I am processing buoy data where two GNSS buoys are separated by about hundreds of metres, and when performing wave direction spectrum analysis, I find that the dominant directions of the two buoys sometimes differ by about 180 degrees, what is the reason for this?
Yes, measurements of the dominant direction of the wave can create a 180-degree ambiguity. This is because a wave can travel in two opposite directions, and the measurements only tell us the direction of the strongest component of the wave. For example, if we measure that a wave is traveling north, it could also be traveling south, but with a weaker amplitude.
There are a few ways to resolve this ambiguity. One way is to use multiple sensors to measure the wave from different angles. This can be done with seismometers, which measure the ground motion caused by waves, or with hydrophones, which measure the sound waves caused by waves.
Another way to resolve the ambiguity is to use a technique called polarization. Polarization refers to the direction of the wave's electric field. Waves that are polarized in the same direction will add together, while waves that are polarized in opposite directions will cancel each other out. This means that if we can measure the polarization of the wave, we can determine its direction.
Finally, we can also use a technique called interferometry. Interferometry is a way of measuring the difference in the arrival times of two waves. This can be used to determine the direction of the wave by measuring the angle between the two waves.
In general, the best way to resolve the 180-degree ambiguity depends on the specific application. For example, if we are trying to determine the direction of an earthquake, we will need to use multiple seismometers to get accurate measurements. However, if we are trying to determine the direction of a sound wave, we may be able to use polarization or interferometry.
Preston Guynn Thank you very much for your answer, but the sampling frequencies of the buoys are 1Hz and 5Hz when we measure. Normally, the frequency of the measured waves is usually 2s-30s. Of course, there will be some high-frequency waves on the sea surface, but their wavelengths are small and have little effect on the buoys, or the buoys are not sensitive to these high-frequency waves.
Preston Guynn I understand what you mean. If the peaks in one direction are significantly higher than those in other directions, will this problem not exist?
This is one of my analysis results, which are the 20-minute directional wave spectra of two buoys, one of which is 1Hz and the other is 5hz, but in the dominant direction of the wave, 270° vs 90°
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