Comparison between CODG TEC and GPS based TEC ... - NOPR

Indian Journal of Radio & Space Physics Vol 41, December 2012, pp 617-623

Comparison between CODG TEC and GPS based TEC observations at Koudougou station in Burkina Faso Frédéric Ouattara1,$,*, Christian Zoundi2,# & Rolland Fleury3,+ 1

Ecole Normale Suprieure (E N S), Université de Koudougou, BP 376, Koudougou, Burkina Faso

2

Direction Régional de l’Enseignement Secondaire – Plateau Central (DRES-PC), Ziniaré, Burkina Faso 3

Telecom Bretagne, Technopole Brest Iroise, 29239 Brest, France

E-mail : [email protected], #[email protected], [email protected] Received 7 March 2012; revised 12 October 2012; re-revised received and accepted 21 December 2012 The paper presents the comparison between total electron content (TEC) map given by CODG model and Rinex TEC obtained from Koudougou GPS station, which started its operations in December 2008. The TEC observations were carried out at Koudougou station during 5-10 December 2008 by ionosphere single layer model (SLM). The observation of TEC profiles showed noon bite out profile with predominance of morning or afternoon peak and night time peak. CODG TEC profiles highlighted dome and reverse profiles. There is no night time peak in TEC map profiles. The comparison between GPS based TEC observation at Koudougou station and TEC map shows that CODG model does not estimate Rinex TEC at Koudougou station. Key words: Ionospheric variability, Single layer model, Total electron content (TEC), GPS TEC map PACS Nos: 94.20.dv; 84.40.Ua

1 Introduction Now a days, due to development of communication and navigation systems, the knowledge of ionospheric variability has become important. Among several methods of ionosphere investigations, total electron content (TEC) estimations, through Global Positioning System (GPS) satellites, play an important role. In fact, many scientists used GPS based TEC observation for studying ionospheric variability1-7. For global and local ionosphere investigations during International Heliophysical Year (IHY) campaign (2007-2009), a number of GPS stations were installed over Africa [Fig. 1(a)], where there is lack of data6 [Fig. 1(b)]. Koudougou GPS station (geographic latitude 12°15’N; geographic longitude -2°20’E; dip = + 8.24) in Burkina Faso (West Africa) [Fig. 1(b)] is one of the multiple stations involved in IHY campaign. The present study is based on the data taken from Koudougou station data base and is the first data utilized after the station started operating in December 2008. GPS based TEC observation can be carried out using several processes, viz. single layer model (SLM)8-10, 10th polynomial function technique11,

combining ground base GPS data and ionosonde data12, etc. Among these processes, in this paper, SLM is used to obtain GPS based TEC observation at Koudougou station. In SLM, TEC measurements were estimated assuming that the ionosphere is a spherical shell at fixed height of 400 km above the Earth’s surface13. The slant TEC (a measure of the total electron content of the ionosphere along the ray path from the satellite to the receiver) is then converted to vertical TEC (VTEC) using a mapping function. The point, where the VTEC is determined, is called the ionospheric pierce point (IPP), which is the intersection of the user line of sight to the tracked satellite with the centre of the ionospheric slab14 (Fig. 2). Koudougou ionospheric variability is studied using these observations and comparing them with TEC map data. The later are derived from Centre for Orbital Determination in Europe (CODE) model (CODG model). TEC map is determined by means of IGS stations, through Global Ionosphere Map, using SLM8,9. CODE is one of the centres of analysis of International GNSS (Global Navigation Satellite Systems) Service (IGS). This centre publishes

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Fig. 1 —(a) Africa GPS network [blue stations are provided their GPS measures during 2011 and the others (red stations) not; there are several GPS networks: AMMA and NIGNET in West Africa and NIGNET and UNAVCO for the others]; (b) IGS GPS network

ionosphere maps and gives the correct GPS orbits, Earth orientation parameters and GPS stations coordinates and global ionosphere map, namely global ionosphere maps (GIMs). CODG permits the determination of GPS TEC in specific region

identified by geographical coordinates determined by the IGS GPS network. The comparison is made with a view to appreciate qualitatively the model estimation for a station that is not involved in IGS network stations.

OUATTARA et al.: COMPARISON OF CODG TEC & GPS BASED TEC OBSERVATIONS AT KOUDOUGOU

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determination procedure, one can access the IGS website (ftp://igscb.jpl.nasa.gov/igscb/data/format/ ionex1.ps). Estimations of TEC from both Rinex files and TEC map have been carried out by Matlab software. GPS provide both carrier phase delays ф and pseudo-range P of the dual frequencies for observation needed. The L-band signal frequencies are f1 = 1575.42 MHz and f2 = 1227.60 MHz. These frequencies are derived from the fundamental frequency f0 = 10.23 MHz as f1 = 154.f0 and f2 = 120.f0. The expression of total electron content (total number of electrons integrated along the path from receiver to each GPS satellite and measured in TEC Unit (TECU), 1 TECU = 1016 electron m-2) can be obtained by using group delay (∆P) and carried phase delay (∆ϕ). The range of error produced by the group delay is ∆R =

40.3TEC f

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Fig. 2 — Ionospheric single layer model (Source: Norsuzila et al. )

2 Data and Methodology 2.1 Data

For this study, two types of GPS based TEC observation, viz. Rinex (Receiver INdependant EXchange) TEC carried out at Koudougou station and IGS (International Geodesy System) GPS TEC map have been used. Rinex TEC used here are obtained from double frequency receiver of Koudougou GPS station (geographic latitude 12°15’N; geographic longitude -2°20’E, dip: + 8.24). The receiver was provided by Ecole Nationale de Télécommunication de Bretagne (ENST-Bretagne, now Télécom-Bretagne) and has been operating since December 2008. This GPS station is a non-IGS station with in situ data obtained in Rinex file format. IGS TEC map are obtained from IGS data base where TEC map files are in IONEX (IONosphere Exchange) file format. It is important to underline that the used Rinex file contains data recorded at 30 seconds interval while IONEX file contains data recorded at an interval of two hours. 2.2 Methodology 2.2.1 Procedure of TEC determination

Only the procedure for determining TEC from Rinex files is presented here. For TEC map

2

with f as the frequency. Therefore, the differential code group delay is expressed as  1 1  ∆P = P1 − P2 = 40.3 TEC  2 − 2   f1 f 2 

where, P1 and P2, designate the pseudo-range for frequencies f1 and f2, respectively. The carrier phase advance is given by δϕ =

40.3TEC c. f

2

where, c, is light velocity in a vacuum; and f, the frequency. Thus, differential carrier phase delay is obtained by ∆ϕ =

40.3TEC  1 1  − 2  2  c f1   f2 .

The determination of TEC by using group delay gives the absolute scale of TEC with less precision10 TEC evaluated by differential phase delay that increases measurement precision but since the actual number of cycles of phase remains unknown in the actual measurement, the derived TEC also shows finite bias. Therefore, the method consists in using TEC derived from code group delay and smoothing GPS pseudorange data with carrier phase measurements15.


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By rewriting code group delay equation one can obtain: TEC =

(P1− P2 ) 40.3

f12 f 22 f 22 − f12

.

As the TEC between the satellite and the user depends on the satellite elevation angle, the TEC obtained from the above equation is called slant TEC (STEC). As the GPS observables are biased on the instrumental delays16, to accurate TEC estimation, differential instrument biases from satellites and receivers are removed from STEC17. It is important to underline that STEC is a measure of the total electron content of the ionosphere along the ray path from the satellite to receiver. Therefore, it depends on the ray path geometry through the ionosphere. To obtain TEC, which is independent of path geometry, STEC is converted to vertical TEC (VTEC) by using a mapping function after considering ionosphere as a single layer. The mapping function f (α) used here is expressed as:   R sin α  2    f (α ) = 1 −  E   RE + hm    

using the database that contains TEC data computed at an interval of 30 seconds. For evaluating the accuracy of the TEC map at Koudougou station, in situ TEC is plotted together with hourly smoothing window and TEC map is interpolated hourly. 3 Results Figures (3-5) show the hourly variation of VTEC for six selected days of December 2008 based on the data availability. Concerned days are days of the year (DOY) 340 (5 December) - 345 (10 December). The experimental plots are done with one hour of window time for smoothing. The TEC map plots are obtained by interpolating TEC map data. The broken green curves show TEC map and red curves denote Rinex TEC. Figure 3 shows plots for 5 and 6 December. Figure 4 shows the graphs of 7 and 8 December 2008.

0.5

where, RE, is the mean Earth radius; hm, is the height corresponding to maximum electron density at the F2 peak16 or the altitude of the thin layer above the surface of the Earth7; and α, the zenith angle at the receiver sites. The α can be obtained when satellite position and the coordinates of receiver location are known. The altitude, hm, ranges from 250 to 350 km at mid latitudes and from 350 to 500 km at equatorial14. For the present study, hm= 450 km (the same as for TEC map) is taken. For this altitude, it is possible to obtain VTEC by means of Matlab program when (i) the latitude and longitude of the receiver point, and (ii) the ray path direction given by the elevation and azimuth angles, are known. As one can obtain VTEC at many IPP points away from the receiver, while one has to calculate the VTEC above the receiver at 15 minutes. Quadratic regression weighted by the angle of the elevation limiting to 25 degrees is used to derive the vertical TEC at the receiver point. This method can incorporate an error of a few TECU in the estimation. 2.2.2 Plotting procedure

In order to highlight time variation of TEC at Koudougou station, daily TEC plot was obtained

Fig. 3 — Local time variation of Rinex TEC and TEC map for: (a) 5 December 2008; and (b) 6 December 2008


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Fig. 5 — Local time variation of Rinex TEC and TEC map for: (a) 9 December 2008; and (b) 10 December 2008 Fig. 4 — Local time variation of Rinex TEC and TEC map for: (a) 7 December 2008; and (b) 8 December 2008

Figure 5 depicts the variation of the TEC for 9 and 10 December. All Rinex TEC graphs show night peak between 20.00 and 21.00 hrs LT while such peak is not seen in CODG graphs. Three local peaks before 20.00 hrs LT are observed in Rinex curves. These local peaks are located at 08.00 LT - ~12.00 hrs LT; ~13.00 – 16.00 hrs LT; and 16.00 - 18.00 hrs LT. In Fig. 3 (5-6 December 2008), CODG curves show dome profile while Rinex graphs exhibit predominant morning peak located at 12.00 hrs LT and two troughs at ~13.00 and ~ 16.00 hrs LT. All CODG TEC and Rinex curves show a decreasing trend from 13.00 to 18.00 hrs LT. Figure 4, devoted to 7-8 December 2008, highlights graphs that combine predominant morning peak at 12.00 hrs LT with afternoon peak at ~16.00 hrs LT. This type of profile

shows two troughs at 13.00 and 16.00 hrs LT. Moreover, night peak can be seen in all the graphs. Rinex TEC for 9 December 2008 [Fig. 5(a)] shows predominant morning peak combined with reverse profile. The morning peak is located at 10.00 hrs LT and the afternoon peak at 17.00 hrs LT. CODG TEC shows reverse profile with the afternoon maximum peak at 15.00 hrs LT. In panel (b) of this figure, Rinex TEC profile exhibits three peaks at 11.00, 14.00 and 17.00 hrs LT. CODG TEC shows here reverse profile with a maximum peak at 14.00 hrs LT. 4 Discussions The closed midday trough of Rinex TEC profile shows the effect of E×B and night time peak expresses the effect of the pre reversal electric field18-22. The dome profile of CODG TEC shown in Fig. 3 expresses the absence of E×B effect in these

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TEC map profiles. The absence of the signature of E×B is also observed in reverse profiles of CODG TEC shown in Fig. 4. The CODG TEC profile of Fig. 5(a) is more marked by E×B effect than that of the Fig. 5(b). The absence of night peak in CODG profiles and the difference of TEC profiles between Rinex TEC profiles and those of CODG show the discrepancy between the two. Fayot & Vila23 gave the different types of foF2 profiles observed at Ouagadougou station (geographic latitude 12.4°N; geographic longitude -1.5°E; dip +8.04), an African equatorial ionization anomaly (EIA) region station. These different types of foF2 profiles are: (1) noon bite out profile or B profile (two peaks with trough around midday); (2) morning peak profile or M profile; (3) dome profile or D profile; (4) plateau profile or P profile; and (5) afternoon peak profile or reversed profile or R profile. As parameter foF2 is able to describe the distribution of E-layer electric current through the study of ground recorded magnetic field variations24-25, Vassal26 relied different types of foF2 profiles of West Africa equatorial region on different types of E-layer electric current (electrojet and counter electrojet). For that it attributed the absence of electrojet to D and P profiles; the presence of intense counter electrojet to R profile, the presence of a mean intense electrojet to M profile and the presence of a strength electrojet to B profile. It is important to point out that while in African sector the relationship between foF2 and electric current is theoretically sound26, in Indian sector an empirical relation of electrojet with VTEC has been established27 and an approximate physical model of this relationship has been given28. By taking into account the relationship Ne = 1.24 × 1010 f0F22 (where Ne is the peak electron density) and by taking into account the results coming from Indian sector researches, it becomes possible to interpret TEC profiles in term of electric current. Therefore, Rinex profiles of Fig. 3 may be related to the presence of a mean intense electrojet while the CODG profiles express the absence of an electrojet. The Rinex TEC profiles in Fig. 4 are related to: (1) mean intense electroject in the morning and (2) intense counter electrojet during the afternoon. The TEC profile of the panel (a) of Fig. 5 is related to mean intense electrojet in the morning and intense electrojet in the afternoon. The TEC profile of the panel (b) of the same figure is related to intense counter electrojet.

5 Conclusion Rinex TEC profiles present night time peak and show morning peak profile combined with reverse profile. TEC map variations do not show night peak and exhibit dome and reverse profiles. The present study shows the discrepancy between TEC map and Rinex TEC values. To well appreciate CODG response in further study, its responses will be investigated during different seasons and under different solar cycle phases in future. Acknowledgements Authors would like to thank the reviewers for their kind help, advice, remarks and suggestions. Authors also thank IGS data providers for TEC map database. References 1

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