CODAR Data
CODAR (Coastal Ocean Dynamics Applications Radar) is a technology that allows the measurement of surface ocean current velocities at a distance using the Doppler shift of reflected radio waves. The term "CODAR" is frequently used as a generic term, but it is strongly associated with CODAR Ocean Sensors, a California company that manufactures them. A more general term for CODAR is "HF (High-Frequency) Coastal Radar"
CODAR Station Locations
There are currently two CODAR stations: one at the Westshore Coal Terminal in Tsawwassen and the other at the Iona Wastewater Treatment Plant, near Vancouver Airport. The stations are designated "VCOL" and "VION", respectively. A third is being planned for the end of the Iona Causeway and will be designated "VCWY".
Each station measures the radial velocities - that is, towards and away from the station's antenna - of ocean currents in the Strait of Georgia. The stations have been placed in locations such that their areas of coverage overlap considerably. In the overlapping region, it is possible to combine the radial data from the two stations into "total" velocities that may be resolved into their north-south and east-west components.
How CODAR Works
CODAR works on the principle of Bragg scattering from ocean surface waves. Bragg scattering returns a signal to the transmitting antenna only when the ocean waves are moving either directly towards or directly away from the antenna.
Bragg scattering from ocean waves is also highly wavelength-specific; any given system is sensitive only to ocean waves with wavelengths one-half the wavelength of the radio waves it transmits. The CODAR system used by is a 25-MHz model. A radio frequency of 25 MHz corresponds to a radio wavelength of 12 metres, so the CODAR system is sensitive to ocean wavelengths of 6 metres. If ocean waves of this wavelength are entirely absent, the CODAR system is unable to measure surface currents. In practice, there is virtually always some wave energy in the 6-metre band, so this is generally not a problem. In extremely calm conditions, however, the data quality can be reduced because of the resulting lower signal-to-noise ratio.
The principle of Bragg scattering dictates that virtually all of the radio signal received by a CODAR antenna is being reflected back from ocean waves of a particular wavelength. The reflected signal is Doppler-shifted to higher or lower frequencies, depending on whether the ocean wave is moving, respectively, toward or away from the antenna. In the absence of ocean currents, this Doppler shift gives a measurement of the wave speed, just as a police radar gun measures the speed of a vehicle on the highway. If ocean currents are present, however, the Doppler shift gives a measurement of the wave speed PLUS the speed of the ocean current towards or away from the antenna. Because the speed of deep-water waves is known for a given wavelength, the wave speed can be subtracted from the total measured speed, resulting in a value for the speed of the ocean current alone.
The equations used in calculating ocean current velocities from the Bragg-scattering signal assume that the ocean waves being measured are "deep-water" waves--that is, that their wavelengths are less than twice the water depth. As depth decreases beyond this point, the waves will increasingly take on the character of "shallow-water" waves, and the assumptions used in calculating the ocean currents will become less and less valid. For a 25m-MHz system, then, ocean currents measured in water depths of less than 3 metres should be viewed with suspicion. Note that the daily tidal range in the Strait of Georgia is on the order of 3 metres, so this effect will be time-varying.
Revision History
- 20150330: Initial CODAR data product released
Data Product Options
Formats
The CODAR data products are archive file returns. There are a three formats which can be selected as described below.
RUV Data
RUV files are encoded in ASCII and contain radial current velocities. They are saved in the archive with file names ending in the source station. An RUV file for the Westshore Coal Terminal would be named with a suffix of -VCOL, e.g. CODARARRAY01_20111121T234733.000Z-VCOL.ruv. The data options allow selection of specific station files or all station files.
TUV Data
RUV files are encoded in ASCII and contain combined station current velocities. They are saved in the archive with file names ending in -TOTALS. An example of a TUV file name would be CODARARRAY01_20111122T234733.000Z-TOTALS.tuv.
PNG
PNG files contain a graphic representation of both station radial and combined total velocities. There are PNG files for both station and combined current velocities. Station files end in the four letter station code and the combined files end in -TOTALS. The data options allow selection of specific station files, combined data files and all files.
Other Formats
Several intermediate data formats are produced en route to generating ocean-current values. Two of these intermediate formats are retained in case it is decided to regenerate ocean-current data with changed parameters. These are known as "CSS" and "Range" files (originally, "CSQ" files were retained rather than the Range files, but the Range format has certain advantages. In December 2012, it was decided to switch to retaining Range files; see "Data Availability", below).
CSS and CSQ/Range files are available on request, but interpreting them requires proprietary software from CODAR Ocean Sensors, so they will be of limited use to most users.
Data Availability
Radial data is available via the VENUS website from the the Westshore Coal Terminal station (VCOL) beginning on on November 24, 2011 and from the Iona Wastewater Treatment Plant (VION) beginning August 1, 2012. "Total" current data files are also available beginning August 1, 2012. Radial and total data files are generated hourly.
VENUS radial and total-current data is also available from HFRNet, a continent-wide repository of HF Radar measurements. An interactive mapping application there provides a graphical way to explore ocean currents, not only from VENUS, but from CODAR stations across the United States (and a single station in Mexico) as well.
CSS and CSQ/Range data are available for each station for the same time period as the corresponding radial data. The exception to this is the CSQ files from the VCOL station in the five and half months between the date that acquisition started and June 10, 2012. An unnoticed default setting on the VCOL computer resulted in the CSQ files from this period being overwritten (apart from the CSQ files from February 6-7, 2012, which were recovered).
The VION station was switched from retaining CSQ files to retaining Range files on December 4, 2012. VCOL was switched to retaining Range files on December 19, 2012. Because VCOL is on a wireless router with limited bandwidth, CSQ/Range files are not fetched over the network, but rather are physically transferred via portable hard drive when site visits are made. As a result, there may be a lag of several months between acquisition and availability of CSQ/Range files from VCOL.
The range of high-frequency radio waves is greatly reduced over fresh water compared to the range over salty, highly-conductive water. For this reason, during the spring freshet, when the surface water in the Strait is much less salty, one or both of the VENUS CODAR stations may return little or no current data in the area of overlap, resulting in no total current vectors being available. This will vary from year to year, but at the time of writing (May, 2013) it appears that total current vectors may be unavailable from early May to as late as July each year.
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