CODAR stands for Coastal Ocean Dynamics Applications Radar. It’s a type of HF radar system that’s used for remote sensing of ocean surface currents and waves. These systems are used in coastal areas and measure the direction and speed of ocean surface currents up to a range of 200 km from the shore.
The CODAR system uses the Doppler effect to measure the velocity of ocean surface currents. The system uses multiple antennas that are arranged in a linear or circular array along the coast. Each antenna radiates a continuous signal at a specific frequency. All the signals from the antennas are combined to create a beam that is directed out to sea. When this beam hits the ocean surface, it is scattered by waves and currents, and the scattered signal is received by the antennas. By analysing the changes in the frequency of the scattered signal, the CODAR system determines the velocity of the surface currents.
CODAR systems are used in a variety of applications, including oceanographic research, search and rescue operations, and maritime navigation. They can provide valuable information about ocean currents and waves, which can help to improve ocean modelling and forecasting, as well as inform decision-making in a variety of marine industries.
CODAR is a specialised radio radar system used to measure ocean surface currents and wave conditions from the coastline. What makes CODAR interesting from a radio engineering point of view is that it operates on HF frequencies, often around 5, 13 or 25 MHz. It uses radio signals reflected and scattered by moving sea waves. Unlike conventional marine radar, which primarily detects ships and objects, CODAR uses the ocean surface itself as the radar target.
How HF radar detects ocean waves
HF radar exploits a phenomenon called Bragg scattering. When an HF radio signal travels across the electrically conductive sea surface, some of its energy is scattered back towards the transmitter by ocean waves. The strongest resonant backscatter occurs when the ocean waves have a wavelength exactly half that of the transmitted radio wave.
For example:
| Radar frequency | Radio wavelength | Resonant ocean wavelength |
|---|---|---|
| 5 MHz | 60 metres | 30 metres |
| 13 MHz | 23.1 metres | 11.5 metres |
| 25 MHz | 12 metres | 6 metres |
This is a fundamental principle behind oceanographic HF radar.
How CODAR measures ocean currents
The reflected signal experiences a small Doppler frequency shift. This shift depends on two things:
- The natural movement of the ocean waves.
- The movement of the underlying surface water current.
Because the natural wave speed can be calculated from its wavelength, the radar processor can separate the influence of the current from the observed Doppler shift. In the radar spectrum, two prominent Bragg peaks appear, corresponding to waves travelling towards and away from the radar.
If the water has no net radial current, those peaks occupy predictable positions. A surface current shifts their positions. A single CODAR station measures the component of current flowing towards or away from its antenna. Two or more stations can combine their measurements to calculate the complete surface current speed and direction.
CODAR frequency monitoring list
| Frequency range | Suggested centre frequency | Priority |
|---|---|---|
| 4.438–4.488 MHz | 4.463 MHz | High |
| 5.250–5.275 MHz | 5.2625 MHz | High |
| 9.305–9.355 MHz | 9.330 MHz | High |
| 13.450–13.550 MHz | 13.500 MHz | High |
| 16.100–16.200 MHz | 16.150 MHz | High |
| 24.450–24.600 MHz | 24.525 MHz | Medium |
| 26.200–26.350 MHz | 26.275 MHz | Medium |
Information from codar.com