Klaus-Werner Gurgel

Wellen Radar (WERA): a new ground-wave HF radar for ocean remote sensing

HF radars can be used to measure surface currents and wave spectra. The Coastal Radar (CODAR) used by the University of Hamburg was designed for current mapping only. It has been operated for 15 field experiments during the past 15 years. Recently, a new HF radar called Wellen Radar (WERA) has been developed at the University of Hamburg. One main advantage of the system is the possibility of connecting different configurations of receive antennas. When operated with a linear array, information on the sea state can be obtained via second-order spectral bands. A further advantage is the flexibility in range resolution between 0.3 and 1.2 km, instead of the fixed resolution of about 2 km of CODAR. This is achieved by transmitting frequency-modulated continuous wave (FMCW) chirps instead of continuous wave (CW) pulses. In addition, this technique avoids the blind range of about 3 km in front of the CODAR. The technical design of WERA is described and first experimental results are presented.

High-frequency radars: physical limitations and recent developments

High-frequency (HF) radars based on ground-wave propagation are used for remotely sensing ocean surface currents and gravity waves. For some 20 years a number of systems have been developed taking advantage of improved electronics and computer techniques. However, the performance of these systems are limited by physical constraints, which are due to HF wave propagation and scattering as well as to the technical design of the measuring system. Attenuation of the HF ground-wave is strongly dependent on the radio frequency and sea-water conductivity. Experimental data confirm the predicted decrease of propagation range with decreasing conductivity. HF radar systems use different methods of spatial resolution both in range and azimuth. Range resolution by means of short pulses and frequency-modulated chirps is compared, as well as azimuthal resolution by means of beam forming and direction finding (phase comparison). The emphasis is placed on recent developments.

On the accuracy of current measurements by means of HF radar

The accuracy of surface current velocities measured by high-frequency (HF) radar is investigated. Data from the two radar systems of the University of Hamburg, CODAR (Coastal Radar) and WERA (Wellen Radar), are compared with in situ data. In one experiment, CODAR and a near-surface current meter were operated simultaneously over a 19-day period. In addition, WERA was operated for 6 days during that period. In the other experiment, WERA and a bottom-mounted current meter were operated simultaneously over a 35-day period. Both radars use frequencies of about 30 MHz where backscattering is due to ocean waves of 5 m wavelength. The influence of the orbital motion of underlying longer waves on radial velocity errors is investigated. In accordance with theory, the measured standard deviations of HF-measured current velocities depend on the sea state. Depending on the sea state, estimated errors range from 3 to 10 cm/spl middot/s/sup -1/ and explain only part of the rms difference of 10-20 cm/spl middot/s/sup -1/ found between HF and in situ current measurements. The rest is assumed to be due the differences of the quantities measured, e.g., the spatial averaging.

Validation and intercomparisons of wave measurements and models during the EuroROSE experiments

The objective of the EuroROSE (European Radar Ocean Sensing) project was to combine area covering ground-based remote-sensed wave and current data with high-resolution numerical forecast models to provide nowcasts and forecasts for coastal marine operators. Two experiments to test and to demonstrate the system took place: one on the coast of Norway, north of Bergen in March 2000 and the second on the north coast of Spain at Gijon in October–November 2000. Qualitative and quantitative intercomparisons of the wave measurements and wave model products from these experiments are presented. These include measurements using the Wellen Radar (WERA) high-frequency (HF) radar, the WaMoS (Wave Monitoring System) Xband radar, a directional Waverider and output from the WAM wave model. Comparisons are made of the full directional spectra and of various derived parameters. This is the first-ever intercomparison between HF and X-band radar wave measurements and between either of these and WAM. It has provided a data set covering a much wider range of storm and swell conditions than had been available previously for radar wave-measurement validation purposes and has clarified a number of limitations of the radars as well as providing a lot of very useful radar wave data for future model-validation applications. The intercomparison has led to improvements in the data quality control procedures of both WaMoS and WERA. The two radar sytems measured significant wave height with mean biases of 3% and 6%, respectively, and mean direction differences of less than 2j in both cases. Limitations in the WAM model implementation are also discussed. D 2002 Elsevier Science B.V. All rights reserved.

Low power High Frequency Surface Wave Radar application for ship detection and tracking

High-frequency (HF) radars are operated in the 3-30 MHz frequency band and are known to cover ranges up to some thousand kilometers. Sky wave over-the-horizon radars (OTHR) utilize reflection by the ionosphere, but they require a transmit power up to 100 kilowatts. Especially for oceanographic applications, low power high frequency surface wave radar (HFSWR) systems have been developed, which use ground wave propagation along the salty ocean surface. The WERA HF radar system transmits a power as low as 30 watts, but achieves detection ranges up to 200 kilometers, which are far beyond the conventional microwave radar coverage. Due to external noise, radio frequency interference, and different kinds of clutter, special techniques for target detection have to be applied. This paper describes a new signal processing approach based on a curvilinear regression analysis for thresholding combined with a constant false-alarm-rate (CFAR) algorithm for detection. The target locations detected by the HF radar are passed to a tracking filter utilizing range, azimuth, as well as radial and azimuthal velocities to track the ship locations. For a 12-hour period real HF radar data from the WERA system were processed and secondary ship locations were recorded from the automatic identification system (AIS). This data set is used to assess the performance of the HF radar detections. Comparisons have been made for a maximum distance of 5 km between AIS and radar detected locations. The deviation between AIS and radar detected locations was below 1 kilometer in 77% of these comparisons. A number of ships was detected and tracked by the radar, but could not be used for comparisons due to the lack of AIS information.

Tidal currents in the northwestern Adriatic: High‐frequency radio observations and numerical model predictions

The results of a two-year long deployment of high frequency radars along the Italian coast of the northern Adriatic are analyzed, to characterize the surface tidal currents. M2 and K1 ellipses are aligned with the basin axis and exhibit large eccentricities in the middle of the basin, decreasing toward the Italian coast. Comparisons are made with a 3D finite-element non-linear numerical model of the tides. Complex correlations between modeled and observed tidal currents show a remarkable agreement in the middle of the basin, with magnitudes reaching 0.985 and average phases of -6.4 deg. However the magnitudes drop to 0.5 within 20-30 km from the Italian coast, where the modeled currents amplitudes are underestimated by 2cm/s and the phases lag the observed phases by 60 deg. This shallow region (less than 30-m deep) is characterized by low-salinity water originating at the Po River and laterally sheared coastal flows. The radars may have captured the influence of stratification or mean sheared flows, both absent from the model. The model parameterization of bottom friction may also inadequately represent the effects of real bottom friction on the vertical current shear.

Shipborne measurement of surface current fields by HF radar

HF radar is a remote sensing technique for measuring surface currents and ocean wave directional spectra. This paper describes the extension of a land based system to enable surface current measurements from a slowly sailing ship. To compensate for the ships motion, the Global Positioning System (GPS) has been used. Due to additional error sources compared to the land based system, the accuracy of measured currents is slightly reduced from 2 to 5 cm/s to 3 to 10 cm/s.<<ETX>>

An Empirical Method to Derive Ocean Waves From Second-Order Bragg Scattering: Prospects and Limitations

High-frequency (HF) radar wave processing is often based on the inversion of the Barrick-Weber equations, introduced in 1977. This theory reaches its limitations if the length of the Bragg-scattering wave raises to the order of the significant waveheight, because some assumptions are no longer met. In this case, the only solution is moving to lower radar frequencies, which is not possible or desirable in all cases. This paper describes work on an empirical solution which intends to overcome this limitation. However, during high sea state, the first-order Bragg peaks sometimes could not be clearly identified which avoids the access to the second-order sidebands. These cases cause problems to the algorithm which have not been solved yet and currently limit the maximum significant waveheight to about the same values as reported for the integral inversion method. The regression parameters of the empirical solution calibrated from the European Radar Ocean Sensing (EuroROSE) data set are constant values for the complete experiment and when applied to the HF radar data they reconstruct the measurements by a colocated wave buoy quite well. When including a radar-frequency-dependent scaling factor to the regression parameters, the new algorithm can also be used at different radar frequencies. The second-order frequency bands used for the empirical solution are sometimes disturbed by radio interference and ship echoes. Investigations are presented to identify and solve these situations

Radio Frequency Interference Suppression Techniques in FMCW Modulated HF Radars

High-frequency (HF) radars are operated in the 3-30 MHz frequency range and need to share the frequency bands with other radio services. Due to their over-the-horizon (OTH) capabilities, HF radars play an important role in remote sensing and surveillance. The propagation conditions of the electromagnetic wave depend on the earths ionosphere and mailnly follow a daily cycle. Communication paths between the HF radar and other radio services, some thousands of kilometres off, open and close with a high variability. Special care must be taken to dynamically adapt the HF radars characteristics to the varying electromagnetic environment. The impact of a frequency modulated continuous wave (FMCW) HF radar on other radio services is not very strong, because of its low transmit power and utilisation of the radio spectrum. However, strong signals from other radio services can significantly reduce the performance of the oceanographic measurements. Several radar control and signal processing steps are discussed in this paper. All together form an effective procedure to reduce the impact of Radio Frequency Interference (RFI) on the oceanographic measurements.

The Surface Expression of Semidiurnal Internal Tides near a Strong Source at Hawaii. Part II: Interactions with Mesoscale Currents*

Abstract Observations of semidiurnal surface currents in the Kauai Channel, Hawaii, are interpreted in the light of the interaction of internal tides with energetic surface-intensified mesoscale currents. The impacts on internal tide propagation of a cyclone of 55-km diameter and ∼100-m vertical decay scale, as well as of vorticity waves of ∼100-km wavelength and 100–200-m vertical decay scales, are investigated using 3D ray tracing. The Doppler-shifted intrinsic frequency is assumed to satisfy the classic hydrostatic internal wave dispersion relation, using the local buoyancy frequency associated with the background currents through thermal-wind or gradient-wind balance. The M2 internal tide rays with initial horizontal wavelength of 50 km and vertical wavelength of O(1000 m) are propagated from possible generation locations at critical topographic slopes through idealized mesoscale currents approximating the observed currents. Despite the lack of scale separation between the internal waves and backgroun...