Wave Measurements

from SeaSondes

Measurements

•Significant Wave Height

•Wave Period

•Peak Wave Direction

•Wind Direction

Wave info•Derived from 2nd

order peaks in Doppler spectra

What Are Second Order Peaks?

•Secondary peaks caused by the orbital velocity created by passing waves

•Characteristics:

•Signal is an order of magnitude weaker

•May occur on both sides of 1st order peak

•May be present in both +/- Doppler

•Occasionally more than one peak is present

Example: 5MHz System

First Order(Currents)

Second Order(Waves)

Where Do Second Order Peaks Come From?

Where Do Second Order Peaks

Come From?

Negative Doppler

Positive Doppler

Ocean Wave ModelPierson-Moskowitz Spectrum

Total ocean-wave spectrum: Pierson-Moskowitz model with cardioid directional factor

S(k,)Z(k)cos4 *2

Z(k)Ae 0.74(kc /k )

2

k 4

Short wave spectrum

S (k , ) cn (k )tfn ( )n 2

2

Process•Calculate model radar spectra for wide

range of total ocean-wave spectral parameters (using Pierson-Moskowitz model)

•Separate 1st order from 2nd order regions

•Determine short-wave ocean spectrum from 1st order to give direction (wave and wind)

•Fit measured radar spectra to models to give wave height, period and direction

Reading References• Barrick, D. (1971), Dependence of second-order Doppler Barrick, D. (1971), Dependence of second-order Doppler

sidebands in HF sea echo on sea state, IEEE G-AP sidebands in HF sea echo on sea state, IEEE G-AP Internat. Symp. DigestInternat. Symp. Digest

• Hasselman, K. (1971), Determination of ocean wave Hasselman, K. (1971), Determination of ocean wave spectra from Doppler radio return from the sea surface, spectra from Doppler radio return from the sea surface, Nature.Nature.

• Barrick, D. (1972), Remote sensing of sea state by radar, Barrick, D. (1972), Remote sensing of sea state by radar, in Remote Sensing of the Tropospherein Remote Sensing of the Troposphere

• Barrick, D. (1977), Extraction of wave parameters from Barrick, D. (1977), Extraction of wave parameters from measured HF radar sea-echo Doppler spectra, Radio Sci.measured HF radar sea-echo Doppler spectra, Radio Sci.

• Lipa, B. (1978), Inversion of second-order radar echoes Lipa, B. (1978), Inversion of second-order radar echoes from the sea, J. Geophys, Res.from the sea, J. Geophys, Res.

• Lipa, B.J., B. Nyden (2005), Directional wave information from the SeaSonde, IEEE Jour. Ocean. Eng., vol 30, no. 1, pp 221-231.

Limitations

•42MHz High-Res•Sensitivity: 0.25m waves

•Saturation: 3m waves

•25MHz High-Res•Sensitivity: 0.5m waves

•Saturation: 4m waves

12MHz Standard•Sensitivity: 0.75m waves

•Saturation: 7m waves

5MHz Long Range•Sensitivity: 1m waves

•Saturation: 20m waves