Tutorial CODAR HF ocean radar

Tutorial: CODAR and HF ocean radar

What it is

HF ocean radars (SeaSonde and similar) measure ocean surface currents and waves by sending a swept signal along the coast. Their signal is a repeating chirp that sounds like a regular buzz or tick on the HF bands. OnAir finds the sweeps as moving lines in a spectrogram and tells you which bands have radars, how wide and how fast they sweep, and when they were heard. It does not claim positions of the radars.

What you need

• A radio that tunes the HF bands: the radars sit between 4 and 50 MHz. A wide radio sees every radar of a band together; a narrow one needs the scan. The mode asks 2 Msps and works from 240 ksps.
• An HF antenna: Tutorial-HF-beacons describes antennas.
• Frequencies (database, ITU Resolution 612 allocations; band centres only): 4.463, 5.2625, 9.33, 13.5, 16.15, 24.55, 26.31 and 40 MHz, with bandwidths from 25 kHz up to 1000 kHz. The radar sits somewhere in the band and sweeps across it.

Before you start

Reduce the gain so that broadcast stations do not overload the receiver. A radar sweep is a regular series of identical chirps, so the AGC of a radio can follow them; use a fixed gain.

Step by step

1. Connect the radio and antenna and start OnAir. Open the group HF utility and choose Radar chirps, HF.
2. Set the centre of the band you want to look at.
3. Press Start. The panel says start the receiver to look for ocean radar sweeps until it runs.
4. Open Radars (the table of radars heard), Waterfall (the chirp waterfall with the sweeps marked) and Heard (the history).
5. To cover several bands with a narrow radio, tick scan radar bands and set the dwell per band.

What you should see

Radars as rows: Centre, Band, Width, Rate, Dir (rising or falling sweep), Repeat, SNR, Sweeps, First heard, Last heard, ID (unverified). In the waterfall the sweeps appear as slanted lines. A status line gives sweeps counted, tracks rejected (not linear, too wide or too slow).

Reading the screen

Radars: the table above; click a row for details and the SNR history (one point per second heard). Waterfall: the chirp waterfall with the sweeps marked. Heard: the history heard on band X at time Y (Time UTC, Radar, Frequency) with Copy.

Options and settings

• scan radar bands: step through the bands; dwell: how long per band.
• Clear: forget every radar and event.
• The Morse call sign some radars send is read from the keyed energy of the whole sweep width and is always labelled unverified.

Troubleshooting

• Nothing heard. Radars are few and regional; try the bands near a coast. Daytime and night change what you hear.
• Several radars overlap. Radars that share a frequency overlap, so the sweep numbers give only a possible hint.
• Broadband bursts counted. Impulse noise from other equipment is skipped.
• The waterfall shows a vertical line. A carrier, not a chirp: it is rejected.
• Narrow radio sees only one radar. Use the scan.

Tips

• Use the history to see when a radar is on air.
• Most are in the 13 MHz and 25 MHz bands.

Legal and privacy notes

HF ocean radars are broadcast radar signals; receiving them is legal in most countries, and OnAir is receive only: nothing is decrypted or transmitted. It makes no claim about the radar's position; the "heard" history is propagation information only. See Legal.

Under the hood

These radars use frequency-modulated continuous waves (FMCW): a linear sweep, typically a few tens of kHz to a few hundred kHz wide, repeated about once a second. OnAir builds a 1 ms spectrogram, looks for linear tracks, measures centre, sweep width and rate (kHz/s), and groups repeated sweeps into a radar. Some radars key a call sign in Morse; the reading is unverified.

Related

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