GIS@development August 2002: "The effects of Ionospheric layers on TEC values using continuous satellite tracking data"

The Global Positioning System (GPS) is the satellite-based method of collecting 3D data for various scientific applications. For most of its trips from the satellite to the receiver antenna, the GPS signals ‘enjoys’ a trip through the virtual vaccum of ‘empty space’. When it gets to the earth’s atmosphere, however, the speed drops by an amount that varies somewhat randomly. Of course, since the calculation of the range to the satellite depends on the speed of the signal, a change in signal speed implies an error in distance, which produces an error in position finding. Significant changes in signal speed occur through the atmosphere is known as atmospheric effects. One of this effect is the ionospheric delays.

The ionising action of the sun’s radiation on the earth’s upper atmosphere produces free electrons which affect the propagation of electromagnetic waves. The ‘ionised’ region of the atmosphere is a plasma and its referred as the ionosphere. Shorter wavelength signals (30 MHz), such as GPS signals, pass through the ionosphere but are affected by it. The ionospheric refraction is one of the major error sources in GPS, which causes signal propagation delays. The major effects the ionospheric can have on GPS are the followings, see Klobuchar, (1996) :

group delay of the signal modulation, or absolute range error;
carrier phase advance, or relative range error,
Doppler shift, or range-rate errors
refraction or bending of the radio wave,
distortion of pulse waveforms,
signal amplitude fading or amplitude scintillation, and phase scintillation

The ionospheric delay is a function of the Total Electron Content (TEC) along the signal path and the frequency of the propagated signals. Since the ionosphere is a dispersive medium for radio waves, a dual-frequency GPS receiver can minimise ionospheric delay through a linear combination of L1 and L2 observables (Lao, 1998). Nowadays, with the decreasing cost of GPS equipment, permanent GPS tracking stations have been installed at a rapid pace over almost all the worlds. The permanent GPS tracking stations are normally set to operate 24 hours a day, all year round, and one of these, is the Malaysian Active Satellite System (MASS). The MASS network is used in a variety of applications in Malaysia involving the determination of precise positions, unified datum, earth rotation parameters and crustal studies as well as for atmospheric (tropospheric and ionospheric) modelling. This paper describes the potential of the dual-frequency GPS observations collected by MASS network in modelling the TEC values with respect to ionospheric layers.

Fig. 1: Illustrates the ionospheric refraction, which causes the GPS signal propagation delays.
The Mass Network and Ionospheric TEC
Current trend indicates that for the most precise, effective, economical and fast applications, some form of permanent GPS network should be established. Therefore, a large number of permanent GPS tracking stations has been built up in the past few years that allows for many scientific researches. In order to realise such requirements, the Directorate of Surveying and Mapping, Malaysia (DSMM) has started establishing permanent GPS tracking stations namely MASS network, at the end of 1998. Currently, the MASS network consists of seventeen (17) permanent GPS stations over the country. This network is also known as the Zero Order Geodetic Network and it complies with international standards to provide the highest precision for positioning in Malaysia. Thus, GPS data of the MASS network seemed to be a very reliable sources for ionospheric studies, particularly in the Equatorial regions. The network is set to operate 24 hours a day, all year round, so that the daily variations of the electron density in the ionosphere can be studied. The continuous GPS data can be collected on a daily basis, centrally archieved at data centers, and are available via the internet to users.

The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the earth, streching from a height of about 50 km to more than 1000 km above the earth’s surface. The existence of the ionosphere is primarily due to the extreme ultraviolet radiation and X-rays from the sun. Different regions of the ionosphere are produced by different chemical species. The ionosphere is composed of the D, E, F1 and F2 regions. Figure 2 illustrates the different regions of the ionosphere.

Fig. 2: The different regions of the ionosphere

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