IJCSNS International Journal of Computer Science and Network Security, VOL.8 No.9, September 2008 154 Manuscript received September 5, 2008. Manuscript revised September 20, 2008. Determination of GPS Total Electron Content using Single Layer Model (SLM) Ionospheric Mapping Function Norsuzila Ya’acob † , Mardina Abdullah †,†† and Mahamod Ismail †,†† † Department of Electrical, Electronics and Systems Engineering, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor. MALAYSIA †† Affiliate fellow, Institute of Space Science, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor. MALAYSIA Summary The ionosphere layer is very important to the communication system. The ions produced from the striking process of the ultra violet light have an active role in reflecting and observing the earth radio waves. This layer also is an unstable medium because it is vulnerable to several distortions which affect its physical condition. Studies of Total Electron Content (TEC) have given important relationship to the ionospheric layers due to high density of electron concentration at F region. Total Electron Content (TEC) measurements from ground stations to Global Positioning System (GPS) satellite provide a rich source of information about the Earth’s ionosphere. This research involves the determination of TEC content in ionosphere based on single layer model in order to determine the appropriate TEC value for Malaysia. This research assessing the errors translated from the code-delay to the carrier-phase ionospheric observable by the so- called “Levelling Process” which is applied to reduce carrier phase ambiguities from the data. Key words: GPS, TEC, ionosphere, levelling process 1. Introduction The ionosphere causes GPS signal delays to be proportional to the total electron content (TEC) along the path from the GPS satellite to a receiver. Total electron content is a key parameter in the mitigation of ionospheric effects on radio system. Total electron observations using Global Positioning System (GPS) satellites are becoming a very powerful and valuable tool for investigating global and local ionospheric structures [1, 2, 3] because the world-wide coverage provided by the 27 satellites constellation and the increasing number of networks available such as the International Standard for the specifications of ionospheric densities and temperatures. The highest TEC in the world occurs in the equatorial region. Comparatively, little corresponding research has been done on the low latitude (equatorial) ionosphere [4]. The major factors that determine the TEC are the solar cycle, season, local time, and geographic and geomagnetic coordinates. The main parameters of the ionospheric information (ionospheric electron content or electron density) are the altitude, local time, intensity of solar activity, season, position of station, and so on. The TEC itself is hard to accurately determine from the slant TEC because this depends on the sunspot activity, seasonal, diurnal and spatial variations and the line of sight which includes knowledge of the elevation and azimuth of the satellite, etc [5]. Modelling the altitude dependency of the electron density by a Chapman profile allows in turn to estimate the fractional TEC above the receiver altitude. The ionospheric path delay for positive elevations is then obtained from a thin layer approximation with a suitably chosen effective height of the residual ionosphere above the receiver. The primary focus is on mitigation of inherent fluctuations in pseudorange due to bandwidth limited precision, receiver noise, and multipath, typical GPS receivers generally employ so-called phase smoothing or levelling of code. In the present study a single thin shell of infinitesimal thickness situated at a median ionospheric height of 400 km above the earth surface was assumed. 2. Measurement of Total Electron Content (TEC) Using Dual Frequency Technique The TEC is defined as the total number of electrons integrated along the path from the receiver to each GPS. TEC is an indicator of ionospheric variability derived from the modified GPS signal through free electrons. It is also the parameter of the ionosphere that produces most of the effects on GPS signals. TEC is measured in unit (TECU) of 10 16 electrons per m 2 [6]. GPS observations provide both carrier phase delays L and pseudoranges P of the dual frequencies. GPS operates on two different frequencies f 1 and f 2 , which are derived from the fundamental frequency of 10.23 MHz: