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Remote Sensing of Surface Currents and Waves by the HF Radar WERA


Presented at the EEO'97 conference in Southampton


Klaus-Werner Gurgel and Georg Antonischki
University of Hamburg, Institute of Oceanography, Tropolwitzstrasse 7, D-22529 Hamburg, Germany.
Tel: +49-40-42838-5742, Fax: +49-40-42838-5713,
email: klaus-werner.gurgel@uni-hamburg.de
WWW: http://wera.cen.uni-hamburg.de/index.shtml

A full paper has been published in the conference proceedings


Abstract

HF radar uses the process of backscattering of electromagnetic waves of 10 m to 50 m wavelength from the rough sea surface and can be used to measure surface currents and ocean wave parameters. The technique has been rapidly developed during the past 25 years. In Germany, the work on HF radar started in 1980 by adopting NOAA's CODAR (COastal raDAR) system, introduced in 1977 by D. E. Barrick. Recent developments lead to an improved design called WERA (WEllen RAdar). This paper describes the different techniques used by CODAR and WERA, their advantages for different applications and the design used by WERA.

Introduction

The basic physics of backscattering of electromagnetic waves by ocean waves have been discovered and described in 1955 by D. D. Crombie. The first commercial application of this physics has been the CODAR system, introduced in 1977 by D. E. Barrick at NOAA. This system has initially been designed for mapping of surface current fields. Based on CODAR, further developments have been made in the UK (Marconi OSCR), the United States (Barrick's Sea Sonde) and in Germany (University of Hamburg WERA). From CODAR independent developments of HF radars have been made in Canada (C-CORE Northern Radar), the UK (University of Birmingham PISCES), France (University of Toulon), Australia and Japan. There also exist several skywave HF radar systems all over the world.

In Germany at the University of Hamburg, the work on HF radar started in 1980. NOAA's CODAR (COastal raDAR) has been bought and was modified until 1985 to reduce internal noise, increase the sensitivity of the system and optimize the algorithms. In 1990, a shipborne CODAR has been developed to measure the circulation at the Arctic Front and the ice edge in the North Atlantic. Several limitations of the CODAR have been identified: The 4-element receiving antenna array did not give direct access to ocean wave measurements and the range resolution technique of CODAR was limited to range cell sizes of minimal 1.2 km. Finally, the design did not allow as much flexibilty as required to adapt the system to new applications.

Based on this experience, WERA (WEllen RAdar) has been designed: Up to 16 receiving antennas can be set up very flexible, e.g. as a 16-element linear array, a 12-element linear array plus the 4-element CODAR array, or even on a circle. The configuration can easily be reflected in processing software. The range resolution technique has been changed from using pulses to a using a frequency chirp, which allows more flexibility in changing the range resolution of the system. To increase the signal-to-noise ratio of WERA, parallel processing of 16 independent receiver channels has been included in the design. The operating frequency can be between 27 MHz and 30 MHz, but other frequencies may be possible. Range resolutions of 1.2 km, 600 m and 300 m can be selected.

Different Techniques for Spatial Resolution

Spatial resolution of radar systems has to be achieved in range and azimuth. Several different techniques have been developed for the HF radars mentioned, which can be described by the following principles:

Range resolution can be done on two different ways:

  1. The time delay between a transmitted pulse and the received echo gives the range, the length of the pulse gives the extent of a range cell. This basically is range resolution in time domain and used by CODAR.
  2. The frequency offset between a transmitted linear frequency chirp and its echo gives the range, the bandwith of the frequency chirp gives the extent of a range cell. This basically is range resolution in frequency domain and used by WERA.
For a selected range resolution (e.g. 1.2 km), a specific bandwidth of the radar signal (125 kHz) is required. This bandwidth is independent of the technique used (8 micro seconds pulsewidth or 125 kHz chirp-sweepwidth). The advantage of range resolution by pulses is the simple design, however a high peak power has to be gained by the transmitter. Range gating in the receiver leads to high data sampling rates to be processed (2 micro seconds at 300 m range resolution). The advantage of range resolution by a linear frequency chirp is the simple way to change the range resolution and a low data rate to be processed in the receiver, however the transmitter and receiver must be designed for high dynamic range and linearity.

Azimuthal resolution can basically be done by

  1. Direction Finding: Looking for the direction of the echo by measuring the phase differences between all antennas (CODAR, WERA).
  2. Beam Forming: Stear a beam to different directions by adding up the echos received by all antennas after applying a phase shift (WERA).
The advantage of Direction Finding is the use of a relatively small receiving antenna, which can more easily be installed at the shore. Beam forming is essential to allow direct access to the second order sea echoes, which carry the information on ocean waves, however significantly increased space is required to set up the array. As WERA stores raw data, a comparison of Direction Finding and Beam Forming results calculated from the same data set is possible.

The Design of WERA

In an frequency chirp (FMCW) radar system, signals from increasing ranges appear at increasing frequency offset. Range resolution is done by fourier transforming each sweep separately, which for a selected frequency offset gives the time series at that range. It appears, that a windowing function is required to avoid smearing of short range echoes to longer ranges by convolution with the phase-incontinuity of the repetition of the sweep.

To achieve a high signal-to-noise ratio, which besides access to the second order sea echoes is important for optimal results of the wave processing algorithms, the signals received by the 16 antennas of the WERA system are processed in separate receiver channels instead of multiplexing them. Amplitude and phase calibration of all 16 receiver channels including all individual cable lengths is very important for the Beam Forming and Direction Finding.

To simplify the design of the WERA system, a direct conversion receiver with I- and Q-channels is used. Before A/D conversion of the received signal, a 7-pole lowpass filter keeps off all signals above Nyquist frequency. To achieve a high dynamic range, a 16 bit A/D converter has been selected. Data are preprocessed inside a VME-bus computer system sweep by sweep and stored on a Unix workstation for final processing.

The linear frequency chirp is generated by a fast DDS synthesizer. To keep the phase noise of the transmitted signal and the local oscillator of the receivers low, an oven controlled high stability crystal oscillator has been selected. This helps to extend the dynamic range of the system when processing strong signals from near by and weak signals from long distances at the same time.

Conclusion

Compared to CODAR, the new design of WERA offeres increased flexibility in spatial resolution and allows simultanous Beam Forming and Direction Finding techniques, as required by the application. Within the EC project SCAWVEX, the WERA system measures surface currents and wave hight directional spectra simultanously, using the University of Hamburg current algorithm and the University of Sheffield wave algorithm. This is a further step in research on current-wave interaction.

Acknowledgement

This work has been supported by the European Commission, DG XII, within the Mast-2 programme, project MAS2-CT94-0103, SCAWVEX (Surface Current And Wave Variability EXperiment). I wish to thank the other members of the HF-radar group, Georg Antonischki, Heinz-Hermann Essen, Florian Schirmer, Thomas Schlick and our technician Monika Hamann for supporting the measurement campaigns, developing algorithms and processing data.
klaus-werner.gurgel@uni-hamburg.de
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