VOL. 10, NO 22, DECEMBER, 2015 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences © 2006-2015 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 10324 VHF/UHF AMPLITUDE SCINTILLATION OBSERVED BY THE LOW- LATITUDE IONOSPHERIC TOMOGRAPHY NETWORK (LITN) Dessy Francisca 1 , Ernest Macalalad 2 , Edgar Vallar 3 , Maria Cecilia Galvez 3 , Lung-Chih Tsai 4 and Tung Yuan Hsiao 5 1 Immanuel Christian School, Pontianak, Kalimantan Barat, Indonesia 2 Department of Physics, Mapúa Institute of Technology, Manila, Philippines 3 Environment and Remote Sensing Research Group, Department of Physics, De La Salle University, Manila, Philippines 4 Center for Space and Remote Sensing Research, National Central University, Jhongli District, Taoyuan City, Taiwan (R.O.C.) 5 Department of Information Technology, Hsing Wu Institute of Technology, Linkou District, New Taipei City, Taiwan (R.O.C.) E-Mail: macalalade@gmail.com ABSTRACT Electron density irregularities in the ionosphere that cause rapid fluctuations in radio signals or scintillation has been studied using the Low-Latitude Ionospheric Tomography Network. The network uses Ionospheric Tomography System (ITS) receivers to retrieve VHF and UHF scintillation data from August 2008 to February 2011. Amplitude scintillation, which mostly occurred at the equatorial anomaly peak, varied with local time, solar activity and magnetic activity. Moreover, night-time scintillation occurred predominantly around local midnight (2100-0200 LT), while most of the daytime scintillation occurred at 0900-1500 LT. Generally, the scintillation occurred under quiet magnetic condition and the occurrence increases as the solar activity increases. Keywords: scintillation, ionospheric irregularity. INTRODUCTION The ionosphere is the part of the upper atmosphere that lies between about 50 km to more than 1000 km. It acts as a reflector and distributor of radio waves. Electron density irregularities in the ionosphere cause a significant problem in radio wave propagation, where it generates rapid fluctuations of amplitude, phase, polarization and angle of arrival of a radio signal (Davis, 1990; Das et al., 2010). This effect, called scintillation, can cause a crucial disturbance for radio systems using satellite to ground links near the magnetic equator and at high-latitudes, which includes the polar caps and auroral regions (Bernhardt et al., 2000). These significant effects of scintillation at equatorial and low latitude regions attracted many researchers to study the scintillation phenomenon. Scintillation is usually quantified through the S4 scintillation index or simply the S4-index, where it describes the variance of the received power fluctuations. Specifically, it is defined as the square root of the variance of received signal intensity fluctuations over the time- averaged received signal intensity in dB P (Bernhardt et al., 2000): P P P S   2 4 (1) In this study, this parameter, commonly known as amplitude scintillation, is calculated over a 60-s period at 50-Hz sampling frequency from the very high frequency (VHF) and ultra high frequency (UHF) bands of the Low- Latitude Ionospheric Tomography Network (LITN). S4- index is related to diurnal and seasonal variations, as well as to geomagnetic and solar activity. The low-latitude ionospheric tomography network The LITN is a network of ground beacon receivers along the 120-130° East longitude. It was re- established in 2006 by the Ionospheric Sounding Laboratory of the Center for Space and Remote Sensing Research, National Central University in Taiwan after the original LITN was retired in 1997 (Hsiao et al., 2009). The LITN ground stations is spread from 34°N to 1°S latitude, as shown in Figure-1. Each station is equipped with an Ionospheric Tomography System (ITS) coherent receiving system from Northwest Research Associates, Inc. (NWRA). It can receive mutually coherent signals (150 MHz, 400 MHz and 1066.7 MHz) from low-earth orbit (LEO) satellites, such as the FORMOSAT3/COSMIC satellites and other NNSS-like satellites such as C/NOFS, OSCAR, GFO, RADCAL, and COSMOS.