Ocean Wave and Current Radars

1. Ocean Wave and Current Radars By Laura Elston

2. Our earth is a very aqueous environment with nearly three quarters of it covered by ocean. So how do we read and understand this environment to better our everyday lives? The answer is ocean radars. With the help of these radars we are better able to calculate wave energy and direction which in return stimulates our economy, warns us of danger, and better educates us on our surrounding environment.

3. What is RADAR

4. What is RADAR • Radio Detection and Ranging

5. What is RADAR • Radio Detection and Ranging • Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects.

6. What is RADAR • Radio Detection and Ranging • Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects. • This includes aircraft, ships, motor vehicles, weather formations, and terrain.

7. There are a wide range of radio wavelengths that can be used for measuring ocean waves

8. There are a wide range of radio wavelengths that can be used for measuring ocean waves • This includes…

9. There are a wide range of radio wavelengths that can be used for measuring ocean waves • This includes… • infrared (IR)

10. There are a wide range of radio wavelengths that can be used for measuring ocean waves • This includes… • infrared (IR) • microwave (MW)

11. There are a wide range of radio wavelengths that can be used for measuring ocean waves • This includes… • infrared (IR) • microwave (MW) • and high frequency (HF).

12. High Frequency RadarCodar • One particular High Frequency radar is CODAR • Codar is used to measure the surface currents of the coastal ocean.

13. A little Background • CODAR (COastal raDAR), was introduced in 1977 at NOAA by D.E.Barrick.

14. How Codar Works • The way this radar works is that a transmitter sends out a radio frequency that scatters off the ocean surface and back to a receiving antenna. • With this information it is able to calculate the speed and direction of the surface current. Normally CODAR employs two sites, since a single station measures only one component of the total horizontal surface current vector. • Spacing for a two radar system is approximately 15 to 40 km for long-range open ocean mode and 8 to 20 km for higher frequency higher resolution shorter range mode.

15. How is this useful? What makes HF RADAR particularly useful for current mapping is that the ocean waves associated with HF wavelengths are always present. The following chart shows three typical HF operating frequencies and the corresponding ocean wavelengths that produce Bragg scattering. 25 MHz transmission -> 12m EM wave -> 6m ocean wave12 MHz transmission -> 25m EM wave -> 12.5m ocean wave5 MHz transmission -> 60m EM wave -> 30m ocean wave

16. So far three facts about the Bragg wave are known: wavelength period and travel direction. By looking at the same patch of water using radars located at two or more different viewing angles, the surface current radial velocity components can be added together to determine the total surface current velocity vector. Since the wavelength of the wave is known, we also know it's speed.

17. High Frequency Radars are also useful in that rain or fog does not affect the signals so weather does not interfere with it’s measurements. However, the wave has to be traveling in the radial path either directly away from or towards the radar.

18. Benefits • Knowing information of ocean surface currents benefits us on many different levels. • Accurate maps of ocean surface currents, both in coastal zones and in the open ocean, are useful by… • Coast Guard search-and-rescue activities • fisheries managers • shipping industries • coastal ecosystem managers • oil-spill response teams • global-climate-change researchers

19. Coast Guard search-and-rescue activities • The U.S. Coast Guard spends nearly $15 million annually on ocean search-and-rescue missions and nearly $4 million is spent determining ocean surface currents, for the purpose of estimating the track of a target whose position was known at some time in the past.

20. Fisheries Managers • Fisheries management involves understanding of oceanographic influences on fish populations, Ocean-surface winds that affect upwelling of nutrients, and warm and cold currents. • With this understanding the annual income for fisheries has risen to roughly $20 billion.

21. Coastal Ecosystem Managers& Oil-spill Response Teams • Knowing ocean surface currents helps us determine where pollution is going to spread. • More than $50 million is spent in the average year to clean up toxic spills at sea. With the help of radars, it can map ocean surface currents with high resolution, through clouds and rain, over very large ocean areas because of this more information about surface currents is what is needed to predict the traveling direction of the pollutants. OTH radar can monitor bursts of strong currents or sudden changes in current direction that can damage offshore oil platforms and cause spills.

22. Global-Climate-Change Researchers • Ocean current radars keep us safe by giving us more of a warning during a storm. • In the average year, approximately $1 billion in property losses are due to high surf and storm surges generated by marine storms. • With better warnings, fixed property can be secured, transportable property moved, and false alarms avoided. Then the residents and industries that must take protective measures can do so. • OTH radar can map parameters of the ocean wave directional spectrum in the open ocean, which can be used to initialize coastal wave models which give us a better understanding of hurricane movement and direction.

23. Conclusion In conclusion, we are able to measure wave length period, direction and speed of ocean waves and currents. We've mapped ocean-surface wind directions over areas large enough to reveal the structure and progress of tropical waves that spawn hurricanes, which in return has saved millions of lives The cost of this research is only a tiny fraction of what taxpayers have already paid for these radars and in return has saved the people millions of dollars annually.

24. The Doppler shift calculated above is assuming that there is no surface currents changing the motion of the waves. So the current can be calculated by measuring the frequency shift from the original Doppler shift caused by the wave motion. If there is no current then the Doppler shift caused by the surface wave motion will not be changed. If however, the surface current is not zero, the frequency will be shifted further depending on the magnitude and direction of the current. • The Doppler equation is used again to calculate the velocity of the target using the frequency shift measured by the receiver antenna. Note that the velocity calculated is only the component of the velocity moving toward or away from the receiver (radial velocity component). CODAR must use radial components from at least one other site to determine the total current vector at a given point. Using this system, CODAR can calculate surface currents with an error of less than . 4 cm/s

25. So how is this signal used to calculate surface currents? • All of the previous equations assume that the surface waves are not moving. In fact the waves are moving and a moving wave will change the frequency of the return signal. This phenomenon is known as the Doppler Shift. • The frequency of a signal scattered by a moving wave will be shifted depending on the velocity of the surface wave. If the wave is approaching the receiver, the return frequency increases. On the other hand, a wave moving away from the receiver will return a lower frequency. Therefore the shift will be positive if the wave is moving toward the receiver and negative if the wave is moving away from the receiver. The following equation is used to measure the magnitude of the frequency shift: