The SCAWVEX Petten Experiment
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.