Initially, automatic identification system (AIS) was proposed to avoid collision between vessels and ensure effective traffic controlling off the coast or in harbors via navigational messages among mobile terminals and base stations in the VHF maritime mobile band.

Since it used typical VHF data exchange devices, AIS terminal equipment has been used for ocean monitoring. However, most ocean monitoring systems rely on satellites and no studies have been conducted on shore-based systems.

In the case of a long distance between the ground station and vessels, the received signal intensity decreases rapidly and there are a number of small cell areas around the base station. The rapid degradation of the strength of received signals due to troposphere propagation and the packet collision due to simultaneous signal transmission are main challenges to widening the communication range of AIS systems.

In order to extend the communication range of AIS ground stations, it is necessary to amplify received signals as much as possible to transfer to feed those with sufficient signal-to-noise ratio into the demodulation part, and the demodulation part should use coherent Viterbi detector and error correction codes to reduce the packet error rate.

O Myong Guk, a researcher at the Faculty of Communications, has proposed a new packet structure using error correction codes to reduce the packet error rate at the shore station, and a scheme to correctly decode it.

The simulation and experimental results show that the proposed scheme is suitable for reducing the packet error rate and increasing the coverage area of long-range AIS systems which depend on the troposphere wave propagation.

Coal-bed hydraulic fracturing is a key process in coal-bed gas production and one of the important ways for improving its efficiency is to make a new kind of proppant. What is important here is to reduce its density as much as possible, maintaining its technical parameters including strength.

Activated water often used as a fracturing liquid in hydraulic fracturing of coal seam is favourable for forming a complex fracture net, with stable performance and low frictional resistance, but it is limited in hydraulic fracturing due to its low viscosity and low capacity of carrying proppants. The use of ultra-low density proppants for hydraulic fracturing can solve problems such as clogging of cracked pores after gel break of the fracturing liquid and low productivity, often encountered in hydraulic fracturing using dense sand or ceramic granular proppants, and can provide sufficient proppant carrying capacity by crystalloid solutions such as clean and cheap activated water.

In order to solve the problems of high settling rate of proppants when low-cost activated water is used as a hydraulic fracturing liquid, Yun Myong Guk, a researcher at the Faculty of Earth Science and Technology, has prepared a resin-coated ultra-low density proppant with solid slag and white fly ash as feedstock.

He has analyzed the effects of the material composition and the resin dip and coating of the proppant on its performance by X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and cylinder pressure methods.

The analysis results are as follows.

The packing effect was best when resin concentration was 25wt.%. When the coating resin content of the proppant dip body was 6%, the visible porosity of particles was reduced to 7.13% and its bulk density was 0.91g/cm³. The apparent density was 1.5g/cm³ and the acid solubility was 0.35%. The breakage rates were 7.3% and 9.0%, respectively under 16MPa and 20MPa closing pressure conditions.

Electricity is clean and efficient energy that is essential for human life and widely used. As the power demand increases, the demand for power quality is constantly increasing. The sudden voltage fluctuations in industrial production hinder the normal operation of automation devices and even cause damage to products.

In the evaluation of power quality, system frequency and voltage at the grid nodes are main indices, and the voltage, symmetry and waveform of three-phase network are auxiliary indices.

The dynamic voltage restorer (DVR) is a power electronic device used to protect the load from the shock caused by voltage fluctuations occurring in the electrical network. This device is very effective in improving power quality, compared to conventional compensating devices.

Aiming to compensate harmonics and unbalances, Jong Il Bok, a researcher at the Faculty of Electrical Engineering, has proposed a controller for standard voltage calculation of DVR and a current controller using PCI method. Then, he has verified the accuracy of a designed controller in MATLAB/Simulink environment.

The simulation results show that the dynamic voltage restorer can cope with sudden voltage fluctuations and effectively prevent factors affecting power quality such as voltage fluctuations.

And the proposed current controller can be used not only for DVR but also for AC sequences control.