GPS-DERIVED LOCAL TEC MAPPING OVER PENINSULA MALAYSIA DURING SOLAR MINIMUM OF SUNSPOT CYCLE 24

S. K. Leong 1, T. A. Musa 1, K. A. Abdullah 1, R. Othman 1, S. Lim 2, C. Rizos 2

1 UTM GNSS & Geodynamics Research Group, Faculty of Geoinformation Science & Engineering,

Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia - tajulariffin@utm.my 2 Satellite Navigation and Positioning Laboratory (SNAP), School of Surveying and Spatial Information

Systems, University of New South Wales, NSW, Australia.

ABSTRACT: The ionosphere is the major contributor of errors in Global Positioning System (GPS), especially during the 11-year sunspot cycle. The incoming 11-year sunspot cycle is expected to peak in 2013. The ionosphere condition is distinctly severe during ionospheric disturbances caused by high solar activity, which raises the question of how will this affect the ionosphere in the equatorial region, especially Malaysia. This study monitors the changes of Total Electron Content (TEC) by generating local TEC maps over Peninsula Malaysia using GPS measurements. The TEC maps show insignificant morphological changes in TEC variations during the period of study. The findings have widened the understanding of TEC variations in equatorial region, particularly during solar minimum. KEYWORDS: TEC variations, equatorial region, Solar Cycle 24

1. INTRODUCTION

In the last two decades, GPS applications have been growing rapidly, proving the availability and reliability of the Global Positioning System (GPS). However, the GPS positioning accuracies are affected by different error sources. A major error source affecting GPS positioning accuracy are the propagation delays as signals pass through the ionosphere layer. This error source can be the dominant bias during periods of disturbed ionospheric conditions. These periods are usually characterised by a significant degradation of positioning accuracy, and reduction of receiver tracking performance. However, the severity of the ionospheric effects is a function of a number of factors, such as the user location, time of day, time of the year, and level of solar activity. In particular the ionospheric electron density profiles are strongly affected by the number of the sunspots. As can be seen from Figure 1, the onset of the next solar cycle, Solar Cycle 24 is underway after the past 11-year sunspot cycle in 2000/2001. This cycle’s peak, which is called solar maximum, is expected in May 2013 (Philips, 2009b). During this stage, the mean Total Electron Content (TEC) value is predicted to be increasing as shown in Figure 2. As a result, single point positioning, differential GPS, and precise positioning applications will experience degradations in accuracy during these periods of high ionospheric activity. Moreover, the GPS receiver may lose lock on phase and/or amplitude of the signal when local irregularities in electron density are present in the ionosphere (Chen et al., 2007). Hence, these phenomena will have direct impact on GPS users in equatorial region (see Figure 3) since the size and variability of the

ionospheric free electron density is usually the largest in this region (Odijk, 2002). The subject of this paper is twofold: first, to help bridge the current understanding of spatial and temporal variation of the TEC over Peninsula Malaysia in the equatorial region, and second, to investigate the changes in ionosphere layer during the solar minimum period of Solar Cycle 24. In this study, GPS data from the Malaysian RTK network (MyRTKnet) are being utilised to extract variations in the TEC, and local vertical TEC maps of Peninsula Malaysia are generated.

Figure 1. Sunspot Number Prediction (After NASA, 2009)

Figure 2. Mean TEC value (After CODE, 2009)

Figure 3. Earth’s geomagnetic regions (After http://www.mike-willis.com/Tutorial/sporadicE.htm)

2. IONOSPHERE AND SUNSPOT CYCLE

The ionosphere is that band of atmosphere extending from about 50 to 1000 kilometres above the earth's surface in which the sun's extreme ultraviolet (EUV) radiation and X-ray emission ionises gas molecules which then lose an electron (Klobuchar, 1991). The ionosphere consists of different layers, namely D, E, F1, and F2 according to their electron density profile that represents varying degree of ionization (Langley, 2000). These layers influence the propagation of GPS signals (speed, direction and polarisation) as they pass through the ionosphere. The level of ionization depends on the radiation emitted from the Sun. The activity of the Sun is associated with the sunspot cycle and solar flares. Solar activity rises and falls periodically, repeated every 11 years known as a solar cycle. The changes in ionospheric electron density highly depend on the solar cycle due to tremendous release of electromagnetic energy from the Sun or commonly known as solar flares and overall enhanced solar radiation (El-Gizawy, 2003). The peak of each cycle is

the period of solar maximum, which occurs when the Sun reverses the direction of its magnetic field every 11 years. The solar maximum causes perturbation in ionospheric conditions, and hence producing disturbances on GPS as discussed by Klobuchar (1996). The more outbursts of energetic electromagnetic radiation and charged particles from the Sun during solar maximum will result in higher ionospheric electron densities and more variable contents.

3. GPS TEC MAPPING

The parameter of the ionosphere that affects on GPS signals is the TEC. TEC is the integral of total electron content along the line of sight path from the broadcasting satellite in space to the receiver on Earth (Hoffmann-Wellenhof, 2008). TEC is measured in units of TEC Units (TECU), which one TECU equals to 1016 electrons/m2. The ionospheric TEC is studied by analysing dual-frequency GPS data from the Malaysian RTK network (MyRTKnet) (see Figure 4). This local dense GPS network provides a promising tool for studying regional ionospheric TEC with high resolution in Peninsula Malaysia as well as the equatorial region. It is also possible to assess the variations in ionospheric TEC during quiet and disturbed period on regional scale by simultaneous observations to monitor the ionosphere, such as the observations carried out by the extensive network of GPS receivers (Komjathy et al., 2002). Data from seventy selected stations of the network have been collected and processed, which covers approximately the area from 0°N to 7.5°N in latitude and from 95°E to 105°E in longitude. Five nearby International GNSS Service (IGS) stations are also included in the processing. All GPS data have been processed using Bernese GPS software version 5.0 for six months in two-year period (2007-2008).

Figure 4. Malaysian RTK network (MyRTKnet)

For mapping TEC, geometry-free (L4) linear combination, which contains ionospheric information is analysed. A single layer model (SLM) ionosphere approximation was used (Schaer, 1999). Slant TEC values along oblique GPS signal paths are quantified from the network of selected stations in MyRTKnet and converted to vertical TEC by means of the single layer mapping function. Following Schaer (1997, 1999), the TEC is developed into a series of spherical harmonics adopting a single-layer model in a sun-fixed reference frame. Based on this model, local vertical TEC maps of Peninsula Malaysia are generated.

4. RESULTS AND DISCUSSION

The monthly averaged TEC maps have been generated as shown in Figure 6. From this figure, it can be noticed that TEC values gradually increases with the decreasing latitude. In the period of study, monthly averaged TEC values are below 15.0 TECU except in May 2007 where slight TEC enhancements were observed over 15.3 TECU. It is also found that May 2008 has higher averaged TEC values compared to January and September in the same year. There is a sign of reduced averaged TEC by 2.7 to 1.9 TECU for the year 2008 compared to 2007, which tallies with NASA scientists that noted the Sun is undergoing a period of very deep solar minimum during 2007 to 2009 (Philips, 2009a). In Figure 7, the diurnal cycle for TEC is plotted such that the maximum occurs two hours after solar noon, and is at minimum before dawn. The maximum and minimum values for averaged value of TEC for each month are listed in Table 1. Table 1. Maximum and minimum averaged monthly TEC

values

Year Month Maximum

(TECU) Minimum (TECU)

January 32.317 0.908 May 36.433 0.067 20

07

September 34.558 0.408 January 33.083 0.092

May 38.583 0.008 2008

September 31.767 0.017 Other than that, Figure 8 shows the average value of daily TEC for each month. In addition, the red line on every graph represents the polynomial trend line for the each month. The trend indicates that no obvious changes in TEC values. However, it is worth to mention that this analysis should clearly illustrates the yearly trend if every month of each year is included. Generally, TEC is a key ionospheric parameter that proved to be efficient in monitoring the ionospheric conditions (Jakowski et al., 2002). It is also most likely

that TEC monitoring can be utilised in improving the quality of network-RTK corrections that depends on ionospheric influence (Musa, 2007).

5. SUMMARY AND FUTURE WORKS

This paper reports on the research effort being undertaken to study the equatorial ionospheric TEC variations during solar minimum period of Solar Cycle 24. Local TEC maps over Peninsula Malaysia using MyRTKnet GPS data were produced together with diurnal variation and daily averaged trends of TEC for six months in two-year period (2007-2008). It is found that low variations in TEC occured during these durations suggested that low solar activity, concisely undergoing solar minimum. Further investigations to monitor the TEC variations will be conducted by processing GPS data for a longer and continuous period. This study is desirable to improve the quality of network-RTK corrections, especially during the period of the 11-year sunspot cycle in the equatorial region.

REFERENCES

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ACKNOWLEDGEMENTS

The authors would like to thank the Department of Survey and Mapping Malaysia (DSMM) for providing MyRTKnet GPS data, School of Professional and Continuing Education (SPACE), Universiti Teknologi Malaysia (UTM) and Ministry of Science, Technology & Innovation, Malaysia (MOSTI) for funding this study.

Figure 6. Monthly average of GPS-derived TEC maps

Figure 7. Bi-hourly average TEC shows diurnal variation of the ionosphere

Figure 8. Daily average TEC variations