Tutorial Ionospheric sounders

Tutorial: Ionospheric sounders (chirp sounders and ionograms)

What it is

Ionosondes and chirp sounders send a signal that sweeps across the HF band and measure the echoes from the ionosphere to find its height and density. They are scientific instruments, not communication, but they are loud: the sweep is a loud buzz that crosses the band every few minutes. OnAir finds the sweeps and draws an ionogram: delay against frequency, which shows how high the layers are and how far shortwave signals can reach.

What you need

• A wide receiver: the mode asks 20 Msps and 20 MHz; a HackRF at 20 Msps sees 18 MHz of the 0 to 30 MHz range. A narrow radio only sees the part it covers and builds up the picture over time (the panel shows the coverage in grey).
• A GPS-locked or calibrated radio is best for absolute timing; without it only frequencies and relative ranges are shown.
• HF antenna, as for the other HF modes (see Tutorial-HF-beacons). Start with the receiver tuned into the HF band (0 to 30 MHz): the panel complains when the captured span is outside it.
• No fixed frequency in the database; the sweeps cover the whole range.

Before you start

Chirp sounders are strong. Reduce the gain so that the broadcast stations in the HF band do not overload a wide radio. Make sure the computer clock is right if you want absolute delays.

Step by step

1. Connect the wide radio and antenna and start OnAir. Open the group Weather & science and choose Chirp sweeps (the entry reads Chirp sweeps, 0 - 30 MHz).
2. Set the centre so that the captured span lies inside 0 to 30 MHz.
3. Press Start. The panel says listening for sweeps ... until it finds one, then lists each sounder (#n with its rate in kHz/s).
4. Pick the Sounder (the latest heard is the default) and watch the ionogram build up as sweeps cross the span.
5. Open Scaling for the values derived from the traces, History and the coverage view.
6. Save PNG or Save CSV to keep the ionogram.

What you should see

The ionogram: 0 to 30 MHz across (50 kHz per column) and group range up (15 km per row, 0 to 3000 km), the brightness is dB over the median. The sweeps have rates such as 200 or 500 kHz/s. The traces mark the layers; the scaled values give foE, foF2, fxF2, h'F, hmF2, M(3000)F2 and MUF(3000)F2 when the sounder is local (vertical incidence). On a distant (oblique) sounder you get the path MUF and the distance.

Reading the screen

Sounder list and info (rate, period, start offset, Timing locked), Range, Scale (colour scale: dB over the median of each frequency column), Traces (draw the echo traces), Clear, Save PNG, Save CSV. The timeline shows one mark per sweep. Scaling and History show the derived values (see below).

Options and settings

• Range, Scale, Traces.
• Timing locked shows whether delays are absolute (sweep starts rounded to whole seconds) or only relative.
• The ? button explains what the sounders are and how to hear them.
• The built-in list of sounders is short and unverified: an identification by the rate alone is a guess.

Troubleshooting

• No sweeps heard. Not every sounder is in range; wait several minutes, as a sounder sweeps only every so often.
• Ionogram stays patchy. Your radio sees only a narrow span; the picture builds up over time.
• Delays look relative. The timing is not locked: you need an accurate clock or a GPS-locked radio.
• The span is outside 0 - 30 MHz. Tune the radio into the HF band.
• The sounder identification is wrong. The list is only a guess; trust the measured rate and period.

Tips

• A HackRF at 20 Msps is the easiest way to see a large part of the HF band at once.
• Save the CSV and plot your own graphs.

Legal and privacy notes

Ionosondes are scientific transmitters; receiving them is allowed. OnAir only receives. See Legal.

Under the hood

A chirp sounder transmits a linear frequency-modulated sweep (FMCW). Mixing the received signal with a copy of the sweep (dechirping) turns the delay of each echo into a beat frequency, so every echo path becomes a peak; stacking the peaks per frequency gives the ionogram. The group range is c times the delay; virtual height is half of it. Scaling follows the standard ionogram parameters (foF2, M(3000)F2 = 1490 / (h' + 176) and MUF(3000)F2 = foF2 times M(3000)F2).

Related

Mode-Ionospheric-sounders, Tutorial-CODAR-HF-ocean-radar, Tutorial-HF-beacons, Radio-HackRF, FAQ, Troubleshooting