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Voltage-mode-control of a phase-shift-modulated fullbridge DC-DC converter, using a digital Proportional-Integral (PI) controller, is described in this paper. For implementing the digital controller, a low cost 16 bit microcontroller, dsPIC30F2010, is used. External hardware requirements are quite minimum, as the microcontroller has an inbuilt PWM module and ADC. Frequency response method is employed here to directly design the digital controller. Alongside, the concern of limit cycle oscillations is addressed, while maintaining converter switching frequency and ADC resolution at comfortably high levels. Performance of the digital feedback controller is checked using a 300 W, 100 V prototype converter.

Two-ship acoustic propagation experiments were conducted in deep waters of the Arabian Sea between 22-25 July and 7-8 August 2002 from onboard INS Sagardhwani and ORV Sagarkanya during ARMEX-I phase. The aim of this experiment was to understand the spatial (up to 30 km) and temporal (up to 24 hr) fluctuations of acoustic intensities within the surface duct. The acoustic transmissions were made from 15m depth at 2.4 KHz and 620 Hz frequencies from one ship and the signals were received at 15m and 40m depths on the second ship. The data were recorded for 10 min duration during each transmission and 30 sec averages were taken for computing acoustic intensities. Simultaneous oceanographic data on temperature (CTD and XBT), currents (ADCP) and waves (SBWR) were also recorded. Steady and homogeneous mixed layer depths of 50-60m offered an ideal environmental condition for surface duct propagation. The sea-state was moderate with significant wave height ranging from 1.5 to 2.3m. The surface and sub-surface currents in the upper 125m water column revealed the predominance of tides with change of current direction at 6 hr intervals. The current speeds were ~ 50 cm/s. The analysis of acoustic data highlighted the importance of temporal variations compared to spatial variations. Around 20 dB fluctuations in acoustic intensity were noticed for temporal variability where as only 7 dB fluctuation was observed for spatial variability.

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Physical oceanographic and acoustic data were simultaneously collected from the coastal waters of the Arabian Sea. Acoustic transmissions were carried out from an anchored vessel using 620 Hz transducer and received by an array of hydrophones moored at ~5 km away from the anchorage. Thermal structure in this region was characterised by a tri-layer structure, ie, a strong thermocline (> 0.4 o C/m) sandwiched between an upper (< 10 m) and bottom (> 25 m) homogeneous layer. High-resolution (sampled at 10 s interval) temperature data from moored sensors revealed intense internal wave activity. The maximum value of Brunt-Vaisala frequency, which is the maximum frequency limit of internal waves in the thermocline, suggests that the upper frequency limit of the internal wave, which can be generated during this period, is 23 cph (2.6 min). High and low frequency waves caused variations of ~3 o C and ~5 o C respectively in the temperature field. But the low frequency internal waves were found to contain maximum energy compared to the high frequency waves. Fluctuations of 8-12 dB were noticed in the measured acoustic intensity values in the presence of low frequency internal waves. Simulation studies carried out using parabolic equation model using 620 Hz source indicated well-defined ducted propagation with minimum transmission loss, when the source was kept within the homogeneous layer. The presence of tri-layer thermal structure, ie, a strong gradient layer sandwiched between an upper and bottom homogeneous layer, caused surface and bottom channel propagation in this region.