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Ionospheric scintillation characteristics in IRNSS L5 and S-band signals

Conference Paper · March 2017

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Indian Journal of Radio & Space Physics
Vol. 46, March 2017, pp. 15-19

Ionospheric scintillation characteristics in IRNSS L5 and S-band signals

M R Sujimol* & K Shahana
Satcom Experiment and Operation Division, SAC, ISRO,
New Delhi 110 021, India

Received 24 April 2017; revised 18 September 2017; accepted 26 February 2018

Ionospheric scintillation is a common phenomenon observed over the low latitude Indian region. It occurs due to the rapid
fluctuations in phase and amplitude of the signal and affects Global Navigation Satellite Systems (GNSS) very severely,
especially in L-band. Severe scintillation leads to loss of lock at the receiver, which affects user position accuracy. Ionospheric
scintillation mainly depends on solar cycle, user latitude, local time, season and elevation angle of the satellite. It also depends
on the frequency of the transmitted signal. Usually, lower frequencies experience higher scintillation at ionosphere. In this
paper, ionospheric amplitude scintillation effect on Indian Regional Navigation Satellite System (IRNSS) L5 and S-band
frequencies is studied on the basis of computed S4 index from Carrier to Noise (C/No) ratio measurements from IRNSS
receivers installed at New Delhi and Ahmedabad. A comparison of L5 and S-band scintillation is done. A confirmation
of scintillation phenomenon is also carried out using co-located GPS Septentrio (PolarRxS REO) receiver at frequency L1.
It is, further, found that the scintillation occurrence is mostly in post sunset periods and may result in frequent loss of lock at
L5 band during these periods.

Keywords: Ionospheric scintillation, Amplitude scintillation, S4 index

1 Introduction signal intensity scintillation can cause deep signal fades

The ionosphere is a highly variable and complex physical and in extreme cases, the receiver is unable to acquire the
system. It is produced by ionizing radiation from the sun. It scintillated signal. There are two types of ionospheric
is controlled by chemical interactions and transported by scintillations in Global Positioning System (GPS)
diffusion and neutral wind. Generally, the region between measurements, viz., amplitude and phase scintillations.
250 and 400 km, known as the F-region of the ionosphere, Amplitude scintillation refers to rapid fluctuation in signal
contains the greatest concentration of free electrons. At intensity (or carrier to noise ratio, C/No) measured by a
times, the F-region of the ionosphere becomes disturbed, and receiver, while phase scintillation refers to rapid fluctuation
small-scale irregularities develop. When sufficiently intense, in the carrier-phase measurements2,3. Levels of amplitude
these irregularities scatter radio waves and generate rapid and phase scintillations are commonly represented by the
fluctuations (or scintillation) in the amplitude and phase of standard deviations of amplitude and phase, S4 and σφ,
radio signals. The impacts of scintillation are not mitigated respectively, in a certain time period (typically 1 min). For
by the same dual-frequency technique that is effective at the amplitude scintillation, rapid sampling of C/No is
mitigating the ionospheric delay. For these reasons, necessary, while rapid carrier-phase measurements are
ionospheric scintillation is one of the most significant threats required for the phase scintillation. Furthermore, GPS
for Global Navigation Satellite Systems (GNSS) operating receivers for phase scintillation measurements need to be
in the near equatorial and polar latitudes. Scintillation equipped with a highly stable clock (oscillator), such as
activity is most severe and frequent in and around the oven-controlled crystal oscillator (OCXO) to distinguish the
equatorial regions, particularly in the hours just after sunset. phase fluctuations due to ionospheric scintillation and clock
(oscillator) noise2. Both types of ionospheric scintillations
1.1 Ionospheric scintillation are caused by plasma irregularities in the ionosphere. In the
Ionospheric scintillation refers to the random variation in low-latitude regions, where the background electron density
the phase and intensity of the received GNSS signals is high and plasma drift velocity is relatively slow, the
resulting from signal propagation though randomly amplitude scintillation is dominant. In this study, therefore,
structured and distributed irregularities in the earth’s the amplitude scintillation is focused on.
ionosphere. The phenomenon is observed primarily in the
equatorial and high latitude region1. The highest level of 1.2 Amplitude scintillation (S4) index
activity is observed in the solar maximum periods. The Amplitude scintillation directly affects the carrier to noise
——————— ratio (C/No) of signals in a GPS receiver, as well as the noise
*Corresponding author (E-mail: sujimol@sac.isro.gov.in) levels in code and phase measurements4. It can be
16 INDIAN J RADIO & SPACE PHYS, MARCH 2017

sufficiently severe that the received GPS signal intensity Indian Regional Satellite Systems (IRNSS) receivers at L5
from a given satellite drops below the receivers tracking and S-bands for the equatorial months of March and
threshold, causing loss of lock on that satellite, and hence, September with a data sampling rate of 1 s interval. In the
the need to re-acquire the GPS signal(s). This results in present analysis, 17 March and 14 September 2015 data is
reduced accuracy navigation solutions and data loss. used for computing S4 index form IRNSS 1A, 1B and 1C
Amplitude scintillation is quantified by the S4 parameter, satellites from the C/No measurements. The algorithm for
which is defined as the square root of the normalized computing the S4 index from C/No, ambient noise factor and
variance of signal intensity over a given interval of time.5-7 corrected S4 index is developed in Matlab (R2014a). The S4
index is calculated for both L5 and S band. A confirmation
S 4  ( I 2    I 2  ) /  I 2  … (1)
of scintillation phenomenon is also done using co-located
multi-frequency – multi-constellation Septentrio PolarRxS
where I is the signal intensity.
GPS receiver system at frequency L1. The S4 index is
For a given C/No level in dB-Hz, the effect of ambient
calculated at frequency L1 using the data from this receiver
noise on S4 index can be calculated and removed via
for 17 March and 14 September 2015. The results are
Ambient noise factor:
analyzed for the occurrence of amplitude scintillation.
Corrected S4 index = S4 – Ambient noise factor
100 500 3 Results and Discussion
(1+ ) Amplitude scintillation index (S4) variation at Delhi Earth
c / ng 19c / ng
… (2) Station (DES), Delhi and Space Application Centre (SAC),
effect of ambient noise Ahmedabad are studied in both L5 and S band. The S4 index
is derived from C/No measurements and the variation of
2 Data Sets and Statistical Results C/No along with S4 index are also plotted for comparison.
Study of ionospheric amplitude scintillation is carried out Figure 1 represents the amplitude scintillation index of L5
for two stations Delhi and Ahmedabad using ACCORD band at Delhi and Ahmedabad for the IRNSS 1A, 1B and

Fig. 1 — S4 index at L5 band on 17 March 2015 at Delhi and Ahmedabad

 SUJIMOL & SHAHANA: IONOSPHERIC SCINTILLATION CHARACTERISTICS IN IRNSS L5 AND S-BAND SIGNALS 17

1C. It can be observed that there is severe ionospheric on GPS. Figure 2 shows the ambient noise factor of L1, L5
scintillation at L5 band of radio signals from all three IRNSS and S-band calculated for a particular case on the same
satellites, post sunset period. The S4 index of L5 band at this date. The ambient noise factor is very negligible, of the
period is found to be greater than or equal to 0.4. Strong order of 10-3.
scintillation is generally considered to occur when S4 is The same analysis is carried out in the S-band. The impact
greater than ~0.6 and is associated with strong scattering of of ionospheric scintillation in S-band of radio signals can be
the signal in the ionosphere. Below this is weak scintillation. noted from Fig. 3, which shows variations in S4 index for
An S4 level below 0.3 is unlikely to have a significant impact IRNSS 1A, 1B, and 1C on 17 March 2015. Here, S4 index is

Fig. 2 — Ambient noise factor on 17 March 2015

Fig. 3 — S4 index at S band on 17 March 2015 at Delhi and Ahmedabad

18 INDIAN J RADIO & SPACE PHYS, MARCH 2017

very less compared to L5 band. A peak in the radio The results are plotted in Fig. 4. The scintillation event is
scintillation, albeit of lesser magnitude, can easily be noted much weaker in Delhi in this month.
around 1500 hrs UT (2030 hrs IST). The September data (14 Figure 5 illustrates the amplitude scintillation in L1 band
September 2015) is also analyzed for S4 index at L5 and S of radio signal recorded by the Septenentrio PolaRxS
band using the IRNSS 1A, 1B, 1C and 1D data from Delhi. receiver, which is a multi-constellation GNSS receiver. The

Fig. 4 — S4 index at S band on 14 September 2015 at Delhi

Fig. 5 — S4 index from Septenentrio- dual frequency receiver at L1 band at Delhi

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SUJIMOL & SHAHANA: IONOSPHERIC SCINTILLATION CHARACTERISTICS IN IRNSS L5 AND S-BAND SIGNALS 19

Fig. 6 — Position error measured in the hybrid mode on 17 March 2015

S4 index from this receiver in the band L1 is plotted for 17 tracked by the GAGAN system and variations and
March 2015 and 14 September 2015. Appreciable amplitude correlations may be efficiently studied.
scintillations in L1 signals during 1400 – 1800 hrs UT can
be clearly noted on 17 March 2015 as the case in IRNSS L5 Acknowledgement
and S. The S4 index of L1 band during this period is found The authors sincerely thank the Director SAC for
to be greater, the magnitude of the scintillation in L1 signal, providing opportunity to carry out this study. They would
however, is less compared to the L5 signal as recorded by also like to acknowledge the support and guidance provided
the IRNSS receiver. On 14 September, the scintillation is by Deputy Directors, SNAA and SNPA. The authors would
weak as it observed from the IRNSS L5 and S band. also like to thank Dr Rajkumar Chaudhary, SPL, VSSC for
Figure 6 shows the position error measured on 17 March providing support. Thanks are also due to Shri A P Shukla,
2015 using hybrid mode (IRNSS L5+S and GPS L1). The Shri Ashish Shukla, Shri S Sunda and Shri Rajat Acharya for
position accuracy has shown an increase in value (> 12 m) giving necessary guidance. Finally, the authors would like to
when the S4 index of 0.8 is observed on the same day in the thank the staff at Delhi Earth Station (DES) for the
L5 band. This is due the loss of lock of the signal of few encouragement throughout the study.
satellites in the L5 and S band, which in turn reduces the
position accuracy. References
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