Thomas Schlick

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.

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.

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.

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.

Evaluation of an HF-radar ship detection and tracking algorithm by comparison to AIS and SAR data

Since several years, High Frequency (HF) Over-The-Horizon (OTH) radar is used to measure océanographie parameters, such as currents, waves, and wind direction over large areas up to 200 km off the coast. Cost effective low power systems transmitting less than 50 Watts have been developed, e.g. the WERA (WEUen RAdar), and are now commercially available. Besides their applications in oceanography within coastal monitoring systems, these systems can also be used to detect and track ships, if they are modified for this functionality. A ship detection and tracking algorithm for HF-radar has been developed at the University of Hamburg (UHH) in cooperation with the Technical University of Hamburg-Harburg (TUHH). A first evaluation of this algorithm has been done using a data set acquired at Figueira, Portugal, using an 8 MHz WERA system. In May 2009, the NATO Undersea Research Centre (NURC) initiated a measurement campaign in the Ligurian Sea off La Spezia, Italy, which involved two WERA HF-radar systems operated at 12.5 MHz, one directional waverider and a meteorological buoy. In addition AIS data were recorded and satellite borne synthetic aperture radar (SAR) images were acquired. This paper presents preliminary results which show the effectiveness of the HF-radar as a long range (∼130 km) continuous-time coverage surveillance system, despite of its low spatial resolution of 1.5 km. The performance of the HF-radar ship detection and tracking algorithms are evaluated by comparison to AIS and SAR. The detection error found for this data set is less than 1 km for ∼68% of the comparisons between HF-radar and (AIS) reported locations, which is comparable to previous results from the Figueira data set.

HF radar observations in the German Bight: Measurements and quality control

In the South-Eastern part of the North Sea, known as the German Bight, the Helmholz-Zentrum Gessthacht (HZG, former GKSS Research Center) is currently installing the experimental observation Network “Coastal Observing System for Northern and Arctic Seas” (COSYNA). The main components of COSYNA include in situ instruments, a network of High-Frequency over-the-horizon (HF) radars currently consisting of three “WEllen RAdar” (WERA) systems installed on the islands of Wangerooge and Sylt, and close to the harbour of Büsum, as well as numerical models which are linked to the radar measurements by data assimilation. The WERA HF radar system was developed at the University of Hamburg in 1996. A commercial version is available since 2000. As WERA uses FMCW modulation for range resolution, there is no blind range in front of the system and data are available very close to the shore line. Azimuthal and range resolutions are ±3° and 1.5 km, respectively with a maximum range of 120 km. Ocean current maps are created three times an hour. Operation of the HF radar systems started during summer 2010. The three radial components of the current field as measured by the WERA radars are transfered to a central server at HZG, where they are quality checked and combined to give 2-D current maps. Radial components and 2-D maps are archived in a database and made available to the numerical model system. COSYNA provides a web-based interface to make the 2-D maps, as well as the model results available to the public. This paper describes details of the HF radar network including the procedures to reduce the impact of Radio Frequency Interference (RFI) on the measured ocean current maps and to control the quality of the data.

Measurement of ocean wave height and direction by means of HF radar: An empirical approach

High-frequency (HF) radar has been used for 20 years for remote sensing of ocean surface currents and ocean waves. Backscattered Doppler spectra contain two discrete lines, whose frequencies (Bragg frequency) determine the current speed, and four continuous side bands enabling inversion techniques to be used for retrieving ocean wave spectra. Recently, a new HF radar has been developed at the University of Hamburg (Germany). Data of a 34-day experiment reveal a high correlation between the standard deviation of the Bragg frequencies and the significant wave height weighted by an azimuthal function. Applying empirical regression curves it is possible to determine the significant wave height and the mean wave direction from intersecting beams of two radar stations. Compared to inversion techniques, the new method is applicable to data with a lower signal-to-noise ratio, i.e. it allows larger ranges. Two radar sites are required for current measurements. The optimum distance between two 30 MHz radars is about 20 km and, with the new method, needs not be reduced for the purpose of simultaneous wave measurements.

A comparison of surface current fields derived by beam forming and direction finding techniques as applied by the HF radar WERA

HF radar in oceanography makes use of backscattering of electromagnetic waves of 10 m to 50 m wavelength from the rough sea surface to measure surface current and ocean wave parameters. In Germany, the work on ground wave HF radar started in 1980, adopting NOAAs CODAR (COastal raDAR). Recent developments within the European project SCAWVEX (Surface Current And Wave Variability Experiment) lead to a new design called WERA (WEllen RAdar). In Spring 1996, two WERA and CODAR systems have been deployed north and south the Rhine mouth at the Dutch coast. While CODAR uses a four-element squared receive antenna array and direction finding technique for azimuthal resolution, WERA in addition can be configured to use a linear array and beam forming. As both systems have been operated simultaneously at the same location, comparisons of the surface current fields measured by the different systems and algorithms are possible. By defining an absolute quality criterion based on spectral analysis of current time series, limitations in the direction finding algorithm concerning ship traffic can be identified.

Simulation of tsunami signatures in ocean surface current maps measured by HF radar

The high frequency (HF) surface wave radar has a unique capability to monitor the coastal environment far beyond the conventional microwave radar coverage. The HF radar could contribute to the development and improvement of Tsunami Early Warning Systems. Bragg-resonant backscattering by ocean waves with half the electromagnetic radar wavelength allows measuring the ocean surface current at distances up to 200 km. The developed software package allows reconstructing an ocean surface current map of the area observed by HF radar based on the radar range-Doppler spectrum processing. In case of an approaching tsunami, a strong ocean surface current signature can be observed by the radar when the tsunami wave enters the shelf edge. In order to simulate the signals seen by an HF radar in case of a tsunami travelling towards the coast, the tsunami induced current velocity is calculated using the oceanographic HAMburg Shelf Ocean Model (HAMSOM) model, then converted into modulating signals, and superposed to the measured radar backscatter signals. After applying conventional signal processing techniques, the radar spectra include the simulated tsunami wave. The surface current map based on these spectra has a pattern, which changes very quickly in the shelf area before the tsunami wave reaches the beach. Specific radial tsunami current signatures are clearly observed in these maps. If the shelf edge is far off the coast sufficiently then the first appearance of such signatures can be monitored by an HF radar system early enough to issue a warning message about an approaching tsunami.