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A new method for ocean wave observat, I on by marine radar

Shintaro Goto
Mitsubishi Research lnstitute lnstitute Ine.
2-3-6,Otemachi Chiyoda -ku, Tokyo, Japan


Abstract
This paper considers the method to determine thc wave direction and wave length analysing the radar echo from the marine radar which is placed on the ground near coastline.

Firstly, the simulating data of radar echo using random numb is made, secondly this data I a analyzed by FFT (Fast. Fourier Transform) and the wave direction. And wavelength is calculated.

A comparison of the results calculated by this method and the parameter using to make simulated data of radar echo shows the accuracy of the method developed here.

Introduction
On the construction of coastal structure and the disaster prevention in the coastal area. Two -dimensional wave observation is the great theme on coastal engineering, presently, the observational method is largely depends on the number of wave gauges and the placing of wave gauges.

The marine radar which uses a plan position indicator (PPI) to display the images make it possible to observe the sea surface condition, As the PPI image is image data the result of marine radar is not used quantitatively. If the PPI image used quantitatively the wave direction and the wavelength is calculated by Image processing technique.

Recently the marine radar image can be quantitatively used to the automatic control ships. Supposing to use this radar, we consider the method the method to determine the direction and the wavelength.

The algolism which makes it possible to determine the wave direction and the wave length by the sea clutter is investigated and the algolism is verified by the simulating data of marine radar .

The method wave observation by marine Tadar
The process of the data analysis shown schematically in Fig.1. Each procedure of the process is as follows.


Fig.1 Flow diagram showing the analysis procedure

  1. Data correction of distance as the received power from the sea surface become more and more weakly as the radar is are applied to correct the difference The correction functions are applied to correct the difference of these power due to the distance from the radar to the sea surface.

  2. Coordinate transformation as the received power from the radar is transformed to x y coordinate system for FFT analysis

  3. Two-dimensional FFT. The fourier transform F(I, m) is defined by

    F (I, m) =1/(2p)2.ò¥-¥h(x ,y).exp (-I (I, m))dx dy ------------(1)

    In which (x ,y) is the received power distribution of marine radar I and m are the wave number of wave number of x-direction and y-direction respectively

  4. Calculation of the wave direction and the wave length From the Fourier Transform F(l , m) the directional spectrum

    s(lm) = | F (l, m) |2

    The wave direction q and wave length l is defined as follows.

    q = tan-1(mo/lo) -------------(3)

    L=Lo/(L2omo2)1/2---------(4)

    in which (l0,m0) is the point where the directional spectrum is it its maximum, and where F (l, m)l is defined.
Result
Fig, 2 shows the sea clutter simulated by random number in the condition of q = 45 ,L=2.82D.d . Fig.3 shows the directional spectrum distribution which is calculated by the eq.2. From the peak of s(l, m), q is 43.18~45.0° and L is 2.83D d 2.92 D d


Fig.2 Simulated sea clutter


Fig.3 Directional spectrum distribution

Conclusion
The result shows the method which is proposed here is applicable to the ocean wave observation

References
skolnik, m,l.: Sea Echo Radar Handbook McGraw Hill p.26 28, 1970.

Table.1 Cases considered in this study