- Email: [email protected]
Ionospheric effects on GPS positioning
Advances in Space Research 38 (2006) 2478–2484 www.elsevier.com/locate/asr
Ionospheric effects on GPS positioning Smita Dubey *, Rashmi Wahi, A.K. Gwal Space Science Laboratory, Department of Physics, Barkatullah University, Bhopal, MP 462026, India Received 1 November 2004; received in revised form 3 July 2005; accepted 8 July 2005
Abstract Ionospheric scintillation results from a single frequency global positioning system (GPS) receiver have been presented in this paper. Ionospheric scintillation is rapid variation in the amplitude and phase of radio signals caused by irregularities in the ionosphere. Ionosphere contains large amplitude variations over spatial scales from few cm to 100s of km. It is observed that VHF–UHF communications as well as automated navigation and precision positioning via GPS are affected by scintillations. Scintillations do not have major effects on mid latitude regions, but low latitude scintillations are the greatest cause of GPS position errors. In the present work, we study the effect of ionospheric scintillations on GPS signal at low latitude station, Chiang Rai, Thailand. The data were analyzed from January 2001 to December 2001. Results show that scintillation has a significant problem at this latitude and position error increases during active scintillation condition, which causes loss of lock on several satellites. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Ionospheric scintillation; Irregularities; GPS
0. Introduction In the absence of the international degradation of the global positioning system (GPS), standard positioning service known as selective ability (SA), the ionosphere represents the largest source of positioning error for the users of the GPS. The main constraint on the use of space based system like GPS arises from the medium of propagation, particularly the ionosphere. Transionospheric signals from GPS are perturbed in two ways: (a) introduction of an error in the estimated range by group delay of the signal and (b) fluctuations in the signal characteristics caused by irregularities in the electron density distribution of the ionosphere. The irregularities cause severe fluctuations known as scintillation in the signal strength. The positioning accuracy capability of GPS has been degraded by the presence of ionospheric scintillation caused by small*
Corresponding author. Tel.: +91 0755 2677722. E-mail address: [email protected] (S. Dubey).
scale irregularities. Since fluctuations present additional stresses to the GPS receiver tracking loop and can induce cycle slips or even complete loss of lock, so this phenomenon becomes a major issue for navigational applications. The scintillation at low latitude is primarily controlled by the generation and growth of irregularities over the magnetic equator. After sunset when the eastward electric field is enhanced, the equatorial F-region irregularities are generated by Rayleigh–Taylor instability mechanism (Dungey, 1956; Basu et al., 1978; Tsunoda, 1981; Kelley, 1989). Ionospheric scintillation, the most significant manifestation of such disturbances, often takes place in equatorial and auroral region. It is well known that ionospheric scintillation has the potential to affect all types of GPS receivers. When a radio wave propagates through a medium containing plasma structures, the signal suffers amplitude and phase fluctuations from VHF to L-band mainly near the geomagnetic equator (Aarons, 1982; Basu et al., 1988). These fluctuations of the radio signals are known as
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.07.030
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
scintillations. The strongest L-band scintillation with signal fades of about 20 dB occurs during solar maximum years, at ±15 dip latitude, i.e., in the equatorial anomaly regions during post sunset period (Aarons, 1982; Basu et al., 1988). Weak or strong levels of scintillation can produce disruptions of the communication and navigation links that use low or high altitude orbiting satellites. The impact of scintillation on GPS navigation has generated a new impetus in view of the increasing reliance on satellite-based positioning systems in critical application (Kinter et al., 2001). Woodman and LaHoz (1976) were the first who detected this plume-like structure in electron density distribution at Jicamarca. These structures give rise to intense scintillations at VHF and UHF (Basu et al., 1977). The plumes development, predominantly showing backscatter irregularities up to 800 km, produce amplitude scintillations. Within ±5 of the magnetic equator 6–7 dB peak-to-peak fluctuations at 1.5 GHz occur (Aarons, 1993). Several workers have studied the formation and dynamics of these ionospheric bubbles and plumes (Aarons et al., 1980; Basu et al., 1983). Radio scintillations due to the presence of moving irregularities in the ionosphere is a major problem in Navigation applications using GPS and in satellite communication, SATCOM, especially at low latitudes, the problem being particularly acute around equatorial anomaly peak region. The resulting amplitude and phase distortions of the wavefront may cause degradation performance in GPS receivers. The decrease in amplitude and the stress due to phase fluctuations may degrade the performance of various tracking loops. Ionospheric scintillation affects positioning, communication, space tracking and surveillance system at low latitude. This paper presents the study of ionospheric scintillation and its effects on GPS positioning.
1. Ionospheric scintillation monitoring and methodology Using an ionospheric scintillation monitor (ISM) single frequency receiver, scintillation activity was monitored at Chiang Rai (lat. 19.57N, lon. 99.52E), Thailand. The ISMs are based on Novatel GPS single frequency (L1) receiver, which has been modified to process raw data, sampled at 50 Hz and calculate various parameters, which characterize the observed scintillation. This receiver is configured to measure amplitude and phase scintillation at the L1=1.57542 GHz from January 2001 to December 2001. The ISMs record processed data automatically at 1-min intervals throughout the day. The measured parameters are the amplitude scintillation index (S4), phase scintillation index (rD /), latitude and longitude in degree. Amplitude scintillation monitoring is traditionally accomplished by monitoring
2479
the index S4. The S4 index is derived from detrended signal intensity of signals received from satellites. Signal intensity is actually received signal power, which is measured in a way that its value does not fluctuate with noise power. Thus, it cannot be based upon signal-tonoise density or ratio. S4 measured at L-band needs to have the effects due to ambient noise being removed, since the ambient noise at the L1 frequency translates to a relatively high S4 at lower frequency VHF and UHP frequencies. The S4 values are normally computed over 60-s intervals. In the ISM, values are stored on a file and displayed for each satellite along with the storage and display of the phase data as described earlier. This is referred to as the Total S4 (or S4T). The normalized S4 index, including the effects of ambient noise, is defined as follows (Van Dierendonck et al., 1993): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 hP 2 i hP i S 4T ¼ . ð1:1Þ hP i2 Unfortunately, the total S4 defined in Eq. (1.1) can have significant values simply due to ambient noise. The amplitude measurements are filtered using a low pass filter (LPF) and the effects of ambient noise removed from the S4T. Since this index would be used in practice by scaling to predict amplitude scintillation at lower frequencies, such as VHF and UHF, any value due to noise at L1 can swamp out low amplitude scintillation that scales to significant levels at VHF and UHF. Thus, it is desirable to remove, as well as one can, the effects of ambient noise. This can be done by estimating the average signal-to-noise density over the entire evaluation interval (60 s), and using that estimate to determine the expected S4 due to ambient noise. This is legitimate since the amplitude scintillation fades do not significantly alter the average signal-to-noise density over a 60-s time interval. Note from Eq. (1.1) that S4 is simply the square root of the normalized variance of signal power. If the signal-to-noise density (S/N) is known, then the predicted S4 due to ambient noise is sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 100 500 S 4N 0 ¼ . ð1:2Þ 1þ S=N 0 19S=N 0 Thus, by replacing the S/N0 with the 60 s, we obtain an estimate of the S4 due to noise S 4N 0 . Subtracting the square of this value from the square of Eq. (1.1) yields the revised value of S4. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðP 2 Þ ðP Þ2 100 500 S4 ¼ 1 þ . ð1:3Þ 2 S=N 0 19S=N 0 ðP Þ When there is no scintillation the value under the radical may go slightly negative. Phase scintillation monitoring is traditionally accomplished by monitoring the standard deviation, rD /, and
2480
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
the power spectrum of detrended carrier phase from signals received from satellites; the capability of recording raw detrended phase data for offline spectral analysis is provided. The rD / are computed (off-line), however, over 1-, 3-, 10-, 30- and 60-s intervals every 60 s. The detrended phase measurements are used to define the spectral parameters strength, T/ 1, and slope, P/ and are given by Eq. (1.4) as a function of the frequency m in rad2/Hz for frequencies greater than 1 Hz, // ðmÞ ¼ T /1 mp ;
ð1:4Þ
where the phase measurements are detrended but by using a high pass filter (HPF). This removes any low frequency effects below the frequency cut-off of 0.1 Hz. The oscillator effects, therefore, should be filtered out, except for any high frequency phase noise. The quality of the oscillator must be such that this unfiltered phase noise is low relative to the desired scintillation monitoring performance. Both the parameters are made available either in raw form or as a corrected S4, for which the effects of ambient noise have been removed.
2. Results To study the evolution of equatorial plasma bubble irregularities, which produce scintillation and its effects on GPS position data, a single frequency GPS receiver is used. Figs. 1(a) and 2(a) show the hourly polar plot of the position path in azimuth-elevation coordinate and scintillation level represented by S4 at 5-min interval for all GPS satellites tracked by ISM on 19 March 2001. Fig. 1(a) shows the polar plot with scintillation intensity from 12:00 to 16:00 UT (19:00–23:00 LT) and Fig. 2(a) displays the plot from 16:00 to 20:00 UT (23:00–03:00). Scintillation intensity is divided into five levels (0–0.1, 0.1–0.3, 0.3–0.5, 0.5–0.7, 0.7–1) and the radius of the circle shows the variation of scintillation intensity. A graph was plotted from the onset of scintillation and after the disappearance of scintillation, i.e., from 12:00 to 20:00 UT (19:00–03:00 LT). The eight polar plots show the dynamics of equatorial ionospheric irregularities and its effects on GPS links. To study the effect of ionospheric scintillation on GPS positioning, Figs. 1(b) and 2(b) show the latitudinal and longitudinal variation in meters, for the time period of 12:00–16:00 UT and 16:00–20:00 UT, respectively. The latitudinal errors are shown in blue, while longitudinal error in green. Fig. 1(a) shows that between 12:00 and 13:00 UT (19:00–20:00 LT) evening hours, only three satellite PRNÕs 5, 9 and 17 experience scintillation up to the level of 0.5. Due to this low scintillation activity, the latitudinal error shows the maximum variation of 9.5 m and longitudinal variation shows 1.78-m error as shown in
Fig. 1(b). From 13:00 to 14:00 UT (20:00–21:00 LT), including the above mentioned three satellites, PRNÕs 18, 23 and 26 show moderate to intense scintillation activity. As seen from 13:00 to 13:30 UT, very low scintillation activity is observed and after that all these satellites experience scintillations up to 0.7. When we compare polar plot to latitudinal and longitudinal error shown in Fig. 1(b), it shows that after 13:30 the latitudinal error reached up to 14 m and longitudinal variation shows the error of 10 m, which is due to the strong scintillation activity. From 14:00 to 15:00 UT (21:00–22:00 LT), normally the time when the irregularities associated patches move eastward the majority of satellites experienced enhanced scintillation activity throughout the period and only one satellite, PRN 17, exhibited no scintillation at all. PRN 18, PRN 23 and PRN 26 show the highest scintillation activity, and the S4 index for these three satellites is 1. The above three satellites are at high elevation angle. So using data from the highest elevation satellite, it was possible to observe that L-band scintillation is not observed during the low altitude irregularity layers observed on range time intensity (RTI) maps during the initial development phase of plumes. Radar plumes are interpreted as a manifestation of large plasma depletions known as ionospheric plasma ‘‘bubbles’’ that originate in the bottom side F-region and may extend over several hundred kilometers in altitude. When the plume develops to higher altitudes, maximum scintillation activities are observed. This behavior is in agreement with the findings of Rodrigues et al. (2004). They also show that when the plumes develop to higher altitudes, stronger radar echoes and simultaneous maximum S4 scintillation index values are observed. During this period, intense scintillation is observed, so the latitudinal error shows the maximum value of 22.153 m and longitudinal error is 14.4825 m. Between 15:00 and 16:00 UT, the bubble moved further eastward and during this period most of the satellites experience moderate scintillation and latitudinal and longitudinal error shows the variation of less than 10 m as shown in Fig. 1(b). Now from 16:00 to 18:00 UT, the scintillation activity decreases and only few satellites experience some level of scintillation as shown in Fig. 2(a). It shows the decaying phase of irregularities. Between 18:00 and 20:00 UT, no satellite scintillates. Fig. 2(b) shows that from 16:00 to 17:00 UT, the longitudinal error presents fluctuation of 3.5 m and longitudinal error of 7.7 m. But after 17:00 UT, the error in latitude and longitude is much lower. It is clear from Figs. 1(a) and 2(a) that moderate to intense scintillations were observed in pre midnight period. Results of Figs. 1(b) and 2(b) show that during the period of strong scintillation activity, the receiver fails to calculate the correct position value.
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
2481
Fig. 1. (a) Polar plot with scintillation intensity from 12:00 to 16:00 UT. (b) Variation of latitudinal and longitudinal error in meters as a function of time.
3. Discussions Scintillation can have a significant impact on the performance of individual tracking loops, the overall navigational performance of a receiver will depend upon a number of factors. Scintillation activity can introduce perturbations into the GPS signal at a level that results in a significant increase in the phase noise and/or pseudorange error observed by the receivers. Severe scintillation can produce perturbations large enough to cause
the carrier tracking loops of a receiver to lose lock, depending upon the extent of the largescale structure of the ionospheric irregularities. Irregularities are most intense at the equatorial anomaly crest and are characterized by a spectrum of scale sizes, which constitute a random diffraction screen to any signal passing through it. At magnetic equator, the plasma instabilities generated in the form of plasma bubbles initially at the bottom side of the F-layer during nighttime develop into scintillation and spread F
2482
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
Fig. 2. (a) Polar plot with scintillation intensity from 16:00 to 18:00 UT. (b) Variation of latitudinal and longitudinal error in meters as a function of time.
producing irregularities in the equatorial and low latitude F-region. It is well known that the E-region electric field is mapped to F-region of the ionosphere at the equator, which interacting with the horizontal component of the earthÕs magnetic field, set up an upward
E · B drift of the plasma. The plasma bubbles rise non-linearly in the F-layer as a result of E · B motion producing plasma irregularities along the magnetic field lines in a wide spectrum of scale sizes on either side of the magnetic equator extending up to 20 and more
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
under the influence of eastward electric fields. This poses a serious problem to the design and operation of satellite radio system in the 100 MHz to several GHz band. To understand the characteristics of these equatorial ionospheric irregularities, many techniques have been used. Many authors have studied and established that at least during the initial and the development phase of an irregularity event, meter to kilometer scale size of irregularities coexist. In the late phase, during post midnight hours the overall strength of the irregularities is eroded, the smaller scale irregularities decaying earlier (Basu et al., 1978, 1980). Available literature concerning amplitude scintillation in the equatorial region has shown that during both sunspot maximum and sunspot minimum, plumes of irregularities develop at the equator. Basu et al. (1977) showed that plumes-like structure give rise to intense scintillation at VHF and UHF. Results shows that moderate to intense scintillation were observed in pre midnight period. In shaping and the development of irregularities, the equatorial electric field plays a dominant role. By using the data from a network of low latitude station in India, Pathan et al. (1991) calculate drifts of plasma cloud on the basis of these time shifts, indicating an eastward drift throughout the night and decreases from 200 to 100 m/s during the course of the night. In this study, we observed that the receiver fails to calculate the correct position value during the scintillation condition. Results show that equatorial scintillation has greater impact on receiver tracking performance during the period of strong scintillation activity, especially during the local time period of 20:00–23:00 UT. At this time, when intense scintillation activity is observed, the position value (latitude and longitude) shows the variation of 22.153-m latitudinal errors and longitudinal error is 14.4825 m, but when scintillation activity is absent there are almost no fluctuations in position values. Equatorial scintillation has greater impact on receiver tracking performance than the high latitude scintillation. In the local time sector 2000–2300, percentage of corrupt observations often exceeds 40% for station located near the equatorial anomaly peak. Receiver tracking performance in the equatorial region exhibits clear seasonal variations with peaks in the winter month and a dependence on the solar cycle. Based on the observed static and a solar maximum in mid 2000, it is anticipated that the high levels of degraded tracking performance will continue in the equatorial region until early 2002 (Skone and Knudsen, 2000). Bandhyopadhyay et al. (1997) also reported some results of degradation in the position accuracy during periods of scintillation activity. Groves et al. (2000) test the performance of several GPS receivers under potentially severe scintillation conditions. Results show that during the active scintillation condition, one of the receiver experienced navigation outages ranging from 20- to 90-min
2483
duration. Such outages may be more routine than anomalous at low latitudes during solar maximum. Thus, the study of occurrence of scintillation and its effects on GPS receiver performance at low latitude stations during high and low solar activity is a growing concept.
Acknowledgements The authors acknowledge the financial support from Indo Russian programme (ILTP) from Department of Science and Technology, Government of India, New Delhi. (No. NP-29/JC-11). We are thankful to Dr. Pian Totarong and Sub. Lt. Ekkaphon Mingkhwan, Military Research and Development Centre, Bangkok, Thailand, for providing us the GPS data and for their constant and needful help in data analysis.
References Aarons, J. Global morphology of ionospheric scintillations. Proc. IEEE 70 (4), 360, 1982. Aarons, J., Mullen, J.P., Koster, J.R., et al. Seasonal and geomagnetic control of equatorial scintillations in two longitude sectors. J. Atmos. Terr. Phys. 42, 681, 1980. Aarons, J. The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence. Space Sci. Rev. 63, 209, 1993. Bandhyopadhyay, T., Guha, A., Dasgupta, A., Banerjee, P., Bose, A. Degradation of navigational accuracy with global positioning system during periods of scintillation at equatorial latitudes. Electron. Lett. 33, 1010, 1997. Basu, S., Basu, S., Aarons, J., McClure, J.P., Cousins, M.D. On the coexistence of kilometers and meter-scale irregularities in the nighttime equatorial F region. Geophys. Res. 83, 4219, 1978. Basu, S., Aarons, J., McClure, J.P., LaHoz, C., Bushby, A., Woodman, R.F. Preliminary comparison of VHF radar maps of F-region irregularities with scintillation in the equatorial region. J. Atmos. Terr. Phys. 39, 1251, 1977. Basu, S., McClure, J.P., Basu, S., Hanson, W.B., Aarons, J. Coordinated study of equatorial scintillation and in situ region irregularities. J. Geophys. Res. 85, 5119, 1980. Basu, S., Basu, S., McClure, J.P., Hanson, W.B. High resolution topside data of electron densities and/GHz scintillations in the equatorial region. J. Geophys. Res. 88, 403, 1983. Basu, S., Mackenzie, E., Basu, S. Ionospheric constraints on VHF/ UHF communications links during solar maximum and minimum periods. Radio Sci. 23, 363, 1988. Dungey, J.W. Convective diffusion in the equatorial F-region. J. Atmos. Terr. Phys. 9, 304, 1956. Groves, K.M., Basu, S., Quinn, J.M., Pedersen, T.R., Falinski, K. A comparision of GPS performance in a scintillation environment at Ascension Island, in: Proceedings of the ION GPS 2000, September 2000, Salt Lake City, UT, 2000. Kelley, M.C. The EarthÕs Ionosphere, Plasma Physics and Electrodynamics. Academic Press Inc., San Diego, CA, 1989, p. 121. Kinter, P.M., Kil, H., Beach, T.L., dePaula, E.R. Fading time scales associated with GPS signals and potential consequences. Radio Sci. 36 (4), 731, 2001. Pathan, B.M., Koparkar, P.V., Rastogi, R.G., et al. Dynamics of ionospheric irregularities producing VHF radio wave scintillations at low latitude. Ann. Geophys. 9, 126, 1991.
2484
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
Rodrigues, F.S., dePaula, E.R., Abdu, M.A., Jardim, A.C., Iyer, K.N., Kintner, P.M., Hysell, D.L. Equatorial spread F irregularity characteristics over Sao Luis, Brazil, using VHF radar and GPS scintillations. Radio Sci. 39, RS1531, 2004. Skone, S., Knudsen, K. Impacts of Ionospheric Scintillation on SBAS Performance, in: Proceedings of the ION GPS 2000, 19–22 September 2000, Salt Lake city, UT, 2000.
Tsunoda, R.T. Time evolution and dynamics of equatorial backscatter plumes.1. Growth phase. J. Geophys. Res. 86, 139, 1981. Van Dierendonck, A.J., Klobuchar, J., QuyenHua. Ionospheric scintillation monitoring using commercial single frequency C/A code receivers, in: Proceedings of ION GPS-93, Salt Lake City UT, 1993. Woodman, R.F., LaHoz, C. Radar observations of F-region equatorial irregularities. J. Geophys. Res. 81, 5447, 1976.
Ionospheric effects on GPS positioning Smita Dubey *, Rashmi Wahi, A.K. Gwal Space Science Laboratory, Department of Physics, Barkatullah University, Bhopal, MP 462026, India Received 1 November 2004; received in revised form 3 July 2005; accepted 8 July 2005
Abstract Ionospheric scintillation results from a single frequency global positioning system (GPS) receiver have been presented in this paper. Ionospheric scintillation is rapid variation in the amplitude and phase of radio signals caused by irregularities in the ionosphere. Ionosphere contains large amplitude variations over spatial scales from few cm to 100s of km. It is observed that VHF–UHF communications as well as automated navigation and precision positioning via GPS are affected by scintillations. Scintillations do not have major effects on mid latitude regions, but low latitude scintillations are the greatest cause of GPS position errors. In the present work, we study the effect of ionospheric scintillations on GPS signal at low latitude station, Chiang Rai, Thailand. The data were analyzed from January 2001 to December 2001. Results show that scintillation has a significant problem at this latitude and position error increases during active scintillation condition, which causes loss of lock on several satellites. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Ionospheric scintillation; Irregularities; GPS
0. Introduction In the absence of the international degradation of the global positioning system (GPS), standard positioning service known as selective ability (SA), the ionosphere represents the largest source of positioning error for the users of the GPS. The main constraint on the use of space based system like GPS arises from the medium of propagation, particularly the ionosphere. Transionospheric signals from GPS are perturbed in two ways: (a) introduction of an error in the estimated range by group delay of the signal and (b) fluctuations in the signal characteristics caused by irregularities in the electron density distribution of the ionosphere. The irregularities cause severe fluctuations known as scintillation in the signal strength. The positioning accuracy capability of GPS has been degraded by the presence of ionospheric scintillation caused by small*
Corresponding author. Tel.: +91 0755 2677722. E-mail address: [email protected] (S. Dubey).
scale irregularities. Since fluctuations present additional stresses to the GPS receiver tracking loop and can induce cycle slips or even complete loss of lock, so this phenomenon becomes a major issue for navigational applications. The scintillation at low latitude is primarily controlled by the generation and growth of irregularities over the magnetic equator. After sunset when the eastward electric field is enhanced, the equatorial F-region irregularities are generated by Rayleigh–Taylor instability mechanism (Dungey, 1956; Basu et al., 1978; Tsunoda, 1981; Kelley, 1989). Ionospheric scintillation, the most significant manifestation of such disturbances, often takes place in equatorial and auroral region. It is well known that ionospheric scintillation has the potential to affect all types of GPS receivers. When a radio wave propagates through a medium containing plasma structures, the signal suffers amplitude and phase fluctuations from VHF to L-band mainly near the geomagnetic equator (Aarons, 1982; Basu et al., 1988). These fluctuations of the radio signals are known as
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.07.030
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
scintillations. The strongest L-band scintillation with signal fades of about 20 dB occurs during solar maximum years, at ±15 dip latitude, i.e., in the equatorial anomaly regions during post sunset period (Aarons, 1982; Basu et al., 1988). Weak or strong levels of scintillation can produce disruptions of the communication and navigation links that use low or high altitude orbiting satellites. The impact of scintillation on GPS navigation has generated a new impetus in view of the increasing reliance on satellite-based positioning systems in critical application (Kinter et al., 2001). Woodman and LaHoz (1976) were the first who detected this plume-like structure in electron density distribution at Jicamarca. These structures give rise to intense scintillations at VHF and UHF (Basu et al., 1977). The plumes development, predominantly showing backscatter irregularities up to 800 km, produce amplitude scintillations. Within ±5 of the magnetic equator 6–7 dB peak-to-peak fluctuations at 1.5 GHz occur (Aarons, 1993). Several workers have studied the formation and dynamics of these ionospheric bubbles and plumes (Aarons et al., 1980; Basu et al., 1983). Radio scintillations due to the presence of moving irregularities in the ionosphere is a major problem in Navigation applications using GPS and in satellite communication, SATCOM, especially at low latitudes, the problem being particularly acute around equatorial anomaly peak region. The resulting amplitude and phase distortions of the wavefront may cause degradation performance in GPS receivers. The decrease in amplitude and the stress due to phase fluctuations may degrade the performance of various tracking loops. Ionospheric scintillation affects positioning, communication, space tracking and surveillance system at low latitude. This paper presents the study of ionospheric scintillation and its effects on GPS positioning.
1. Ionospheric scintillation monitoring and methodology Using an ionospheric scintillation monitor (ISM) single frequency receiver, scintillation activity was monitored at Chiang Rai (lat. 19.57N, lon. 99.52E), Thailand. The ISMs are based on Novatel GPS single frequency (L1) receiver, which has been modified to process raw data, sampled at 50 Hz and calculate various parameters, which characterize the observed scintillation. This receiver is configured to measure amplitude and phase scintillation at the L1=1.57542 GHz from January 2001 to December 2001. The ISMs record processed data automatically at 1-min intervals throughout the day. The measured parameters are the amplitude scintillation index (S4), phase scintillation index (rD /), latitude and longitude in degree. Amplitude scintillation monitoring is traditionally accomplished by monitoring
2479
the index S4. The S4 index is derived from detrended signal intensity of signals received from satellites. Signal intensity is actually received signal power, which is measured in a way that its value does not fluctuate with noise power. Thus, it cannot be based upon signal-tonoise density or ratio. S4 measured at L-band needs to have the effects due to ambient noise being removed, since the ambient noise at the L1 frequency translates to a relatively high S4 at lower frequency VHF and UHP frequencies. The S4 values are normally computed over 60-s intervals. In the ISM, values are stored on a file and displayed for each satellite along with the storage and display of the phase data as described earlier. This is referred to as the Total S4 (or S4T). The normalized S4 index, including the effects of ambient noise, is defined as follows (Van Dierendonck et al., 1993): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 hP 2 i hP i S 4T ¼ . ð1:1Þ hP i2 Unfortunately, the total S4 defined in Eq. (1.1) can have significant values simply due to ambient noise. The amplitude measurements are filtered using a low pass filter (LPF) and the effects of ambient noise removed from the S4T. Since this index would be used in practice by scaling to predict amplitude scintillation at lower frequencies, such as VHF and UHF, any value due to noise at L1 can swamp out low amplitude scintillation that scales to significant levels at VHF and UHF. Thus, it is desirable to remove, as well as one can, the effects of ambient noise. This can be done by estimating the average signal-to-noise density over the entire evaluation interval (60 s), and using that estimate to determine the expected S4 due to ambient noise. This is legitimate since the amplitude scintillation fades do not significantly alter the average signal-to-noise density over a 60-s time interval. Note from Eq. (1.1) that S4 is simply the square root of the normalized variance of signal power. If the signal-to-noise density (S/N) is known, then the predicted S4 due to ambient noise is sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 100 500 S 4N 0 ¼ . ð1:2Þ 1þ S=N 0 19S=N 0 Thus, by replacing the S/N0 with the 60 s, we obtain an estimate of the S4 due to noise S 4N 0 . Subtracting the square of this value from the square of Eq. (1.1) yields the revised value of S4. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðP 2 Þ ðP Þ2 100 500 S4 ¼ 1 þ . ð1:3Þ 2 S=N 0 19S=N 0 ðP Þ When there is no scintillation the value under the radical may go slightly negative. Phase scintillation monitoring is traditionally accomplished by monitoring the standard deviation, rD /, and
2480
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
the power spectrum of detrended carrier phase from signals received from satellites; the capability of recording raw detrended phase data for offline spectral analysis is provided. The rD / are computed (off-line), however, over 1-, 3-, 10-, 30- and 60-s intervals every 60 s. The detrended phase measurements are used to define the spectral parameters strength, T/ 1, and slope, P/ and are given by Eq. (1.4) as a function of the frequency m in rad2/Hz for frequencies greater than 1 Hz, // ðmÞ ¼ T /1 mp ;
ð1:4Þ
where the phase measurements are detrended but by using a high pass filter (HPF). This removes any low frequency effects below the frequency cut-off of 0.1 Hz. The oscillator effects, therefore, should be filtered out, except for any high frequency phase noise. The quality of the oscillator must be such that this unfiltered phase noise is low relative to the desired scintillation monitoring performance. Both the parameters are made available either in raw form or as a corrected S4, for which the effects of ambient noise have been removed.
2. Results To study the evolution of equatorial plasma bubble irregularities, which produce scintillation and its effects on GPS position data, a single frequency GPS receiver is used. Figs. 1(a) and 2(a) show the hourly polar plot of the position path in azimuth-elevation coordinate and scintillation level represented by S4 at 5-min interval for all GPS satellites tracked by ISM on 19 March 2001. Fig. 1(a) shows the polar plot with scintillation intensity from 12:00 to 16:00 UT (19:00–23:00 LT) and Fig. 2(a) displays the plot from 16:00 to 20:00 UT (23:00–03:00). Scintillation intensity is divided into five levels (0–0.1, 0.1–0.3, 0.3–0.5, 0.5–0.7, 0.7–1) and the radius of the circle shows the variation of scintillation intensity. A graph was plotted from the onset of scintillation and after the disappearance of scintillation, i.e., from 12:00 to 20:00 UT (19:00–03:00 LT). The eight polar plots show the dynamics of equatorial ionospheric irregularities and its effects on GPS links. To study the effect of ionospheric scintillation on GPS positioning, Figs. 1(b) and 2(b) show the latitudinal and longitudinal variation in meters, for the time period of 12:00–16:00 UT and 16:00–20:00 UT, respectively. The latitudinal errors are shown in blue, while longitudinal error in green. Fig. 1(a) shows that between 12:00 and 13:00 UT (19:00–20:00 LT) evening hours, only three satellite PRNÕs 5, 9 and 17 experience scintillation up to the level of 0.5. Due to this low scintillation activity, the latitudinal error shows the maximum variation of 9.5 m and longitudinal variation shows 1.78-m error as shown in
Fig. 1(b). From 13:00 to 14:00 UT (20:00–21:00 LT), including the above mentioned three satellites, PRNÕs 18, 23 and 26 show moderate to intense scintillation activity. As seen from 13:00 to 13:30 UT, very low scintillation activity is observed and after that all these satellites experience scintillations up to 0.7. When we compare polar plot to latitudinal and longitudinal error shown in Fig. 1(b), it shows that after 13:30 the latitudinal error reached up to 14 m and longitudinal variation shows the error of 10 m, which is due to the strong scintillation activity. From 14:00 to 15:00 UT (21:00–22:00 LT), normally the time when the irregularities associated patches move eastward the majority of satellites experienced enhanced scintillation activity throughout the period and only one satellite, PRN 17, exhibited no scintillation at all. PRN 18, PRN 23 and PRN 26 show the highest scintillation activity, and the S4 index for these three satellites is 1. The above three satellites are at high elevation angle. So using data from the highest elevation satellite, it was possible to observe that L-band scintillation is not observed during the low altitude irregularity layers observed on range time intensity (RTI) maps during the initial development phase of plumes. Radar plumes are interpreted as a manifestation of large plasma depletions known as ionospheric plasma ‘‘bubbles’’ that originate in the bottom side F-region and may extend over several hundred kilometers in altitude. When the plume develops to higher altitudes, maximum scintillation activities are observed. This behavior is in agreement with the findings of Rodrigues et al. (2004). They also show that when the plumes develop to higher altitudes, stronger radar echoes and simultaneous maximum S4 scintillation index values are observed. During this period, intense scintillation is observed, so the latitudinal error shows the maximum value of 22.153 m and longitudinal error is 14.4825 m. Between 15:00 and 16:00 UT, the bubble moved further eastward and during this period most of the satellites experience moderate scintillation and latitudinal and longitudinal error shows the variation of less than 10 m as shown in Fig. 1(b). Now from 16:00 to 18:00 UT, the scintillation activity decreases and only few satellites experience some level of scintillation as shown in Fig. 2(a). It shows the decaying phase of irregularities. Between 18:00 and 20:00 UT, no satellite scintillates. Fig. 2(b) shows that from 16:00 to 17:00 UT, the longitudinal error presents fluctuation of 3.5 m and longitudinal error of 7.7 m. But after 17:00 UT, the error in latitude and longitude is much lower. It is clear from Figs. 1(a) and 2(a) that moderate to intense scintillations were observed in pre midnight period. Results of Figs. 1(b) and 2(b) show that during the period of strong scintillation activity, the receiver fails to calculate the correct position value.
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
2481
Fig. 1. (a) Polar plot with scintillation intensity from 12:00 to 16:00 UT. (b) Variation of latitudinal and longitudinal error in meters as a function of time.
3. Discussions Scintillation can have a significant impact on the performance of individual tracking loops, the overall navigational performance of a receiver will depend upon a number of factors. Scintillation activity can introduce perturbations into the GPS signal at a level that results in a significant increase in the phase noise and/or pseudorange error observed by the receivers. Severe scintillation can produce perturbations large enough to cause
the carrier tracking loops of a receiver to lose lock, depending upon the extent of the largescale structure of the ionospheric irregularities. Irregularities are most intense at the equatorial anomaly crest and are characterized by a spectrum of scale sizes, which constitute a random diffraction screen to any signal passing through it. At magnetic equator, the plasma instabilities generated in the form of plasma bubbles initially at the bottom side of the F-layer during nighttime develop into scintillation and spread F
2482
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
Fig. 2. (a) Polar plot with scintillation intensity from 16:00 to 18:00 UT. (b) Variation of latitudinal and longitudinal error in meters as a function of time.
producing irregularities in the equatorial and low latitude F-region. It is well known that the E-region electric field is mapped to F-region of the ionosphere at the equator, which interacting with the horizontal component of the earthÕs magnetic field, set up an upward
E · B drift of the plasma. The plasma bubbles rise non-linearly in the F-layer as a result of E · B motion producing plasma irregularities along the magnetic field lines in a wide spectrum of scale sizes on either side of the magnetic equator extending up to 20 and more
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
under the influence of eastward electric fields. This poses a serious problem to the design and operation of satellite radio system in the 100 MHz to several GHz band. To understand the characteristics of these equatorial ionospheric irregularities, many techniques have been used. Many authors have studied and established that at least during the initial and the development phase of an irregularity event, meter to kilometer scale size of irregularities coexist. In the late phase, during post midnight hours the overall strength of the irregularities is eroded, the smaller scale irregularities decaying earlier (Basu et al., 1978, 1980). Available literature concerning amplitude scintillation in the equatorial region has shown that during both sunspot maximum and sunspot minimum, plumes of irregularities develop at the equator. Basu et al. (1977) showed that plumes-like structure give rise to intense scintillation at VHF and UHF. Results shows that moderate to intense scintillation were observed in pre midnight period. In shaping and the development of irregularities, the equatorial electric field plays a dominant role. By using the data from a network of low latitude station in India, Pathan et al. (1991) calculate drifts of plasma cloud on the basis of these time shifts, indicating an eastward drift throughout the night and decreases from 200 to 100 m/s during the course of the night. In this study, we observed that the receiver fails to calculate the correct position value during the scintillation condition. Results show that equatorial scintillation has greater impact on receiver tracking performance during the period of strong scintillation activity, especially during the local time period of 20:00–23:00 UT. At this time, when intense scintillation activity is observed, the position value (latitude and longitude) shows the variation of 22.153-m latitudinal errors and longitudinal error is 14.4825 m, but when scintillation activity is absent there are almost no fluctuations in position values. Equatorial scintillation has greater impact on receiver tracking performance than the high latitude scintillation. In the local time sector 2000–2300, percentage of corrupt observations often exceeds 40% for station located near the equatorial anomaly peak. Receiver tracking performance in the equatorial region exhibits clear seasonal variations with peaks in the winter month and a dependence on the solar cycle. Based on the observed static and a solar maximum in mid 2000, it is anticipated that the high levels of degraded tracking performance will continue in the equatorial region until early 2002 (Skone and Knudsen, 2000). Bandhyopadhyay et al. (1997) also reported some results of degradation in the position accuracy during periods of scintillation activity. Groves et al. (2000) test the performance of several GPS receivers under potentially severe scintillation conditions. Results show that during the active scintillation condition, one of the receiver experienced navigation outages ranging from 20- to 90-min
2483
duration. Such outages may be more routine than anomalous at low latitudes during solar maximum. Thus, the study of occurrence of scintillation and its effects on GPS receiver performance at low latitude stations during high and low solar activity is a growing concept.
Acknowledgements The authors acknowledge the financial support from Indo Russian programme (ILTP) from Department of Science and Technology, Government of India, New Delhi. (No. NP-29/JC-11). We are thankful to Dr. Pian Totarong and Sub. Lt. Ekkaphon Mingkhwan, Military Research and Development Centre, Bangkok, Thailand, for providing us the GPS data and for their constant and needful help in data analysis.
References Aarons, J. Global morphology of ionospheric scintillations. Proc. IEEE 70 (4), 360, 1982. Aarons, J., Mullen, J.P., Koster, J.R., et al. Seasonal and geomagnetic control of equatorial scintillations in two longitude sectors. J. Atmos. Terr. Phys. 42, 681, 1980. Aarons, J. The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence. Space Sci. Rev. 63, 209, 1993. Bandhyopadhyay, T., Guha, A., Dasgupta, A., Banerjee, P., Bose, A. Degradation of navigational accuracy with global positioning system during periods of scintillation at equatorial latitudes. Electron. Lett. 33, 1010, 1997. Basu, S., Basu, S., Aarons, J., McClure, J.P., Cousins, M.D. On the coexistence of kilometers and meter-scale irregularities in the nighttime equatorial F region. Geophys. Res. 83, 4219, 1978. Basu, S., Aarons, J., McClure, J.P., LaHoz, C., Bushby, A., Woodman, R.F. Preliminary comparison of VHF radar maps of F-region irregularities with scintillation in the equatorial region. J. Atmos. Terr. Phys. 39, 1251, 1977. Basu, S., McClure, J.P., Basu, S., Hanson, W.B., Aarons, J. Coordinated study of equatorial scintillation and in situ region irregularities. J. Geophys. Res. 85, 5119, 1980. Basu, S., Basu, S., McClure, J.P., Hanson, W.B. High resolution topside data of electron densities and/GHz scintillations in the equatorial region. J. Geophys. Res. 88, 403, 1983. Basu, S., Mackenzie, E., Basu, S. Ionospheric constraints on VHF/ UHF communications links during solar maximum and minimum periods. Radio Sci. 23, 363, 1988. Dungey, J.W. Convective diffusion in the equatorial F-region. J. Atmos. Terr. Phys. 9, 304, 1956. Groves, K.M., Basu, S., Quinn, J.M., Pedersen, T.R., Falinski, K. A comparision of GPS performance in a scintillation environment at Ascension Island, in: Proceedings of the ION GPS 2000, September 2000, Salt Lake City, UT, 2000. Kelley, M.C. The EarthÕs Ionosphere, Plasma Physics and Electrodynamics. Academic Press Inc., San Diego, CA, 1989, p. 121. Kinter, P.M., Kil, H., Beach, T.L., dePaula, E.R. Fading time scales associated with GPS signals and potential consequences. Radio Sci. 36 (4), 731, 2001. Pathan, B.M., Koparkar, P.V., Rastogi, R.G., et al. Dynamics of ionospheric irregularities producing VHF radio wave scintillations at low latitude. Ann. Geophys. 9, 126, 1991.
2484
S. Dubey et al. / Advances in Space Research 38 (2006) 2478–2484
Rodrigues, F.S., dePaula, E.R., Abdu, M.A., Jardim, A.C., Iyer, K.N., Kintner, P.M., Hysell, D.L. Equatorial spread F irregularity characteristics over Sao Luis, Brazil, using VHF radar and GPS scintillations. Radio Sci. 39, RS1531, 2004. Skone, S., Knudsen, K. Impacts of Ionospheric Scintillation on SBAS Performance, in: Proceedings of the ION GPS 2000, 19–22 September 2000, Salt Lake city, UT, 2000.
Tsunoda, R.T. Time evolution and dynamics of equatorial backscatter plumes.1. Growth phase. J. Geophys. Res. 86, 139, 1981. Van Dierendonck, A.J., Klobuchar, J., QuyenHua. Ionospheric scintillation monitoring using commercial single frequency C/A code receivers, in: Proceedings of ION GPS-93, Salt Lake City UT, 1993. Woodman, R.F., LaHoz, C. Radar observations of F-region equatorial irregularities. J. Geophys. Res. 81, 5447, 1976.