Typically, an ionosonde probes by sweeping frequencies between 2 to 12 MHz.

As you can see at the right above, reflections come back at what is called the critical frequency for a particular ionospheric layer, e.g. foE, foF1 and foF2. You can imagine a radar that sends out a pulse, listens for a reflection, then moves to the next frequency in the sweep. You get a fresh ionogram every fifteen minutes.

Ionosphere layers of interest between 70 to 300 km. Height determined by reflection time delay. Lots of number crunching as electron density of layers is quite variable. Accuracy of phase and amplitude measurements depends on the signal to noise ratio of the reflections.

More than sixty ionosonde probes are located around the world. You can find the results of their work at the NOAA Real Time Ionosonde Data Mirror and from Lowell Digisonde International LDI.

Ionosonde Probes Online – A Great Leap Forwards

Historically, we have relied on statistical estimates and correlations for our propagation forecasting. These have been based on static historical ionograms and algorithms driven by sunspot numbers (SSN) or solar flux. These are decent estimates, but a far cry from real-time measurements.

With ionosonde probes online, we get actual measurements of ionospheric activity across the world. In future articles, we will describe how to read these results and move away from guesses to facts.

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