26 
An instrument for the survey of ocean wave fields 
F. Ziemer 
GKSS-Research Centre 
Institute of Hydrophysics 
Department of coastal Hydrography 
21502 Geesthacht, Germany 
Tel: +49-4152-87-1567, Fax: +49-4152-87-1565 
E-mail: ziemcr@gkss.de 
Abstract 
Ocean wave fields are often dominated by different propagation directions of waves 
with close by periods or frequencies. This feature of multimodality , which is more often 
present within the nature of wave fields than unimodality , is effecting all kinds of 
human offshore operations. Therefore the measurement of the directionality within the 
actual seastate onboard a ship, a platform or a coastal tower is a long challenging task. 
For the description of the directionality of ocean wave fields it needs the two 
dimensional wave energy spectra. The system presented in this paper has been 
developed to provide these spectra. 
The input signal of the system is the radar backscatter, that is close to the noise 
background ("seaclutter"), in the near range of a navigation radar. Therefore, almost 
all navigation radar may be used as a sensor. A special hardware was designed for the 
A/D conversion of the signal and for the storage of a time series of radar images. The 
analysis for the computation of the wave field analysis is done by a specially developed 
algorithm. From the beginning of the measurement to the final analysed product it takes 
less than 4 minutes to compute the complete two dimensional asymmetrical wave 
energy. From there it is easy to deduce the parameters that are commonly to present 
time series measurement (as e.g.: from buoys). 
1. Introduction 
It is the multimodal structure within wave fields, which causes frequent desasters in all kind 
of offshore operations. Therefore it is very important to detect the detailed structures within 
the directionality of a wavefield. To fullfill this task it needs the quick and continous 
measurement of the two dimensional wave energy spectrum. From this spectrum all other 
spectral presentations as the integrated wave paramters (as e.g.: Hs) may easily be deduced 
(see: fig. 3.). 
The motivation to develop the Wave Monitoring Sytem (WaMoS) at GKSS was, to have an 
instnament to validate the directionality within the two dimensional wave energy spectra as it 
is either forecasted by numerical wave models or measured by imaging from board airplanes 
or satellites. In this field the instrument has already proven its powerfulness in a series of 
international field studies like e.g.: LEWEX'87 [1], SAXON'90 or ERS-1 CAL/VAL'91 [2]. 
During the scientific analysis period of these experiments, the capacity of 'WaMoS', 
especially to detect multimodal structures within the wavefields, stressed the valuable 
contribution provided by this system.