You can easily receive HF coastal radar signals on your SDR, and sometimes even read the radar return signal under the right conditions.
HF coastal radar has proliferated over the past twenty years. These are low-power, over-the-horizon radars that operate between 3-30 MHz. Scientists use HF coastal radar to measure waves, winds and currents over a few hundred kilometers.
My location is far from any coast, but HF propagation allows me to receive lots of these radars, particularly around 12.2 MHz (just above the 25 meter band) and 13.4 MHz (just below 19 meters.) During darkness, we can also get reception of these radars around 4.8 MHz.
You can identify these signals easily as wideband swept carriers, or “chirps”. Generally, operators use low power in the range of 25 to 100 watts. Modulation is a linear frequency “chirp” or signal ramp. Chirp duration is between 0.5 to 2.0 seconds.
Two of the more popular radar types are CODAR or WERA. Coastal Ocean Dynamics Application Radar, CODAR, uses a direction finding approach. Modulation is FMICW or frequency modulated interrupted continuous wave with a 50% duty cycle. Equipment is typically a vertical transmit antenna with two orthogonal reception loops or dipoles. On the other hand, Wave Radar (WERA) uses FMCW with a beamforming array.
Here is a video explanation of CODAR, as well as a video demonstration of what the signals look and sound like. DF covers a broader area, while beamforming is steered through narrow radials of ocean and provides finer resolution.
HF Coastal Radar – How It Works
Equipment is placed right at the shore and generates a very low angle signal. HF coastal radar works because of the Bragg Scattering, the resonance set up with waves spaced at half the transmit wavelength which improves backscatter reception. In effect, this generates a coherent signal or constructive addition of the reflection. Backscatter is due to the roughness of the ocean surface.
By using a signal ramp or chirp, technicians are able to convert the time delay of reception into a frequency shift. This is achieved by mixing the transmitted and received signals together. By using an FFT to measure and average the reflections, radar operators measure radial velocity of currents, distance and angular direction of the reflected signal.
HF coastal radar works over ranges from 60 to 200 kilometers, well beyond a 12 kilometer horizon. Standard range systems typically work in 10-30 MHz while longer ranges are achieved in 3-10 MHz. Sweep bandwidths range from 10 to 200 kHz. Measurement or acquisition time typically falls into 10 to 30 minutes of data collection.