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Directional spreading function of the gravity-capillary wave spectrum can provide the high-wavenumber wave energy distribution among different directions on the sea surface. The existing directional spreading functions have been mainly developed for the low-wavenumber gravity wave with buoy data. In this paper, we use radar observations to derive the directional spreading function of the gravity-capillary wave spectrum, which is expressed as the second-order Fourier series expansion. So far the standard form of the second-order harmonic coefficient has not been proposed to correctly unify the gravity and gravity-capillary wave. Our strategy is to introduce a correcting term to replace the inaccurate gravity-capillary spectral component in Elfouhaily's directional spreading function. The second-order harmonic coefficient at L, C and Ku band calculated by the radar observation is used to fit the correcting term to obtain one at the full gravity-capillary wave region. According to our proposed the directional spreading function, there is a spectral region between the gravity and gravity-capillary range where it signifies the negative upwind-crosswind asymmetry at low and moderate speed range. And this is not reflected by the previous models, but has been confirmed by radar observations. The Root Mean Square Difference (RMSD) of the proposed second-order harmonic coefficient versus the radar-observed one at L, C band Ku band is 0.0438, 0.0263 and 0.0382, respectively. The overall bias and RMSD are −0.0029 and 0.0433 for the whole second-order harmonic coefficient range, respectively. The result verifies the accuracy of the proposed directional spreading function at L, C band Ku band.

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Modulation model of radar backscatters is an important topic in the remote sensing of oceanic internal wave by synthetic aperture radar (SAR). Previous studies related with the modulation models were analyzed mainly based on the hypothesis that ocean surface waves are Gaussian distributed. However, this is not always true for the complicated ocean environment. Research has showed that the measurements are usually larger than the values predicted by modulation models for the high frequency radars (X-band and above). In this paper, a new modulation model was proposed which takes the third-order statistics of the ocean surface into account. It takes the situation into consideration that the surface waves are Non-Gaussian distributed under some conditions. The model can explain the discrepancy between the measurements and the values calculated by the traditional models in theory. Furthermore, it can accurately predict the modulation for the higher frequency band. The model was verified by the experimental measurements recorded in a wind wave tank. Further discussion was made about applicability of this model that it performs better in the prediction of radar backscatter modulation compared with the traditional modulation model for the high frequency band radar or under lager wind speeds.

In modern complex battlefield environment, it is important to obtain the frequency spectrum precisely. We propose an operational method of instantaneous frequency measurement (IFM) assisted by photonics, which can achieve high precision based on stimulated Brillouin scattering (SBS), giving the credit to the narrow linewidth of gain spectrum of SBS. We use MZM and DPMZM cascade to generate a tunable continuous optical signal and modulate the measured signal to the continuous optical signal and a DPMZM is used to generate pump light, the two beams of light are injected into the fiber, the stimulated Brillouin scattering (SBS) effect occurs in the optical fiber and the Brillouin gain spectrum (BGS) is formed. By setting the scanning frequency, the Brillouin gain varies with frequency and the amplitude comparison function (ACF) can be formed in the narrow line band of BGS. And the measurement of full frequency band is realized through a reference signal, the measured frequency range is limited only by photoelectric device. Estimation of multiple radio-frequency (RF) signals can also be achieved with a resolution of 250 MHz. In the numerical simulation, the average measurement error less than 1 MHz is achieved in this scheme.