OnAirDecode CODAR / HF ocean radar with an SDR
CODAR / HF ocean radar: the coastal radar chirps on the HF bands (receive only).
- Group
- HF utility
- Start frequency
- 13.5 MHz
- In the mode list
- Radar chirps, HF
What CODAR / HF ocean radar is
SeaSonde-class coastal radars sweep the sea with FMCW chirps in the ITU oceanographic radar bands (4.4 to 42.5 MHz). OnAir finds and measures the sweeps; it is a monitor of the signals, and never claims a position.
Frequencies and settings
The ITU oceanographic radar bands: 4.4 - 4.65, 5.25, 12 - 14, 24.45 - 24.65 and 41.5 - 42.5 MHz, 250 ksps and up (2 Msps takes in 1.8 MHz; a wide radio sees a whole band, a narrower one steps through the band presets with the scan option).
| Start frequency | 13.5 MHz |
|---|---|
| Tuning range of the mode table | 3 to 50 MHz |
| Sample rate asked | 2 Msps |
| Lowest working rate | 240 ksps |
| Channel width | 2 MHz |
OnAir takes any frequency you type; the numbers above are the mode's starting point and the range its table describes. In the app the shortcut label is Radar chirps, HF.
What OnAir shows
Finds the FMCW sweeps of SeaSonde-class radars as moving lines in a 1 ms spectrogram and measures centre, sweep width, rate, direction (rising / falling), repetition and level; a table of the radars heard (first / last heard, SNR history), a chirp waterfall with the sweeps marked, "heard on band X at time Y" history (propagation only, no position is claimed), band presets and a scan. Radars that share a frequency overlap, so the sweep numbers give only a "possible" hint. The Morse call sign that some radars send is read from the keyed energy of the whole sweep width and is always labelled unverified: it follows the published description, not a recording. Receive only: these are broadcast radar signals and nothing is decrypted or transmitted.
The mode's own tab has these views: Radars, Waterfall, Heard. The other tabs (Overview, Receiver, Scan, History) are described on the main window page.
Limits and what it was checked with
Checked with generated signals and impairments; recordings of real signals welcome.
Which radio you need
This is a shortwave mode, so the radio has to tune HF. An SDRplay, Airspy HF+ or an RTL-SDR in direct-sampling mode do that directly; with a HackRF an HF upconverter or a direct-sampling receiver is the usual route. A long wire or loop antenna matters more than the radio.
Receiving and the law
Questions about CODAR / HF ocean radar
Can I decode CODAR / HF ocean radar with an RTL-SDR?
An RTL-SDR can receive HF only in direct-sampling mode, which OnAir offers in its radio settings; results depend on the antenna. SDRplay and Airspy HF+ radios are better suited.
Is it legal to receive CODAR / HF ocean radar?
Some HF utility traffic belongs to governments or the military. Receiving is legal in many countries, but rules differ. Check them where you live.
Can I try CODAR / HF ocean radar in OnAir without a radio?
Yes. Most modes include a built-in synthetic test signal, so you can open the screen and try the controls before you own a radio.