Hourly surface currents measured by High Frequency (HF) Wellen radars (WERA) off western Oahu, Hawaii, from September 2002 to May 2003 (NODC Accession 0013113)

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What does this data set describe?

Title:
Hourly surface currents measured by High Frequency (HF) Wellen radars (WERA) off western Oahu, Hawaii, from September 2002 to May 2003 (NODC Accession 0013113)
Abstract:
A pair of High Frequency Wellen radars (WERA) shore-based at southwest Oahu (Ko'Olina) and northwest Oahu (Kaena), Hawaii measured surface currents over a nine-month period from September 2002 to May 2003. Vector currents were combined from measured radial currents on a 5-km cartesian grid by least-square fitting zonal and meridional components to radial components within a 5-km search radius. The resultant hourly data set was calibrated and quality controlled. Data provided by the originators as a MATLAB file. NODC has exported the data into ASCII files.
Supplemental_Information:
Entry_ID Unknown Sensor_Name WERA HF radar Source_Name manual Project_Campaign: Hawaii Ocean Mixing Experiment Storage_Medium Matlab binary, ASCII Reference None Online_size: 117,873 kbytes

Resource Description: NODC Accession Number 0013113

  1. How might this data set be cited?
    Flament, Dr. Pierre, and Chavanne, Mr. Cedric, Unknown, Hourly surface currents measured by High Frequency (HF) Wellen radars (WERA) off western Oahu, Hawaii, from September 2002 to May 2003 (NODC Accession 0013113).

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -158.8105
    East_Bounding_Coordinate: -158.1363
    North_Bounding_Coordinate: 21.5473
    South_Bounding_Coordinate: 20.9627
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 06-Sep-2002
    Ending_Date: 22-May-2003
    Currentness_Reference: ground condition
  5. What is the general form of this data set?
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
    2. What coordinate system is used to represent geographic features?
  7. How does the data set describe geographic features?
    Entity_and_Attribute_Overview:
    Under directory /data:

    0-data/: files as received by the originators file comment email_originator.txt email correspondence wera_oahu_bis.mat matlab file containing all data

    1-data/: files created by NODC file comment 1readme.txt overview of files on directory matlab_export.txt steps in exporting data from Matlab wera_U.txt u-component velocity data wera_UU.txt zonal variance factor wera_UV.txt zonal and meridional covariance factor wera_V.txt v-component velocity data wera_VV.txt meridional variance factor wera_x.txt longitude positions wera_y.txt latitude positions wera_t.txt date/time

    Under directory about/: misc_doc/ powerpoint presentation and jpeg copies of select slides

    Entity_and_Attribute_Detail_Citation: None

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
  2. Who also contributed to the data set?
    Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii
  3. To whom should users address questions about the data?
    Mr. Cedric Chavanne
    Department of Oceanography, University of Hawaii
    Researcher
    Department of Oceanography
    Honolulu, HI
    USA

    808-956-6663 (voice)
    cedric@hawaii.edu

Why was the data set created?

To monitor coastal currents in support to ocean mixing studies.

How was the data set created?

  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
    Date: Unknown (process 1 of 1)
    High Frequency (HF) Wellen Radars (WERA) have 16Rx antennas with 16 MHz and 125 kHz continuous chirp. There are 9-minute acquisitions every 20 minutes. Validation has been possible by comparison with moored ADCPs. The spatial coverage is about 130 km during the day and 90 km at night.

    Vector currents were combined from measured radial currents on a 5-km cartesian grid by least-square fitting zonal and meridional components to radial components within a 5-km search radius. (UU,UV,VV) are the components of the Geometrical Dilution Of Precision (GDOP) matrix.

    Radar calibration Data from HF radars, whether based on linear phased arrays or on direction-finding configurations, must be corrected for unavoidable distortion of the electromagnetic plane waves at the shore boundary and over land, and by obstacles such as trees and fences. Large phase errors may degrade the azimuthal accuracy of the radars. Bi-static time series of the direct path Ka'ena->Ko'olina and Ko'olina->Ka'ena were acquired to map the evolution of the phase error of antennae with environmental conditions, such as rain, and tidal level. A ship survey was conducted in May 2003, with a transmitter installed on the R/V Wyrtki. The ship was steered based on differential GPS, on radial lines originating at the center of the receive arrays, and on circles of 5 km radius. These measurements indicate that: (i) the temporal phase stability of the receive channels was better than 1 deg., (ii) the beamforming stability, over 24 hours, was better than 2 deg. in azimuth, (iii) wavefronts at Ko'olina were linear with less than 10 deg phase errors, (iv) wave fronts at Ka'ena were parabolically curved, due to topographic influence. Data reprocessing over the length of the deployment has been performed based on these phase calibrations.

    The following was copied on January 24, 2007 from http://www.satlab.hawaii.edu/hfradar/introdocs/principles.html

    High Frequency Radar for mapping ocean currents Physics and principles of operation © Pierre Flament, University of Hawai'i Dec 26 2003

    Oceanographic High Frequency Radars are simple in concept: electromagnetic waves (EM) sent to the ocean are backscattered on surface waves of exactly half the radio wavelength, just like X-rays are scattered in crystals.

    Since the ocean is generally covered by waves of many different wavelengths and directions (continuous spectrum), there are always trains of waves propagating toward and away from the radar. The return signal from either train will be Doppler-shifted by the wave velocity, which is known exactly by the gravity wave dispersion relationship. Thus the spectrum of the return echoes consists of two peaks, symmetric with respect to the transmit frequency, in the absence of currents.

    If the waves ride on an ocean current, the return signal is further Doppler-shifted by the radial component of the current, which can be readily estimated. With two radars some distance apart along the coast, vector currents can be computed.

    This is a precise concept, which surprisingly has taken more than three decades to be accepted by the oceanographic community (a list of early references compiled by Robert Stewart can be found here). Yet unlike Acoustic Doppler Current Profilers (ADCP), the scattering targets are well known, and well understood theoretically.

    Wind direction can be inferred from the amplitude of the main peaks, and multiple scattering allows the extraction of spectral information on the wave field.

    Although they have historically been called "radars", they have nothing in common with microwave navigation radars; they in fact use very low power transmitters in the high frequency band (3 to 50 MHz), much like Citizen Band radios. Various HF radars differ on several aspects:

    1. whether they use ground wave or sky wave 2. how the signal is modulated 3. how are the antennas configured, the beam steered and the echo direction estimated

    (1) Do they use ground wave or sky wave?

    At HF, EM wave can propagate in a "trapped" ground wave along the conductive surface of the ocean and a "free" wave in the atmosphere.

    The useable range of the ground wave depends on attenuation, and increases with decreasing frequency, approximately range(km) * frequency(MHz) = 2000, thus 32 MHz = 60 km, 16 MHz = 125 km, 8 MHz = 250 km; the actual range varies greatly depending on EM background noise, and on the type of signal modulation. Attenuation increases and thus range decreases as salinity decreases, and as sea state increases.

    The free wave propagates to much longer distances and circles the earth, bouncing between the ionosphere and the earth surface ("sky" wave). However it is subject to Doppler shifts by the ionosphere. Therefore interpreting distant echos from the ocean is rarely practical, as the Doppler shifts due to currents is hidden in the ionospheric Doppler shifts.

    All modern ocean current mapping HF radars use ground waves. Large "over-the-horizon" military radars, which have been used in the past to monitor the ocean, in particular wind/waves, are sky wave radars. Below 6 MHz, propagation is also affected by the D-layer of the ionosphere, which has a strong diurnal cycle.

    (2) How the signal is modulated?

    Just like ADCPs which may use short pulses ("pings"), broad band chirps, or pseudo-random code phase modulation, so do HF radars. The more recent their design, the more complex the signal encoding. The intent is to lower the instantaneous transmit energy as much as possible, by transmitting with a larger duty cycle (ideally 100%).

    The technology of short pulses is simplest, but there is a price to pay: the instantaneous power needed to achieve spatial resolution through short pulses can be quite large (often 1 kW or more), increasing the risk to personnel (ie. sparks); there is always a blank area in front of the transmitter, as the receiver must wait for the direct transmit signal to die out before listening to the echoes; and range resolution is limited by how short one is willing to make the pulse, thus by how large instanteanous power is acceptable.

    Range resolution can also be obtained by "chirping" the signal, i.e. ramping the frequency. This allows to lower the power and increase significantly (x 1000 or more) the pulse length, without loosing resolution. The spatial resolution in this case is governed not by the pulse length, but by the bandwidth of the chirps. Except for the very high grade components needed (for example, dynamic range reaching 120 dB), this is a simple and easy to configure technology, easy to debug with standard lab equipment such as spectrum analyzers.

    A new avenue is to use pseudo-random code phase modulation, as done in the US HiFAR. It is still under development, and requires sophisticated demodulation correlating the transmit and receive signals.

    In any radar, the strong direct signal from the transmit to the receive antennas or circuit must be rejected as much as possible, to avoid saturating the receiver when listening to the target echoes. In chirped radars, which have large duty cycles, possibly of unity, inevitably the radar must listen to the echoes while still transmitting the chirp.

    Of course the simplest is to ensure physical separation between transmit and receive antennas, at the expense of long cables.

    A second approach is to superimpose a repeated 50% gate, typically 1 kHz, on long continuous chirps; through this artifact, silent periods are created during which the echoes can be listened to, unaffected by the TX, while maintaining spectral and spatial resolution. This is the principle of the US CODAR/SeaSonde and UK PISCES; its advantage is that there is no need for much physical separation of the TX and RX antennas, and that 360 deg. coverage can be obtained with isotropic antennas. However this approach complicates the signal generation, amplification, detection and spectral estimation.

    If a true continuous FMCW (Frequency Modulated Continous Wave) signal is desired, which simplifies some aspects of the electronics and spectral estimation, another approach is needed.

    One way is to form an azimuthal beam on the transmit antenna, and ensure that the receive antenna lies in a null of the transmit pattern. This is how the German WERA is solving the problem; with 4 properly phased antennas, a null is created along the axis of the array, with better than 80 dB (power) rejection, and the signal is enhanced forward, to improve the illumination of the target area and reduce signal reflection on the back topography (cliffs, mountains,...). Another way, of course, is the ensure a sufficient physical separation between the transmit and receive antennas, of a few hundred meters or more.

    (3) how are the antennas configured, the beam steered and the echo direction estimated

    Receive antennas can be monopoles or loops, and can be configured in beam steering mode, or in direction-finding mode.

    In direction-finding mode, four monopole antennas in a square (the old NOAA-CODAR, the WERA in DF configuration), or two orthogonal coils and a single monopole (the new CODAR-SeaSonde), are used to receive the signal. For each range, it is assumed that one set of current-induced spectral shift come from only one direction, which is estimated through a least-square procedure.

    This solution achieves extreme electronic simplicity, but two major shortcomings: drastic assumption on ocean current patterns (this is often underlooked), and impossibility to estimate wave spectrum since second order backscatter spectrum is not separated from first order spectrum.

    In beam steering mode, a rake of 8, 12, 16, ... antennas at half wavelength spacing (thus 10 m at 16 MHz, 20 m at 8 MHz) is used to create a synthetic large aperture antenna; the longer the rake, the narrower the beam; beam width in radians scale as 2/n, thus for 16 antennas, about 1/8 rad, or 7 deg. The antenna is steered by introducing variable phase delays in each antenna channels.

    This steering can be done in hardware, by switching sequentially different lengths of coax for each antenna (COSRAD), or in software, by A/Ding all antennas in parallel, and combining their signals with appropriate phase shifts in software (WERA). Hardware phasing has a major shortcoming: one must sequentially look in different directions, thus reducing the size of the statistical average, or reducing the temporal resolution to obtain the same number of degrees of freedom.

    Beam steering achieves optimum signal sampling and allows estimation of all possible ocean parameters, at the expense of real estate. A 16 antenna rake at 8 MHz would measure 320 m long, not easy to find along a beach! Compromises between radio frequency thus antenna spacing, yielding maximum range, and rake length yielding angular resolutions, must be found depending on the requirement of each application/deployment. Not to speak about cable handling: a 8 MHz 16 antenna array requires handling about 8 km of finger-thick RG-213 coax.

    With software beamsteering, one can easily run both configurations in parallel: a 12-antenna beam steered rake, next to a 4 antenna radio-direction-finding square, all acquired in parallel.

    REFERENCES: Chavanne, C., I. Janekovic, P. Flament, M. Kuzmic, P .-M. Poulain, and K.-W. Gurgel, "Tidal Currents in the Northern Adriatic Sea: High Frequency Radar Observations and Numerical Model Predictions", JGR, 2007.

    C. Chavanne, P. Flament, E. Zaron, G. Egbert, D. Luther and K.-W. Gurgel, "Kauai Channel Tides and Mesoscale Interactions", JPO, in preparation

    Gurgel, K.W., H.H. Essen and S.P. Kinglsey (1999) High Frequency radars: physical limitations and recent developments, Coastal Engineering, 37, 201-218.

    Gurgel, K.W., G. Antonischki and T. Schlick (1999) Wellen radar (WERA): a new ground-wave radar for ocean remote sensing, Coastal Engineering, 37, 219-234. Person who carried out this activity:

    Mr. Cedric Chavanne
    Department of Oceanography, University of Hawaii
    Researcher
    Department of Oceanography
    Honolulu, HI
    USA

    808-956-6663 (voice)
    cedric@hawaii.edu
  3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?
  2. How accurate are the geographic locations?
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
    The 2003 sets were 100% complete
  5. How consistent are the relationships among the observations, including topology?
    see Process Step

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?
Access_Constraints: None
Use_Constraints: Dataset credit required
  1. Who distributes the data set? (Distributor 1 of 1)
    NOAA/NESDIS/National Oceanographic Data Center
    Attn: Data Access Group, User Services Team
    SSMC-3 Fourth Floor
    Silver Spring, MD
    USA

    301-713-3277 (voice)
    301-713-3302 (FAX)
    services@nodc.noaa.gov
    Hours_of_Service: 8am-5pm, Monday through Friday
  2. What's the catalog number I need to order this data set? Downloadable Data
  3. What legal disclaimers am I supposed to read?
    NOAA makes no warranty regarding these data,expressed or implied, nor does the fact of distribution constitute such a warranty. NOAA, NESDIS, NODC and NCDDC cannot assume liability for any damages caused by any errors or omissions in these data, nor as a result of the failure of these data to function on a particular system.
  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 06-Jan-2021
Last Reviewed: 01-Sep-2009
Metadata author:
Mr. Patrick C. Caldwell
NOAA/NESDIS/NODC/NCDDC
Hawaii/US Pacific Liaison
1000 Pope Road, MSB 316
Honolulu, Hawaii
USA

(808)-956-4105 (voice)
(808) 956-2352 (FAX)
caldwell@hawaii.edu
Hours_of_Service: 8 AM to 5 PM weekdays
Contact_Instructions: check services@nodc.noaa.gov if not available
Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

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