Eq. 1:
Where, 
PR_{L2} 
= 
L2 pseudorange in meters 
PR_{L1} 
= 
L1 pseudorange in meters 
Δ 
= 
Input bias between the C/A and P code chip transitions in meters 
CAL 
= 
TEC result due to internal receiver L1/L2 delay and the offset 
In other words, the TEC introduced in Eq. 1 (slant TEC) is
defined as the total number of electrons integrated along the path from the
receiver to each GPS satellite in a column having a cross sectional area of
one m^{2} and is measured at different elevation angles. The vertical
TEC (at elevation angle of 90°) or simply vTEC, as seen in Fig.
1, is modelled using a mapping function^{[15,18]} as below:
with
Where, 
χ and χ’ 
= 
Zenith angles at the receiver site and at the ionospheric
pierce point, IPP 
R_{E} 
= 
Mean earth radius 
h_{m} 
= 
Height of maximum electron density (450 km) 
 Fig. 1: 
Ionosphere single layer model^{[17]} 
Amplitude scintillation from C/A code GPS receiver: Amplitude scintillation monitoring is traditionally accomplished by monitoring index S4. The S4 index is derived from signal intensity of signals received from satellites. Signal intensity is actually received signal power, which is measured in a way that its value doesn’t fluctuate with noise power. When the S4 index is normalized, the receiver’s absolute gain is not important, as long as it is relatively constant during the period. It is also important that the intensity measurements be linear with respect to the signal power over its entire range including deep scintillation fades. S4 measured at L band (L1 and L2 frequencies) needs to have the effects due to removal of ambient noise. It is because the ambient noise at the L1 frequency translates to a relatively high S4 at lower frequency VHF and UHF frequencies band.
The total amplitude scintillation index, S4_{T}, including the effects
of ambient noise, is defined as follows:
S4_{T} 
= 
Total amplitude scintillation index 
SI 
= 
Satellites signal intensity 
where S4_{T} represents the expected (or average) value over the interval of interest (i.e., 60 sec)^{[5,12]}. Unfortunately, the total S4 defined in Eq. 3 can have significant values simply due to ambient noise. Thus, it is desirable to remove. The ambient noise of Eq. 3 can be removed by estimating the average signaltonoise density over the entire evaluation interval (60 sec) and using that estimate to determine the expected S4 due to ambient noise. This is legitimate since the amplitude scintillation fades do significantly alter the average signaltonoise density over a 60 sec time interval.
If the signaltonoise density (S/N) is known, the predicted S4 due to ambient
noise is:
Thus, by replacing the S/N with the 60 sec estimate
an estimate of signaltonoise density, we obtain an estimate of the S4 due
to noise .
Subtracting the Eq. 5 from Eq. 3 yields
the corrected value of S4 as following:
RESULTS The results of the observed vTEC and S4 parameters are presented in this section. In both cases, comparisons are made between the campaign location (Sipitang) and the main observatory (Parit Raja). The daily maxima vTEC and S4 parameter are shown and discussed. vTEC: The onemonth results obtained during the campaign have been summarized as below. Figure 2 shows the daily maxima vTEC for GPS week 1351 to 1354 (Nov. 30thDec 23rd). Maxima vTEC was chosen from the maximum value of vTEC during the 24 h period. It is noted that the daily maxima vTEC for Parit Raja (PR) with values ranging from 38100 TECU during the campaign period. However, vTEC values for Sipitang are generally lower as compared to Parit Raja, ranging from 3042 TECU. It is also of interest to note that a general consistency for both stations at Parit Raja and Sipitang Hill can be seen during the 1 month campaign period.
Scintillation parameter, S4: The scintillation parameter, S4 index,
was computed as a standard deviation of the received signal power normalized
to average signal power every 60 sec, based on 50 Hz sampling data rate (3,000
data samples) on L1.
 Fig. 2: 
The maxima vTEC for GPS week from 13511354 at Parit Raja
and Sipitang 
 Fig. 3: 
Corrected scintillation parameter, S4 for GPS week 1351 at
Parit Raja and Sipitang (Parit Raja = Pink Line, Sipitang = Black Line) 
 Fig. 4: 
Corrected scintillation parameter, S4 for GPS week 1352 at
Parit Raja and Sipitang (Parit Raja = Pink Line, Sipitang = Black Line) 
 Fig. 5: 
Corrected scintillation parameter, S4 for GPS week 1353 at
Parit Raja and Sipitang (Parit Raja = Pink Line, Sipitang = Black Line) 
 Fig. 6: 
Corrected scintillation parameter, S4 for GPS week 1354 at
Parit Raja and Sipitang (Parit Raja = Pink Line, Sipitang = Black Line) 
Base on GISM Chart (Global Ionosphere Scintillation Model), scintillation activity
indicated by S4 has four categories i.e. S4≤0.25 is quiet, S4>0.25
and S4≤0.5 is moderate, S4>0.5 and S4≤1 is disturbed, then S4>1
is severe^{[6]}. Figure 36 show
the scintillation parameter, S4 for both stations during the campaign period.
Results indicate the overall S4 values for both stations are quite consistent.
discussion The TEC parameters maxima vTEC observed at Sipitang are lower as compared to the station at Parit Raja. It is because of the location of Sipitang is closer to the magnetic equator as compared to Parit Raja. The equatorial fountain effect makes the lower electron density for such location near to magnetic equator. This can also be shown in Fig. 1, that the Sipitang vTEC is much lower than the Parit Raja vTEC shows the presence of equatorial anomaly^{[1]}. However, both observation stations are showing a general consistency for their TEC parameters. It is because the TEC parameters are mainly due to solar and geomagnetic activities of the earth. The geomagnetic index that is the Kp index is undisturbed condition during the observation which is less than 5 (The index runs from 09, where 9 is the most disturbed). CONCLUSION From the results, quiet scintillation event was observed at most of the week. Observations are taken on December 2005 (GPS week 13511354) where the geomagnetic is undisturbed condition during observations. Although the duration of this observation was only for almost a month, results confirm the presence of the equatorial anomaly. More campaigns should be carried out to properly understand and characterize the ionosphere over Malaysia and the equator. acknowledgment The researchers would like to thank WARAS for providing the raw data at both stations for this study. WARAS operates the first and only digital ionosonde in Malaysia. " target="_blank">View Fulltext
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