NOPR Collection:

Characterization of underwater acoustic communication channel

Sat, 01 Aug 2020 00:00:00 GMT

Title: Characterization of underwater acoustic communication channel Authors: Malarkodi, A; Latha, G; Srinivasan, S Abstract: The objective of this work is to study the multipath underwater channel characteristics by estimating the channel impulse response and its derived functions such as scattering functions, power delay profile and Doppler spread. This work performs the channel characterization from the data measured from south- west Bay of Bengal during July 2017 at a range of about 1 km and 3 km in a depth of approximately 20 m. To estimate the channel impulse response, Linear Frequency Modulated (LFM) pulse with the bandwidth of 4 kHz and the center frequency of 11 kHz is used as a probe signal. Experimental data analysis shows the variations between two channels of 1 km and 3 km ranges. Other characterization functions such as multipath intensity profile and Doppler power spectrum are estimated from the channel impulse response. The estimated channel parameters convey that the channels are quasi stationary and the Doppler frequency spreads are due to the movement of the transmitter and receiver positions. Page(s): 1323-1329

Predicting wave force on vertically submerged rectangular thin plate in intermediate depth of water using second-order perturbation equation

Sat, 01 Aug 2020 00:00:00 GMT

Title: Predicting wave force on vertically submerged rectangular thin plate in intermediate depth of water using second-order perturbation equation Authors: Roy, P Deb Abstract: The present paper studies the nonlinearity effect of an ocean wave on a thin rectangular plate under two geometrical configurations in the intermediate water. The perturbation approximation method was derived analytically up to the second-order. Analytical results was validated by the numerical method of Simpson's 1/3 rule. Results showed that the horizontal force of the wave on a plate recorded at the water surface (z/d = 0) was significantly high for ε = 0.175 and d/L = 0.24 as compared to the low value of relative depth. The results also showed that the wave forces are gradually converging to each other under two types of geometrical configurations with the decrease of relative depth. Nonlinear effect of the wave forces on the plate in the form of double peaks was found in the graph at a low value of d/L = 0.10 and wave steepness ε (= 0.070). This study revealed that due to the effect of nonlinearity, greater wave force occurred at a depth d = 3 m and T = 3 s and d/L = 0.24 on a thin plate and also implied that this force does not occur at the stage of double peaks form. Page(s): 1330-1340

Albedo variations in relation with solar altitude and transmission factor over Arabian Sea at different weather conditions

Sat, 01 Aug 2020 00:00:00 GMT

Title: Albedo variations in relation with solar altitude and transmission factor over Arabian Sea at different weather conditions Authors: Subrahmanyam, M V Abstract: Albedo variations with respect to different weather conditions with the observed shortwave radiation data over Arabian Sea using Pyranometer, during June to August 2002, and March -June 2003. The hourly albedo variations are analysed and presented in relation with solar altitude (SinH) and transmission factor (TF). TF classified into three groups: under clear sky (TF > 0.65), partly cloudy (0.4 < TF < 0.65) and overcast condition (TF < 0.4). As per classification, the mean albedo (TF) values are 0.07 (0.71), 0.06 (0.59) and 0.07 (0.30), respectively. The study period includes different weather conditions such as clear sky to overcast conditions and different wind conditions. During the lower wind speeds, albedo is varying between 0.01 and 0.17 and TF varied between 0.16 and 0.73. However, during higher winds (> 12 m/s), there is no significant changes observed in albedo. Variations in TF and albedo during the overcast conditions are significant, however wind also dominating. Page(s): 1341-1349

Improving numerical current prediction with Model Tree

Sat, 01 Aug 2020 00:00:00 GMT

Title: Improving numerical current prediction with Model Tree Authors: Dauji, S; Deo, M C Abstract: A method to improve the real time predictions of ocean currents on the basis of a machine learning technique called model tree is proposed. It consists of forming an error time series obtained as the difference between the numerical prediction and the actual measurement of the current at a given time step, carrying out time series prediction as per the technique of model tree and predicting the error for a future time step. Subtraction of such error from the numerically predicted current produces the improved current magnitude for the next time step. The suggested procedure is applied at two deepwater locations in the Indian Ocean. The numerical current model under investigation is code named: HYCOM, while corresponding current observations are those coming from a measurement program called: RAMA. It was found that such method of error subtraction yielded more accurate predictions than those based only on the numerical modelling. This is judged from analysing certain error statistics as well as by comparison with the random walk time series prediction method. The predictions up to five days in advance are satisfactorily done in this manner. Page(s): 1350-1358