These pop up anywhere from 3MHz to 50MHz and may be on the air for hours to months on any given frequency. I see on
CODAR Ocean Sensors - The Leaders in HF Radar Technology that the WRC has set forth some specific bands for the equipment. That might help keep it corralled some in the future.
I have had a couple of complaints that I have QRN on my Globaltuners radio on specific frequencies, I'll generally try to listen to the same frequency on an east coast and west coast radio and then point the listener to the fact that the signal is higher on one coast or the other.
The radars are supposed to be transmitting on a secondary basis, but most of the users don't seem to monitor the frequencies for too long before they push the TX switch.
In the FCC database each CODAR user has specific frequency allocations. I imagine during site establishment they do try a few frequencies to find what works best for them there, but even those tend to fall in freq ranges used by other CODARs.
For the most part they are very consistent in frequency range. Yes, the 4300 to 5400 is one of the heavy ranges. But there are chunks of spectrum in use by CODAR throughout the HF range (4300 - 5400, 11500 - 14000, 24000- 27000, and 40000 - 44000 are the most frequent ranges I see CODAR in, with a few scattered on other freqs), and other pieces of spectrum where I have never seen a CODAR at all (and I have a separate log file just for CODAR and other radars/sounders).
As for confusing a CODAR for a time station, probably not once you think about it for a second, or hear an example for comparison. Yes, many of them do cycle one sweep per second, but they don’t have to be at that rate, others are 2, 3, or even 5 sweeps per second, and I have seen one that was 30 sweeps per minute (other rates can be used, I just picked those as common examples I have seen). CODARs can sweep either up or down in frequency, although the majority seem to sweep down. You can tell the sweep direction on a traditional receiver by listening in USB mode, a CODAR sweeping down will have an audio signature that starts high and goes low, a CODAR sweeping up will sound low to high. Listening in LSB mode would reverse those sounds. Listening in AM mode would make it impossible to tell direction.
By the way, CODARs seem to be GPS time synced for the most part. This, combined with Doppler as the detection technique and fairly tight Doppler bins, allow multiple CODARs to work in the same frequency range with minimal interference. The second of the below videos are an interleaved set of CODARs. While I don’t have any examples online at this time I have seen CODARs in overlapping frequency ranges that sweep in opposite directions.
Radar, CODAR, 4435 kHz cf, Ocotober 10, 2010, 1720 UTC - YouTube
Radar, CODAR x2, 4550 kHz cf, March 08, 2009, 0822 UTC - YouTube
If you hear periodic QRN that just shows up from time to time I would bet it is not CODAR. For the most part CODAR is predictable, if you heard it on that freq last night you will probably hear it tonight, for example.
Other radars and sounders, ones that are frequency agile on the fly, do tend to be invasive and pop up almost anyplace. However, as far as I know they all include the ability to exclude frequencies and frequency ranges. Some obviously do not always do this, as you can occasionally find one hitting a frequency range it normally avoids.
Other examples of radars and sounders can be found on my Youtube channel.
T!
