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The SCAWVEX Petten Experiment

Animations of surface current fields measured by HF radar are available now.

The Petten experiment was mainly aimed on wave data intercomparison and scheduled in autumn 1996. As the operation of the WERA systems has been verified in spring, no CODARs have been set up this time. Because of potential interference with the wave bouys' telemetry, the operation frequency of WERA has been chaned from 29.85 MHz to 27.65 MHz.

The WERA sites have been set up at Petten on the dyke directly at the starting point of the line of several masts and wave bouys, and 10 km north in the dunes at Voordijk. The short baseline between the radars has been chosen to ensure a good signal to noise ratio of the radar signal at the wave bouys' positions. For large scale current measurements, this configuration is not optimal, because the influence of geometry is decreasing the accuracy of the 2-dimensional current vector at narrow angles between the radars. WERA has been set up using a 16-element linear array, as the current algorithm used for the WERA 12-element linear array during the Maasmond experiment have shown good results.

A photo of the 16-element linear array at the Petten site (100 dpi), Petten site (150 dpi), a picture of the 16-element linear array at the Voordijk site (100 dpi), Voordijk site (150 dpi), and of the WERA system can be seen here.

To ensure proper operation of the 16-element linear array, the amplitudes and phases of all antennas have been calibrated internally. In addition, a ship supplied by Rijkswaterstaat has been used to confirm the calibration values and to measure the influence of the environment. Slightly different calibration values have been measured at that time and those have been used during the experiment. The two WERAs have been run alternately to reduce the risk of interference between the systems, as they have been only 10 km apart. The time schedule of the systems and the duration of the measurements can be found in table 1.

The WERA operated at Petten had some problems with interference with the transmitter of the wave bouys. It finally turned out, that the bouys in the near field (200 m) of the WERA transmit antenna have been picking up the radar signal, modulating it with the bouy's signal, and then retransmitting it to the WERA receiver. This on one hand increased the noise floor in the WERA signal, and on the other hand was responsible for some "blind" ranges of the radar. By carrying a wave bouy on a truck to the beach at the Voordijk WERA site, the interference could be reproduced at that site too.

During the antenna calibration with the ship, the weather was very calm. With the start of the measurements, the wind increased and waves up to hm0 = 4.61 m have been measured by the wavebouy at MP A at the 6th of Novemver 1996. The spectra of the sea echos at that time have shown very strong second-order peaks and the direction finding algorithm used for currents gave strange results. To supress the second-order peaks, a beam forming algorithm has been developed during the first weeks of the experiment.

Figure 1 shows the radial component of the surface current field measured at the Voordijk WERA site. The working range of WERA increased to 55 km during medium sea states. The topography shown has been supplied by Rijkswaterstaat. The three wave bouys shown in the figure are at MP 2, 3.5 km off Petten, MP 1, 8 km off Petten, and MP A, 10 km off the coast between Petten and Voordijk.

Figure 2 shows the surface current field measured by both WERAs. The red arrows result from the blind ranges of the Petten WERA and are interpolated data, using the radial component measured at Voordijk and the current field at surrounding measurement points. A coastal jet can be seen in this figure, which appeared and disappeared depending on tide and wind conditions.

Figure 3 shows an example of a WERA spectrum measured from the Voordijk site at wave bouy position MP A. The first order Bragg lines show 50 dB signal to noise ratio and the second order reflections can clearly be seen.

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