Intercomparison of a WaveGuide radar and two Directional Waveriders
Down-looking FMCW radars for wave measurements are in use already for 25 years. They have proved to be very robust and maintenance is hardly needed. Now the prices of FMCW radars came down they are being more widely applied for water level, tide, harbour oscillations and wave height at sea, lakes and rivers.
Radacs WaveGuide was introduced in 1998 based upon a commercial tank radar, first Enrafs Smartradar 873 and since 2000 with Smartradar 973. Enrafs Smartradar is designed for tank storage measurements and therefore approved by the authorities for weight and measures up to an accuracy of 1mm. So the instrument itself is very stable and accurate. However the sea surface in wind force 12 differs from a liquid surface in a storage tank. In this report the quality of WaveGuide information is described for sea states from twenty centimetres up to six metres.
In describing the quality of a wave sensor we have to realise that the sea is chaotic and its conditions are constantly changing. Even with very good instruments we will obtain different information if we are not measuring at exactly the same position and time.
WaveGuide information is extracted from the signal reflected by the water surface. Therefore the quality of information could depend on the conditions of the water surface. This raises questions about the quality of the information as function of the sea conditions. In this paper we will discuss the following questions:
How do the wave height parameters derived with the WaveGuide compare with those derived with buoys in different sea states? The Directional Waverider is the de facto standard for wave measurements.
Is data from bad quality or even lost in certain sea conditions e.g. due to spray or steep slopes?
If data is being lost, does that happen in large sequences? Individual errors are mostly no problem for the processing but gaps longer than 1 or 2 seconds will cause problems.
During the winter of 2003/2004 an extensive measuring experiment was done at the Measuring Platform Noorwijk, the research facility of Rijkswaterstaat In this experiment very detailed information is being collected from three WaveGuides and two Directional Waveriders. This data is used for this study and will also be used for the development of new versions of the WaveGuide e.g. for the development of other processing schemes eg. for measuring high frequency waves or for wave direction.