Ionospheric TEC estimations using dual frequency coherent L1/L5 signals from the geostationary SBAS satellites.

Kurbatov G.A.1,*, Kunitsyn V.E. 1, Padokhin A.M. 1, Yasyukevich Yu.V. 2

1Lomonosov Moscow State University, Faculty of Physics, Leninskie Gory, Moscow 119991, Russia

kurbatov_ga@physics.msu.ru, kunitsyn@phys.msu.ru, padokhin@physics.msu.ru,

2 Institute of Solar-Terrestrial Physics, Russian Academy of Sciences, Siberian Branch, Irkutsk, Russia, yasukevich@iszf.irk.ru

Abstract

In this work we study coherent L1/L5 signals of satellite based augmentation systems (SBAS) geostationary

satellites observed with geodetic GNSS receivers located at equatorial and mid-latitudes and estimate corresponding geostationary total electron content (TEC) and errors of such estimations. We also provide the comparison of geostationary TEC with the data of nearest ionosondes, as well as wavelet analysis of geostationary TEC, providing typical periods of observed variations at different time scales and discuss the capabilities of SBAS TEC observations in connection with ionospheric effects of solar flares.

1. Introduction

Geostationary satellites have been used for ionospheric TEC estimations for decades. Until recently those were

mostly the measurements based on Faraday rotation of polarization plane. This method was used particularly for the studies of ionospheric effects of solar flares [1,2], statistical characteristics of travelling ionospheric disturbances [3], acoustic gravity waves [4], etc.

With the SBAS development the dual frequency L1/L5 observations from a number of geostationary satellites are now available. Nowadays there are six such sattellites: SES-5, GSAT-8, GSAT-10, Intelsat Galaxy 15, TeleSat Anik F1R, Inmarsat 4-F3. It provides the possibility to retrieve ionospheric TEC from these observations using the same approach as for dual frequency L1/L2 GPS/GLONASS observations [5].

Along with the advantages of geostationary TEC observations, such as almost motionless ionospheric pierce point, there are issues should be taken into account when analyzing geostationary TEC data, including larger amount of plasmaspheric electron content in geostationary TEC compared to GNSS observations due to higher geostationary orbits, and very low elevation angles of geostationary satellites already at midlatitudes, causing significant level of noise and cycle slips in data. Also the spatial gradients of electron density should be considered.

2. Examples of observations

In this work we consider results obtained at equatorial IGS LMMF (14.59N, 61W) station and at two mid-latitude stations: MSU (55.75N, 37.62E) and ISTP (52.28N, 104.30E). In Fig. 1 the geometry of SBAS observations at these stations is presented including the pierce points and the directions to Inmarsat 4-F3 (prn133), GSAT-8 (prn127) and GSAT-10 (prn128) satellites.

30 32 34 36 38 40 42 44 46 48 50

46

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58

Moscow (MSU station)

ionospheric pierce point

to prn127 (lon=55, el=25)

latit

ude

longitude96 98 100 102 104 106 108 110 112 114

46

48

50

52

54

56

58

to prn128, el=25

ipp

Irkutsk (ISTP station)

ipp

to prn127, el=15

latit

ude

longitude-68 -66 -64 -62 -60

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12

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16

18

20

ionospheric pierce point

latit

ude

longitude

PRJ18

lmmf

to prn133 (lon=-98)

Fig. 1. Geometry of the experiment

978-1-4673-5225-3/14/$31.00 ©2014 IEEE

First important issue is to analyze the level of noise in SBAS TEC data. In Fig. 2 typical noise figures (100sec RMS) of geostationary TEC observations at MSU and ISTP stations are presented. Left and middle pannels present observations from GSAT-8 at elevation angles 250 and 150 for MSU and ISTP stations correspondingly. Right pannel presents observations from GSAT-10 at elevation angle 150 for ISTP station. Note that TEC RMS can reach up to 1 TECU which is times greater than for typical GNSS observations at these elevations thus limiting the application of the data. TEC RMS for the same satellite GSAT-8 at both receiving sites manifests the same diurnal variability which does not depend on the local time of the observation sites and differs from those for GSAT-10. That suggests that the observed diurnal variability in TEC RMS is rather due to the motion of the satellites than due to ionospheric variability. Taking into account mentioned above issues, smoothing of TEC time series makes the SBAS L1/L5 data potential.

0 2 4 6 8 10 12 14 16 18 20 22 240.00.10.20.30.40.50.60.70.80.91.01.11.2

GrAnt Antennaprn 127, el=250

RM

S[M

SU-p

rn12

7], T

ECU

time, UT

Typical noise figure for Moscow (MSU) station

0 2 4 6 8 10 12 14 16 18 20 22 240.00.10.20.30.40.50.60.70.80.91.01.11.2

RingAnt Antennaprn 127, el=150

Typical noise figure for Irkutsk (ISTP) station

RM

S[IS

TP-p

rn12

7], T

ECU

time, UT0 2 4 6 8 10 12 14 16 18 20 22 24

0.00.10.20.30.40.50.60.70.80.91.01.11.2

Typical noise figure for Irkutsk (ISTP) station

RM

S[IS

TP-p

rn12

8], T

ECU

time, UT

RingAnt Antennaprn 128, el=250

Fig. 2. Typical noise figure for Moscow and Irkutsk stations.

In Fig 3 (bottom left pannel) geostationary TEC observations for IGS station LMMF and Inmarsat 4-F3 are

presented in comparison with Puerto Rico ionosonde (PRJ18) data. Here we can see good correspondence in observed geostationary TEC and foF2. Still note the difference in TEC and foF2 behavior in the evening conditions which we explain by the variations in the thickness and the height of the F2 layer. In Fig 3 (bottom middle pannel) we also present the wavelet spectrum of geostationary TEC variations for the range of periods up to 1h. It is interesting to see ~40min periodical components repeating from day to day with the time of appearance shifted by 3-4 hours from the sunrise and sunset terminator times. This interesting fact should be carefully investigated in further work.

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LMMF geostationary TEC variations

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tive

GEO

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(prn

133)

, TEC

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DOY 2012

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rela

tive

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, TEC

U

Comparison of LMMF and Puerto Rico (PRJ18) Ionosonde

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foF2, MH

z

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1.0Wavelet spectrum for lmmf geoTEC

days, March 2012

perio

d, h

ours

0.0000.025000.050000.075000.10000.12500.15000.17500.20000.22500.25000.27500.30000.32500.35000.37500.40000.42500.45000.47500.5000

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101214161820

Wavelet spectrum for lmmf geoTEC

DOY 2012

perio

d, d

ays

1.000

1.900

2.8003.7004.600

5.500

6.4007.300

8.2009.100

10.00

Fig. 3. Geostationary TEC variations and their wavelet spectrum

In Fig. 3 (upper pannel) we also present long time variations of SBAS TEC from LMMF station and Inmarsat 4-F3 satellite, its LPF filtered values and wavelet spectrum of TEC variations (bottom right pannel). Note that along with distinct diurnal periodic, periods close to 2, 5, 10 and 15 days are observed in the spectrum possibly corresponding to atmospheric planetary waves [6].

Finally let us demonstrate the capabilities of SBAS TEC observations in connection with ionospheric effects of Solar Flares. A chromospheric flare causing a sharp burst in the intensity of X-ray and UV solar radiation results in the height-specific enhancement of electron concentration in the ionosphere of the Earth [7], and in the sudden increase in TEC (SITEC), which can be detected using L1/L5 SBAS signals similar to GPS signals [8,9].

7.5 7.6 7.7 7.8 7.9 8.0 8.1 8.2 8.3 8.4 8.513

14

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16

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19 geo TEC [MSU and prn 127] geo TEC [ISTP and prn127] geo TEC [ISTP and prn 128]

rela

tive

slan

t geo

TEC

, TEC

U

time, UT

1.70x10-3

1.75x10-3

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EU

V Fl

ux [2

0-65

nm],

W/m

2nm

0.0

1.0x10-4

2.0x10-4

X-R

ay F

lux

[0.1

-0.8

nm],

W/m

2

Fig. 4. Solar EUV (purple curve) and X-Ray (black curve) fluxes and geoTEC during X1.7 Solar Flare,

25 October 2013.

Fig. 4 presents the example of processing the SBAS data recorded at MSU and ISTP stations during one of the recent X-class flares of the 24th solar cycle, the X1.7-class flare on October 25, 2013. The maximum of the flare occured at 8:01UT – at 12:01LT and 17:01LT at MSU and ISTP stations correspondingly. SITEC corresponding to variations in the X-ray and UV radiation during the flare is observed at both stations: SITEC ~4 TECU within 10 min is observed at MSU station, SITEC at ISTP station is ~2.5TECU for prn 127 and less than 0.5TECU for prn 128 depending on the elevation angle of the Sun at the satellites ionospheric pierce points. Note also that here low elevation angles of geostationary satellites used in the observations is to some extent an advantage providing longer ray path in ionosphere and thus larger values of SITEC.

4. Conclusion

The results presented above show the possibility to use L1/L5 SBAS signals for continuous monitoring of ionospheric TEC providing an additional instrument for TEC climatology. Provided comparison with ionosondes data shows good agreement having in mind low elevation angles of geostationary satellites already at midlatitudes and necessity to take into account spatial gradients of electron density. We also show the capabilities of SBAS TEC observations in connection with ionospheric effects of Solar Flares.

Intensively growing number of receivers capable to work with L1/L5 SBAS signals and increasing number of dual-frequency satellites in SBAS constellation provide the opportunity in future to incorporate these types of measurements to ionospheric tomography [10,11] and interferometery routines [12], especially at low latitudes.

5. Acknowledgments

The authors are grateful to NGDC and IGS for the data used in presented research and to JAVAD GNSS for

custom receiver firmware. The authors acknowledge the support of the Russian Foundation for Basic Research (grants № 13-05-01122,

14-05-31445, 14-05-00855, 14-05-10069), grants of the President of Russian Federation (MK-2670.2014.5, МК-3771.2013.5) and Lomonosov Moscow State University Program of Development.

6. References

1. M. Mendillo, J.A. Klobuchar, R.B. Fritz, A.V. da Rosa, L. Kersley, K.C. Yeh, B.J. Flaherty, S. Rangaswamy, P.E. Schmid, J.V. Evans, J.P. Schnoedel, D.A. Matsoukas, J.R. Koster, A.R. Webster, P. Chin, Behavior of the

ionospheric F region during the Great Solar Flare of August 7, 1972. Journal of Geophysical Research, 79(4), 665-672, 1974 2. K. Davies, Recent progress in satellite radio beacon studies with particular emphasis on the ATS-6 radio beacon experiment. Space Science Review, 25(4), 357-430, 1980 3. Z.T. Katamzi, N.D. Smith, C.N. Mitchell, P. Spalla, M. Materassi, Statistical analysis of travelling ionospheric disturbances using TEC observations from geostationary satellites. Journal of Atmospheric and Solar-Terrestrial Physics, 74, 64-80, 2011 4. W. Dieminger, J. Schodel, G. Schmidt, G. Hartmann, Recording gravity waves by means of geostationary beacon-satellites. Journal of Atmospheric and Terrestrial Physics, 32(9), 1615-1617, 1970 5. V.E. Kunitsyn, G.A. Kurbatov, Yu.V. Yasyukevich and A.M. Padokhin, Investigation of SBAS L1/L5 signals and their application to the ionospheric TEC studies. IEEE Geoscience and Remote Sensing Letters, submited for publication, 2014 6. I. Silberman, Planetary waves in the atmosphere. Journal of Meteorology,11, 27-34, 1954 7. A.P. Mitra, Ionospheric Effects of Solar Flares, Norwell, MA: D. Reidel, 1974 8. E.L. Afraimovich, GPS global detection of the ionospheric response to solar flares. Radio Science, 35(6), 1417-1424, 2000 9. V.E. Kunitsyn and A.M. Padokhin, Determining the intensity of solar flare ionizing radiation from data of the GPS/GLONASS navigation systems. Moscow University Physics Bulletin, 62(5), 334-337, 2007 10. I.A. Nesterov and V.E. Kunitsyn, GNSS radio tomography of the ionosphere: The problem with essentially incomplete data. Advances in Space Research, 47(10), 1789-1803, 2011 11. V.E. Kunitsyn, I.A. Nesterov, A.M. Padokhin and Yu.S. Tumanova, Ionospheric radio tomography based on the GPS/GLONASS navigation systems. Journal of Communications Technology and Electronics, 56(11), 1269-1281, 2011 12. E.L. Afraimovich, K.S. Palamartchouk and N.P. Perevalova, GPS radio interferometry of travelling ionospheric disturbances. Journal of Atmospheric and Terrestrial Physics, 60(12), 1205-1223, 1998