CODAR Grid Files
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 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 Salish Sea / Strait of Georgia CODAR system (device details) used by ONC 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 25 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 for the current deployment.
CODAR Stations and Combiners
The individual CODAR stations are listed as individual devices. For example, the original two CODAR stations were at the Westshore Coal Terminal in Tsawwassen and at the Iona Wastewater Treatment Plant, near Vancouver Airport. The stations are displayed in the search tree under “Westshore Coal Terminal (VCOL)” and “Iona (VION)”, respectively. Each station measures the radial velocities of ocean surface currents; that is, towards and away from the station's antenna. 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 true north-south and east-west components. In this way, the stations have radial and raw data, while we list the combined data products on a separate, virtual, device, nominally found in the search tree as "Oceanographic Radar System" at the same level as the station search tree locations (for example: https://data.oceannetworks.ca/DataSearch?location=SOGCS&deviceCategory=OCEANOGRAPHICRADAR).
CODAR Data Products Overview
The CODAR stations and combiner virtual device have different data products available to them. The CODAR stations have manufacturer produced .RUV files (radial surface velocity data) and raw data formats: CODAR Raw RNG Data, CODAR Raw CSS Data, CODAR Raw RNG Data. The CODAR combiner virtual devices have manufacturer produced .TUV files (total surface current data). Both types of devices have manufacturer produced PNG and GIF plots of their respective radial and total surface velocities and, recently, we've added the ONC-developed CODAR Quality-Controlled Surface Currents data products.
Oceans 2.0 API filter:
The data formats and processing algorithms are described in detail below.
This data product is used extensively by the CODAR Quality Controlled Surface Currents data product. These are files are archived with a dateFrom corresponding to their creation or applicability date and the data product code uses only the dateFrom to determine which grid file is applicable to use or return (the applicability is taken as from the file's dateFrom to the dateFrom of the next grid file, if none, the last file is applicable). These files are archived on the combiner / totals virtual device, but are available on the associated CODAR stations, as long as the combiner device is one level up in the Oceans2.0 search-tree, as an example, see the Strait of Georgia CODAR system in the search-tree, where the combiner is in bold:
- 20191001: Initial CODAR Quality Controlled Surface Currents data product released
Data Product Options
GRID data files
CODAR grid (.grd) files are ASCII text files that contain metadata defining the CODAR combiner grid upon to which the CODAR radial velocity data is combined and interpolated into total surface currents.
Oceans 2.0 API filter: extension=grd
Straight of Georgia Deployment
Radial data is available from 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.
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 of Georgia is much less salty, one or both of the 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.
As of May 2016, the data acquisition framework was switched to the stations/combiners model. All of the formats will be available from then onward. The older files will migrate from the old scheme to the new devices over the next few months. The old scheme used a single device with multiple file mode fields to distinguish the stations. The new scheme is more flexible for expansion (we have added CODAR stations in Prince Rupert and there's potential for much more).
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