Radar pulses and sounders
Other radar signals and 'sounders': equipment that sweeps across frequencies to measure the ionosphere, or measures ocean currents.
Radar pulses and sounders sound like a quick 'zip' or swoosh as a sweep passes through your receiver, or a steady rapid ticking or buzzing that stays on one frequency.
What does it sound like?
A quick 'zip' or chirp as a sweep passes by, or regular clicks and pulses.
12.0-second example. Recording: Japan Ministry of Defense / JMSDF, via Wikimedia Commons (CC BY 4.0).
Who uses it?
Research agencies, weather and ocean monitoring, and defense organizations.
Where and when can I hear it?
Across shortwave (ionosondes sweep the whole range) and on VHF/UHF.
Easily confused with
- Over-the-horizon radar, from the same family
How to recognize it
These signals do not carry messages; they measure things. Their sound depends on how they work:
- Chirp sounders sweep smoothly across the whole shortwave range. Your receiver only hears a slice a few kilohertz wide, so the sweep flashes past as a short rising or falling "zip," then silence until the next sweep minutes later.
- Ionosondes step through frequencies sending short pulses, heard as a brief burst of ticks or a rough buzz as they pass.
- Ocean current radars near coasts repeat a sweep about once or twice a second, giving a regular "whoosh... whoosh".
- Over-the-horizon radars send powerful pulses many times a second, heard as a loud, wide knocking, tapping or buzz that can blanket part of a band.
Easily confused with: electrical interference, which is usually steadier and tied to the power line's 60 or 120 Hz buzz, and data modes, which have a message-like rhythm rather than a pure repeating pattern.
Where and when to hear it
Anywhere on shortwave. Ocean radars have their own allocations, set by the 2012 World Radiocommunication Conference, including 4.438-4.488, 5.250-5.275, 13.450-13.550 and 24.450-24.650 MHz, and you can hear them strongly near coastlines. Over-the-horizon radars often intrude into the ham bands; the IARU Region 1 Monitoring System publishes monthly reports of the ones it logs.
Chirp and ionosonde sounders run day and night from research sites and military stations worldwide, so patient listening on any quiet frequency will eventually catch one passing through.
A little history
Ionosondes have measured the ionosphere, the electrically charged upper atmosphere that bends shortwave signals back to Earth, since the 1920s and 1930s; their readings tell forecasters which frequencies will work. The most famous radar sound in radio history is the "Russian Woodpecker," a Soviet over-the-horizon radar (Duga) whose loud 10-pulse-per-second tapping disrupted shortwave listening from 1976 to 1989. Modern systems such as Australia's Jindalee Operational Radar Network keep the technique alive.
Coastal ocean radars became widespread from the 1990s and now map surface currents for search and rescue, oil spill response and forecasting along many coastlines.
Common questions
Is it legal to listen to radar and sounder signals?
Yes. They carry no messages, and hearing them is ordinary radio listening. Many hobbyists log them as part of monitoring propagation.
What was the Russian Woodpecker?
A Soviet over-the-horizon radar, known as Duga, whose repetitive tapping at about 10 pulses per second was heard worldwide on shortwave from 1976 until 1989. Its huge antenna still stands near Chernobyl in Ukraine.
Why do I hear a 'zip' every few minutes on a quiet frequency?
Most likely a chirp sounder sweeping across the shortwave spectrum. It passes through your receiver's narrow listening window in a fraction of a second.
Can I decode a chirp sounder?
There is no message to decode, but with a software-defined radio, open-source tools such as the chirpsounder2 project can detect sounders and turn them into pictures of the ionosphere.