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Study of space weather effects using GPS
C. D. Reddy
Indian Institute of Geomagnetism,
Colaba (P.O), Mumbai, 400005
Email: cdreddy@iig.iigm.res.in
C. D. Reddy
Indian Institute of Geomagnetism,
Colaba (P.O), Mumbai, 400005
Email: cdreddy@iig.iigm.res.in
Abstract
Extensive research in studying the influence of the propagation media viz troposphere, ionosphere, plasmasphere on GPS signals led to effective use of GPS for atmospheric research and thus contributing to global space weather monitoring. Ionosphere is the ionized part of the upper atmosphere between 70 and 1000 km altitude and the Ionospheric electron content introduces delay in GPS signal propagation. Irregularities in the ionosphere due to space weather events caused by solar flares and coronal mass ejection can scatter trans - ionospheric radio signals producing fluctuations in both amplitude and phase and GPS cycle slips disrupting satellite communications and navigation. This paper discusses some space weather effects on GPS signal propagation, Total Electron Content (TEC) variation and ionospheric scintillations, based on GPS data collected at Indian low latitude region during severe magnetic storms.
Introducing GPS technology
The Global Positioning System (GPS) is a space based navigation system, consisting of a constellation of 24 satellites, in six orbital planes with 55° inclination to the equator. The satellites are placed at a height of about 20,200 km with 12 hours orbital period and operated by the United States Department of Defense (DOD) for accurate determination of position, velocity and time. All the GPS satellites are controlled by system tracking stations, ground antennae and the master control station.
In each satellite two rubidium and two cesium atomic clocks with stability 1013 to 1014 are used to derive the fundamental frequency fo = 10.23 MHz. The GPS signals are transmitted at two L-Band coherent frequencies, designated L1 (154 fo = 1575.42 MHz) and L2 (120 fo =1227.6 MHz), which are derived from the fundamental frequency (fo). Two codes are used, one of which is called C/A (coarse acquisition code, fo /10) and the other is called P (precise, fo) code. As the rate of P code is 10 times the rate of C/A code, its precision is 10 times better than C/A code. The L1 and L2 are modulated by Pseudo Random Noise (PRN) code, (each satellite is identified by this code) and transmitted after biphase modulation with the carrier.
The distance to GPS satellite is estimated by measuring the time a radio signal takes to reach us from the satellite. This is accomplished by cross -correlation of pseudo-random code generated by the satellite and the receiver. The distances from receiver to satellite measured in this way are called code pseudo ranges. Minimum four satellites are required for estimating the coordinates of a point on the Earth's surface. The position accuracy that can be estimated this way depends on our ability to account for various error sources (Reddy, 2001). The textbooks, such as Seeber (1993), Hofmann et al. (1994), Leick (1995), Parkinson and Spilker (1996) provide very good reference on this subject.
While the use of the GPS is extensive in defense, navigation and surveying applications, it is being used in geo-science, ionospheric & atmospheric studies, global climate changes, observing polar motion & earth rotation rate, mapping the gravity field, detecting seismo ionospheirc effects, transport and communications, environment management, for accurate time and frequency etc.
The space weather
The term "Space weather" refers to the conditions on the Sun and in the solar wind, magnetosphere, ionosphere and thermosphere that affect the performance and the reliability of technological systems on Earth and in space. Space Weather processes can include changes in the interplanetary magnetic field, coronal mass ejection from the sun, and disturbances in the Earth’s magnetic field. With the increased sunspot activity still going on, interest in the possibility of severe space weather exits. Many organizations around the world are now actively engaged in monitoring, modeling and predicting the space weather. Violent eruption in the sun’s outer atmosphere on November 8, 2000, spewed billions of tons of charged particles towards the earth and caused severe turbulence in the space around the earth (James, 2001). One of the consequences of severe space weather is often the disruption to satellite communications and even disruption of power grid on the Earth. GPS navigation accuracy is greatly affected by the space weather, as it is particularly sensitive to changes in the electron density in the ionosphere and described in the following section. The ionosphere under the purview of the GPS studies is shown in Fig.1.
Fig.1. The Earth ’s ionospheric environment within the purview of GPS measurements
Ionosphere and its estimation
The UV radiation from sun ionizes a fraction of Earth’s neutral atmosphere. This ionized environment, which envelops the Earth at the height of 70-1000 km and forms an interface between the atmosphere and space is called ionosphere. At mid-latitudes, the electron density, temperature and magnetic field strength are 105 per cc, 103 K and 104. nT respectively (Baumjohann and Treumann, 1977).
The ionosphere is divided into D (50-90 km), E (90-140km ) and F ( 140-600 km) layers based on the electron density and ion composition, and their variability. The electron density is maximum at F layer and the density profile with height is shown in Fig.2 (in red color). During daytime the F layer bifurcates in F1 and F2 layers. During night, the D and F1 layers vanish and the E region becomes much weaker. Beyond 1000 Km, the ionosphere thinning into the plasmasphere (or protono- sphere), magnetosphere and subsequently the inter-planetary plasma (Fig 1). Study of the ionospheric behavior is of scientific interest as it can facilitate improving ionospheric communication. Some of the traditional methods such as Incoherent Scatter Radars (also called Thomson-Scattering-Sounding) and Ionosondes (ionospheric sounder) and satellite based techniques such as Faraday rotation and GPS are used for ionospheric estimation.
As the ionosphere is dispersive in nature, the delay in the GPS signals is proportional to the inverse of the squared frequency and directly proportional to the refractive index of Total Electron Content (TEC) i.e. the free electrons in a column of 1 m2 cross sectional area centered on the signal path (Fig. 2). The TEC in turn depends on the geographic latitude, longitude, local time, season, geomagnetic activity and viewing direction. By forming the linear combination of the GPS measurement on L1 and L2 frequencies, the TEC can be estimated, either by using GPS carrier phase or pseudo-ranges observables. The TEC estimated by the carrier phase values will be less noisy than that from pseudo-ranges, but biased by the carrier phase integer ambiguity. The TEC from pseudo-ranges is absolute and used to fix the absolute level of the carrier phase TEC (called phase leveling). Some of these aspects have been reviewed by Klobuchar (1991). And the methods used to obtain the TEC are given by (Lanyi and Roth, 1988).
Fig. 2 The ionosphere is represented as thin shell. The variation of electron
density shown in red color and the peak value represent the F layer
(show as yellow line) (Fedrizzi et al, 2002)
Effects of ionosphere
Irregularly structured ionosphere (i.e. inhomogenities in refractive index) can cause fluctuations (due to refraction effects) on the radio signal that is passing through it. These fluctuations are called ionospheric scintillations. Low latitude region is suitable for studying these scintillations; we analyzed the GPS data collected on Nov 24, 2001 at Tirunelveli (Lat. 8o 37’ N, Long. 77o 49’ E) located in south India. The data was collected at 30 sec sampling interval using Trimble 4000 SSI duel frequency receiver. The x-axis in the Fig.3 shows number of epochs and the entire axis represents 24 hours duration with starting time 00 UT. On this day, an intense geomagnetic storm with Ap index 104 occurred. Rate of TEC (ROT) is often used to describe the ionospheric scintillations (Bhattacharyya et al, 1999).
In Fig 3 ROT is plotted for each satellite in view (shown in different colors). The ROT values are relative with its level fixing at zero (at 0000 UT). From the Fig 3, it is seen that the scintillation activity started around 1400 UT and lasted for about 8 hours. The pattern of scintillations is characterized by a central minimum and oscillatory amplitude fluctuations on both the side of the minimum. This type of scintillations is caused by a mechanism intrinsically deferent with that of random scintillations (Hajkowica, 1994). It is clear from the Fig.3 that the maximum number of cycle slips occurred during the time of intense scintillation activity. Cycle slip is indication of loss of lock to the satellite and is represented as vertical line extended upwards in the Fig 3.
Fig. 3 Rate of TEC variations on Nov 24, 2001 at Tirunelveli.
GPS cycle slips are seen as the vertical lines extended to the top of the figure.
Conclusions
The radio signals are affected by space weather. The ionosphere imposes the most detrimental effects on the radio signal passing through it. Dispersive nature of the ionosphere and use of two frequencies in GPS facilitate estimation of ionospheric TEC which is an important parameter in the study of L-Band communication though the ionosphere. Rate of TEC (ROT) from GPS can illustrate the features of the ionospheric scintillations. Highly disturbed ionsosphere can cause cycle slips in GPS data. Very good correlation is found between the ionospheric scintillations and the number of cycle slips in GPS data.
The US Dept. of Defense planning to introduce L3 frequency in GPS, which further enhances the use of GPS in space weather monitoring. Like the weather forecasting from meteorological departments, the space weather forecasting is gaining popularity and many agencies already started space weather forecasting.
References
Extensive research in studying the influence of the propagation media viz troposphere, ionosphere, plasmasphere on GPS signals led to effective use of GPS for atmospheric research and thus contributing to global space weather monitoring. Ionosphere is the ionized part of the upper atmosphere between 70 and 1000 km altitude and the Ionospheric electron content introduces delay in GPS signal propagation. Irregularities in the ionosphere due to space weather events caused by solar flares and coronal mass ejection can scatter trans - ionospheric radio signals producing fluctuations in both amplitude and phase and GPS cycle slips disrupting satellite communications and navigation. This paper discusses some space weather effects on GPS signal propagation, Total Electron Content (TEC) variation and ionospheric scintillations, based on GPS data collected at Indian low latitude region during severe magnetic storms.
Introducing GPS technology
The Global Positioning System (GPS) is a space based navigation system, consisting of a constellation of 24 satellites, in six orbital planes with 55° inclination to the equator. The satellites are placed at a height of about 20,200 km with 12 hours orbital period and operated by the United States Department of Defense (DOD) for accurate determination of position, velocity and time. All the GPS satellites are controlled by system tracking stations, ground antennae and the master control station.
In each satellite two rubidium and two cesium atomic clocks with stability 1013 to 1014 are used to derive the fundamental frequency fo = 10.23 MHz. The GPS signals are transmitted at two L-Band coherent frequencies, designated L1 (154 fo = 1575.42 MHz) and L2 (120 fo =1227.6 MHz), which are derived from the fundamental frequency (fo). Two codes are used, one of which is called C/A (coarse acquisition code, fo /10) and the other is called P (precise, fo) code. As the rate of P code is 10 times the rate of C/A code, its precision is 10 times better than C/A code. The L1 and L2 are modulated by Pseudo Random Noise (PRN) code, (each satellite is identified by this code) and transmitted after biphase modulation with the carrier.
The distance to GPS satellite is estimated by measuring the time a radio signal takes to reach us from the satellite. This is accomplished by cross -correlation of pseudo-random code generated by the satellite and the receiver. The distances from receiver to satellite measured in this way are called code pseudo ranges. Minimum four satellites are required for estimating the coordinates of a point on the Earth's surface. The position accuracy that can be estimated this way depends on our ability to account for various error sources (Reddy, 2001). The textbooks, such as Seeber (1993), Hofmann et al. (1994), Leick (1995), Parkinson and Spilker (1996) provide very good reference on this subject.
While the use of the GPS is extensive in defense, navigation and surveying applications, it is being used in geo-science, ionospheric & atmospheric studies, global climate changes, observing polar motion & earth rotation rate, mapping the gravity field, detecting seismo ionospheirc effects, transport and communications, environment management, for accurate time and frequency etc.
The space weather
The term "Space weather" refers to the conditions on the Sun and in the solar wind, magnetosphere, ionosphere and thermosphere that affect the performance and the reliability of technological systems on Earth and in space. Space Weather processes can include changes in the interplanetary magnetic field, coronal mass ejection from the sun, and disturbances in the Earth’s magnetic field. With the increased sunspot activity still going on, interest in the possibility of severe space weather exits. Many organizations around the world are now actively engaged in monitoring, modeling and predicting the space weather. Violent eruption in the sun’s outer atmosphere on November 8, 2000, spewed billions of tons of charged particles towards the earth and caused severe turbulence in the space around the earth (James, 2001). One of the consequences of severe space weather is often the disruption to satellite communications and even disruption of power grid on the Earth. GPS navigation accuracy is greatly affected by the space weather, as it is particularly sensitive to changes in the electron density in the ionosphere and described in the following section. The ionosphere under the purview of the GPS studies is shown in Fig.1.
Fig.1. The Earth ’s ionospheric environment within the purview of GPS measurements
Ionosphere and its estimation
The UV radiation from sun ionizes a fraction of Earth’s neutral atmosphere. This ionized environment, which envelops the Earth at the height of 70-1000 km and forms an interface between the atmosphere and space is called ionosphere. At mid-latitudes, the electron density, temperature and magnetic field strength are 105 per cc, 103 K and 104. nT respectively (Baumjohann and Treumann, 1977).
The ionosphere is divided into D (50-90 km), E (90-140km ) and F ( 140-600 km) layers based on the electron density and ion composition, and their variability. The electron density is maximum at F layer and the density profile with height is shown in Fig.2 (in red color). During daytime the F layer bifurcates in F1 and F2 layers. During night, the D and F1 layers vanish and the E region becomes much weaker. Beyond 1000 Km, the ionosphere thinning into the plasmasphere (or protono- sphere), magnetosphere and subsequently the inter-planetary plasma (Fig 1). Study of the ionospheric behavior is of scientific interest as it can facilitate improving ionospheric communication. Some of the traditional methods such as Incoherent Scatter Radars (also called Thomson-Scattering-Sounding) and Ionosondes (ionospheric sounder) and satellite based techniques such as Faraday rotation and GPS are used for ionospheric estimation.
As the ionosphere is dispersive in nature, the delay in the GPS signals is proportional to the inverse of the squared frequency and directly proportional to the refractive index of Total Electron Content (TEC) i.e. the free electrons in a column of 1 m2 cross sectional area centered on the signal path (Fig. 2). The TEC in turn depends on the geographic latitude, longitude, local time, season, geomagnetic activity and viewing direction. By forming the linear combination of the GPS measurement on L1 and L2 frequencies, the TEC can be estimated, either by using GPS carrier phase or pseudo-ranges observables. The TEC estimated by the carrier phase values will be less noisy than that from pseudo-ranges, but biased by the carrier phase integer ambiguity. The TEC from pseudo-ranges is absolute and used to fix the absolute level of the carrier phase TEC (called phase leveling). Some of these aspects have been reviewed by Klobuchar (1991). And the methods used to obtain the TEC are given by (Lanyi and Roth, 1988).
Fig. 2 The ionosphere is represented as thin shell. The variation of electron
density shown in red color and the peak value represent the F layer
(show as yellow line) (Fedrizzi et al, 2002)
Effects of ionosphere
Irregularly structured ionosphere (i.e. inhomogenities in refractive index) can cause fluctuations (due to refraction effects) on the radio signal that is passing through it. These fluctuations are called ionospheric scintillations. Low latitude region is suitable for studying these scintillations; we analyzed the GPS data collected on Nov 24, 2001 at Tirunelveli (Lat. 8o 37’ N, Long. 77o 49’ E) located in south India. The data was collected at 30 sec sampling interval using Trimble 4000 SSI duel frequency receiver. The x-axis in the Fig.3 shows number of epochs and the entire axis represents 24 hours duration with starting time 00 UT. On this day, an intense geomagnetic storm with Ap index 104 occurred. Rate of TEC (ROT) is often used to describe the ionospheric scintillations (Bhattacharyya et al, 1999).
In Fig 3 ROT is plotted for each satellite in view (shown in different colors). The ROT values are relative with its level fixing at zero (at 0000 UT). From the Fig 3, it is seen that the scintillation activity started around 1400 UT and lasted for about 8 hours. The pattern of scintillations is characterized by a central minimum and oscillatory amplitude fluctuations on both the side of the minimum. This type of scintillations is caused by a mechanism intrinsically deferent with that of random scintillations (Hajkowica, 1994). It is clear from the Fig.3 that the maximum number of cycle slips occurred during the time of intense scintillation activity. Cycle slip is indication of loss of lock to the satellite and is represented as vertical line extended upwards in the Fig 3.
Fig. 3 Rate of TEC variations on Nov 24, 2001 at Tirunelveli.
GPS cycle slips are seen as the vertical lines extended to the top of the figure.
Conclusions
The radio signals are affected by space weather. The ionosphere imposes the most detrimental effects on the radio signal passing through it. Dispersive nature of the ionosphere and use of two frequencies in GPS facilitate estimation of ionospheric TEC which is an important parameter in the study of L-Band communication though the ionosphere. Rate of TEC (ROT) from GPS can illustrate the features of the ionospheric scintillations. Highly disturbed ionsosphere can cause cycle slips in GPS data. Very good correlation is found between the ionospheric scintillations and the number of cycle slips in GPS data.
The US Dept. of Defense planning to introduce L3 frequency in GPS, which further enhances the use of GPS in space weather monitoring. Like the weather forecasting from meteorological departments, the space weather forecasting is gaining popularity and many agencies already started space weather forecasting.
References
- Bhattacharyya A., T. beach and S. Basu, 1999, GPS studies of TEC variations and equatorial scintillations, 1999 Ionospheric Effects Symposium, Alexandria, Virginia, May 4-6, 1999.
- Baumjohann, W. and Treumann, R.A. 1997. Basic Space Plasma Physics, Imperial College Press, London.
- Brunner, F.K. and Gu, M. 1991 An improved model for the dual frequency ionospheric correction of GPS observations, Manuscripta Geodaetica, v.16, pp. 205-214.
- Fredrizzi, M., Paula, E.R, Kantor, I,J, Langley, R.B, Santos, C. and Komjathy, A., 2002, Mapping the Low-Latitude ionosphere with GPS, GPS world, February 2002, 41-47.
- Gunter Seeber 1993 In: Satellite Geodesy Foundations, Methods, and Applications. Berlin, New York.
- Hajakowicz, L.A., 1994, Multi-station observation of the an ionopsheric quasiperiodic scintillation event, Int. Beacon Satellite Symp, University of Wales, Aberystwyth, UK, 104-107.
- Hofmann-Wellenhop,B., Lichtenegger, H. and Collins, J. 1994 Global Positioning System: Theory and Practice. Springer-Verlag Wien, New York.
- James L. Burch, 2001, The fury of space, Scientific American, April 2001, 72-80.
- Kelley, M.C., 1989. The Earth’s Ionosphere. Plasma Physics and Electrodynamics. International Series, Volume 43, Academic Press, New York.
- Klobuchar,J.A.. 1991 Ionospheric effects on GPS. GPS World, v. 2 (4), pp. 43-51.
- Leick, A. 1995 GPS satellite surveying. John Wiley, New York.
- Lanyi, G.E., 1988, A comparision of mappped and measured total electron content Global Positioning System and beacon satellite observations, Radio Science, 23, 483-492.
- Parkinson, B.W. and Spilder Jr.J.J.,(ed) 1996 Global Positioning System: Theory and Applications. Published by the American Institute of Aeronautics and Astronautics, Inc. Washington, v. I and II.
- Reddy,C.D. 2001. Error contributors and accuracy in GPS measurements. Ind. Geol. Cong., vol. 2, 235-244.