Indian Journal of Radio & Space Physics Vol. 34, December 2005, pp. 402-407 Study of amplitude and phase scintillation at GPS frequency Smita Dubei , Rashmi Wahi 1 , Ekkaphon Mingkhwan 2 & A K Gwal 1 1 Space Science Laboratory, Department of Physics, Barkatullah University, Bhopal462 026, India 2 Military Research and Development Centre, Bangkok, Thailand Email: [email protected]Received 3 September 2004; revised 23 February 2005; accepted 29 July 2005 When a radio wave at L-band frequency passes through the entire ionospheric region, it is affected by the electron density irregularities, which are present in the ionosphere and generate amplitude and phase scintillation of GPS signals. This is especially true during the ni ghttime. The magnitude of amplitude and phase scintillation and the temporal structure of sc intillations need to be specified and predicted to provide support to operational communication and navigation system. Amplitude and phase flu ctuation at the GPS frequency at Chiang Rai (19.57°N, 99.52°E), Thailand is described in thi s paper. It was found th at higher amplitude fluctuation level could be observed on ly in equinox and winter, while S4 is less than 0.4 in summer month s. Keywords: Ionosphere, F-region, GPS frequency, GPS PACS No: 94.20.S; 94.20.1 1 Introduction The ionosphere is a dispersive medium in which radio frequency signals are refracted by an amount depending upon signal frequency and ionospheric electron density. When a radio signal, be it from a satellite or radio star, interacts with the di sturbed ionosphere, the received signal will show rapid fluctuations in amplitude and phase, not consistent with the source strength or ·nodulation. This so called scintillation is attributable to electron density irregularities, which· in turn manifest themselves as change in refractive index ;. Fading (and enhancements) in the amplitude of the signal, or scintillation of the amplitude of the received signal, is caused by irregularities of scale size from hundred meters to kilometers in the electron density of th e ionosphere. Rapid changes in signal phase, called phase scintillation, are attributable largely to rapid but very small changes in electron concentration of the ionosphere. During the time of severe phase scintillation, the phase will not change in a consistent, rapid manner to yield greater ionospheric Doppler shifts, but the phase of the incoming radio frequency signal will have large random fluctuation superimposed upon the changes associated with the normal rate of change in Total Electron Content (TEC). Knowledge of phase scintillation rate is required to determine the spread of the receiver signal phase. Normally, those regions of the Earth where strong phase scintillation effects occur are li mited to the near equatorial latitudes. A physical picture that emerges for the generation of the plasma irregularities that cause scintillation is that after sunset the E-region begins to recombine, thereby decreasing its conductivity. The effect of recombination and E x B drift on the bottom side F- region provides a steep electron density gradient. When the altitude of the F-region is high enough to overcome recombination effects or the bottom side electron density gradient large enough, the Rayleigh- Taylor Instability mechanism initiates a growth in plasma fluctuations. An upward moving bubble of depleted plasma is produced, which rises and eventually transforms itself into a plethora of smaller irregularities, generally associated with the bubble walls. These irregularities map down the magnetic field lines towards the crests of the equatorial anomaly. The occurrence of scintillation is thus essentially an evening phenomenon 2 • In th is region, during the solar cycle maxima periods, amplitude fading at 1.5 GHz may exceed 20 dB for several hours after sunset 3 . Many authors 4 - 9 have studied the morphology of GPS L-band scintillation. These studies show that scintillation activity varies with operating frequency, geographical location, local time, season, magnetic activity and the 11-year solar cycle.
Transcript
Indian Journal of Radio & Space Physics Vol. 34, December 2005, pp. 402-407
Study of amplitude and phase scintillation at GPS frequency
Smita Dubei , Rashmi Wahi 1, Ekkaphon Mingkhwan2 & A K Gwal 1
1Space Science Laboratory, Department of Physics, Barkatullah University, Bhopal462 026, India 2Military Research and Development Centre, Bangkok, Thailand
Received 3 September 2004; revised 23 February 2005; accepted 29 July 2005
When a radio wave at L-band frequency passes through the entire ionospheric region, it is affected by the electron density irregularities, which are present in the ionosphere and generate amplitude and phase scintillation of GPS signals. This is especially true during the nighttime. The magnitude of amplitude and phase scintill ation and the temporal structure of sc intillations need to be specified and predicted to provide support to operational communication and nav igation system. Amplitude and phase fluctuation at the GPS frequency at Chiang Rai (19.57°N, 99.52°E), Thailand is described in thi s paper. It was found that higher amplitude fluctuation level could be observed only in equinox and winter, while S4 is less than 0.4 in summer months.
1 Introduction The ionosphere is a dispersive medium in which
radio frequency signals are refracted by an amount depending upon signal frequency and ionospheric electron density. When a radio signal, be it from a satellite or radio star, interacts with the di sturbed ionosphere, the received signal will show rapid fluctuations in amplitude and phase, not consistent with the source strength or ·nodulation. This so called scintillation is attributable to electron density irregularities, which· in turn manifest themselves as change in refractive index;. Fading (and enhancements) in the amplitude of the signal, or scintillation of the amplitude of the received signal, is caused by irregularities of scale size from hundred meters to kilometers in the electron density of the ionosphere.
Rapid changes in signal phase, called phase scintillation, are attributable largely to rapid but very small changes in electron concentration of the ionosphere. During the time of severe phase scintillation, the phase will not change in a consistent, rapid manner to yield greater ionospheric Doppler shifts, but the phase of the incoming radio frequency signal will have large random fluctuation superimposed upon the changes associated with the normal rate of change in Total Electron Content (TEC). Knowledge of phase scintillation rate is required to determine the spread of the receiver signal
phase. Normally, those regions of the Earth where strong phase scintillation effects occur are limited to the near equatorial latitudes.
A physical picture that emerges for the generation of the plasma irregularities that cause scintillation is that after sunset the E-region begins to recombine, thereby decreasing its conductivity. The effect of recombination and E x B drift on the bottom side Fregion provides a steep electron density gradient. When the altitude of the F-region is high enough to overcome recombination effects or the bottom side electron density gradient large enough, the RayleighTaylor Instability mechanism initiates a growth in plasma fluctuations. An upward moving bubble of depleted plasma is produced, which rises and eventually transforms itself into a plethora of smaller irregularities, generally associated with the bubble walls. These irregularities map down the magnetic field lines towards the crests of the equatorial anomaly. The occurrence of scintillation is thus essentially an evening phenomenon2
• In this region, during the solar cycle maxima periods, amplitude fading at 1.5 GHz may exceed 20 dB for several hours after sunset3
. Many authors4-9 have studied the
morphology of GPS L-band scintillation. These studies show that scintillation activity varies with operating frequency, geographical location, local time, season, magnetic activity and the 11-year solar cycle.
DUBEY et al.; AMPLITUDE & PHASE SCINTILLATION AT GPS FREQUENCY 403
Ionospheric scintillation caused by irregularities in the electron density can disturb the amplitude and phase of a GPS signal, as it travels to the GPS receiver. GPS satellites offer a unique source for measurements of amplitude and phase scintillation on a global scale. One receiver can record scintillation magnitudes and spectra at multiple propagation paths in the overhead sky. The data can be used to study ionospheric plasma structures, develop weather models of scintillation and can be scaled in frequency to support many operational systems. The aim of this paper is to present the available database for understanding the morphology of amplitude and phase scintillation, to show the nature of the fading when scintillation is present.
2 Ionospheric scintillation monitoring and methodology
Scintillation activity was monitored at Chiang Rai (lat. 19.57°N, long. 99.52°E), Thailand using an Ionospheric Scintillation Monitor (ISM) single frequency receiver configured to measure amplitude and phase scintillation at the L1=1.57542 GHz from January 2001 to December 2001. The ISMs are based on Novate! GPS single frequency (Ll) receiver, which has been modified to process raw data, sampled at 50 Hz and calculate various parameters, which characterize the observed scintillation. The ISMs record processed data automatically at one-minute intervals throughout the day. Amplitude scintillation index (S4) and phase scintillation (cr,..q,) parameters are made available either in raw form or as a corrected S4, for which the effects of ambient noise have been removed. We work with the corrected S4, in subsequent analysis of our recorded data. In the absence of scintillation and multipath, corrected S4 values should lie below 0.05. In scintillation conditions, the value from 0.05 to I may be obtained.
3 Results
3.1 Occurrence characteristic of scintillation The observations of amplitude and phase
scintillation for one year is described in this paper. The measurements used to calculate the statistics of scintillations include those recorded during 18:00-06:00 hrs LT, when scintillation occurs in the equatorial region. The statistics were also limited to scintillation measurement made from satellite that were locked on to ISM for more than 4 min., to allow the time for detrending filter to stabilize. Statistical calculations were further limited to those
measurements made from satellite with elevation viewing angle greater than 30 deg, to limit multipath interference. From the measurements that pass the above three tests, we then computed the percentage of occurrence of amplitude scintillation S4 and phase scintillation crM. The occurrence of ionospheric scintillation has been studied in terms of percentage of occurrence of the S4 index for amplitude scintillation of the GHz signal and the crt.<~~ index for phase scintillation. It was seen that L-band amplitude scintillation at this latitude, was basically a nighttime phenomenon. The annual average variation of the percentage of occurrence of amplitude scintillation. is presented in Fig.1. It shows the month-to-month variation of hourly percentage of occurrence of amplitude scintillation. From Fig. 1 it is clear that scintillation occurrence is maximum in April and September and minimum in May-June. A prominent feature to note is that the peak during March-April is higher than the peak during September-October.
To study the nocturnal variation in the percentage occurrence of scintillation for the different seasons, first we grouped all the months in three categories corresponding to Equinox orE-months (March, April, September, October), Summer or J-months (May, June, July, August), and Winter or D-months (November, December, January, February). Figure 2 shows a remarkable seasonal variation. It shows that scintillation activity is maximum during equinox months while minimum in summer months. The seasonal variation in the scintillation activity shows a peak in the pre-midnight hours (2000-2200 hrs LT)
Fig. !-Annual variation of percentage of occurrence of amplitude scintillation from January 2001 to December 2001
404 INDIAN J RADIO & SPACE PHYS, DECEMBER 2005
during equinox and winter but shifts to post-midnight hours during summer.
Figure 3 shows the monthly mean percentage of occurrence of scintillation during pre-midnight (1800-2400 hrs L T) and post midnight (2400-0600 hrs LT) period for each month. It is seen that pre-midnight occurrence of scintillation was predominant in most of the months except in the month of June and December, when post-midnight occurrence was predominant. Pre-midnight scintillation was found to be maximum in September, while post-midnight scintillation duration maximized in December. February and October showed comparable premidnight and post-midnight durations. In these two months pre-midnight and post-midnight durations were nearly the same.
z 50 0 f= :5~
40 .....J --UJ I- () ~z u w (/)0:: wO:: o::J ::J () J- U 20 :JO a.. ~ <!
10
TIME, hrs LT
Fig. 2- Nocl;Jrnal variation of percentage of occurrence of amplitude scintillation for different seasons
Fig. 3-Monthly percentage of the occurrence of scintillation during pre-midnight and post-midnight
To study the intensity of amplitude scintillation index S4 and phase scintillation crM we consider four scintillation levels shown in Table 1. Then, we computed the percentage of scintillation occurrence for four distinct thresholds of scintillation according to S4 index and crM (Table 1). The statistics were computed for every one-minute interval from 1800 hrs LT to 0600 hrs LT.
Figure 4 shows the percentage occurrence of S4 index for each month in the measurement period for very weak, weak, moderate and strong levels of scintillation. The statistics shown in Fig. 4 clearly illustrate the seasonal dependence of scintillation intensity. On comparing the scintillation strength of above four classes, we found that the higher amplitude fluctuation level can be observed only in equinox months, especially in the months of April and September. On the other hand, in winter and summer months the higher fluctuation level is rarely observed. Scintillation intensity levels do not exceed 0.4 in the summer months, while in winter S4 index varies between 0.05 and 0.6. Only in the month of February strong scintillation index is seen.
Figure 5 shows the percentage occurrence of crM for each month in the measurement period for very weak, weak, moderate and strong levels of scintillation. From
Table !-Case consideration for amplitude and phase scintillation intensity
CASE
Strong Moderate Weak _Yery Weak
70 z 0 60 f= :5 ~ 50 .....J -- W 1- 0 ~z 40 o w (J)C:: wO:: o=> :::> () 1-() ::J O a.. ~ <(
DUBEY et al.; AMPLITUDE & PHASE SCINTILLATION AT GPS FREQUENCY 405
80
70 .+: 0 .05SigJ!la60<0 .2 • : 0 .2 Sigma 60<0 .4
z 60 X: 0.4 Sigma60.<0.6
Q ~ o : 0.6 Sigma60<1 1- 0
50 ~-...J w ...JU - z 40 1-w :?;a: u o: (/)::::> wu cnu ~0 I a. 10
0 Jan. Mar. May. July Sep. Nov.
MONTH
Fig. 5-Percentage of occurrence of phase scintillation
Fig. 5 it is clear that the strong phase scintillation was observed only in equinox month, especially in the month of April and September, while no trace of strong phase scintillation is found in summer. Moderate phase scintillation is observed in winter month.
4 Summary and discussion A number of scientific workers have tried to
explain how scintillations can occur at high frequencies. By taking simultaneous recording of scintillation at Ascension Island at VHF, L and SHF bands during the solar maximum, Basu et al.10 showed the fading characteristics of scintillation at different frequencies. Results show that strong scattering is observed at 244 and 257 MHz signal but at 1.541 and 3.954 GHz recording shows less fluctuations. Taru 11
using 4 and 6 GHz data found maximum occurrence in February-April and September-November. He worked with global statistics. By taking the data of Ascension Island near the anomaly crest Mullen et al. 12 showed that percentage of occurrence of scintillation at 1.54 GHz scintillation is maximum during equinox month. Dasgupta et al. 13 showed the same results at 1.54 GHz at Huancayo located near the magnetic equator.
Scintillation morphology can be considerably different if measurements are made at VHF frequencies, such as 137 and 250 MHz. Dasgupta et al. 13 showed that particularly in the December solstice, it is possible to observe uninterrupted patches of scintillation of 5-6 h. In fact, these long patches of scintillations are so numerous in the Huancayo sector that one gets a single overall maximum in scintillation occurrence in the December solstice at VHF, rather
than the two-equinoctical maxima that appear at GHz frequencies . Basu et al. 14 have established that these scintillation patches are due to bottom side sinusoidal (BSS) irregularities, discussed at length by Valladares et al. 15 and as a rule, they do not give rise to GHz scintillations.
The general features of amplitude and phase scintillation at GPS frequency conform well to the nature of scintillation reported at the other low latitude stations. Statistics on the occurrence of scintillation in the equatorial region have a welldefined seasonal dependence with a preference for activity in the equinoctial seasons and with virtually no activity in the summer month at Chaing Rai. Rastogi et al. 16 studied the radio wave scintillation at equatorial stations in Indian and American zones. For Indian zones they found the largest occurrence of scintillation during the month of equinox and least during the summer months; whereas for American zone, they found the largest occurrence of scintillation during the winter months and least during the summer months . Pathan et al. 17 have shown similar variations in scintillation occurrence for equatorial stations in Indian zone.
During solar maximum period as in the present study, scintillations are much enhanced. Dasgupta et al. 18 have shown that the occurrence of scintillation at Calcutta (16.8°N dip lat.) depends highly on solar activity during equinox and winter months, but not so during summer months. They suggest that the scintillation occurrence during winter and equinoctial months are of equatorial origin and during summer months of local ongm. Equatorial density irregularities in the equatoria 1 ionosphere are generated at the magnetic ~t:J'l 'll Ji ciuring post sunset hours and several authors credit its generation to the generation of ·Rayleigh-Taylor (RT) plasma instability19
-24
. Several studies were made using radio signals transmitted by geostationary satellite, in order to understand the morphology of irregularities25
·26
.
The occurrence of ionospheric scintillation can be used as an indicator of presence of irregularities of specific scale size in the ionosphere.
In the present study, the seasonal pattern of occurrence of phase scintillation shows maximum in equinox months and least during summer months, similar to other equatorial regions. Doherty et al. 27
showed that in equatorial region frequent amplitude scintillation activity were observed with negligible occurrence of phase scintillation. By using FLEETSAT satellite at VHF, Sushi! Kumar and
406 INDIAN J RADIO & SPACE PHYS, DECEMBER 2005
Gwaf8 reported similar results of amplitude scintillation for another low latitude station, Bhopal. They conclude that the characteristics of amplitude scintillation during equinox and winter months are similar to equatorial stations, whereas those of summer months are similar to mid-latitude. S4 index increased after local midnight as expected29
.30 and
decay slowly afterward. In the present study, it is seen that pre-midnight
occurrence of scintillation at Chiang Rai was predominant in most of the months except in the months of June and December, when post-midnight occurrence was predominant. Basu et al. 8 studied the statistics of occurrence of scintillation for solar maximum in the equatorial anomaly (AmericanAtlantic sector). They found that at solar maximum during 2000-2400 hrs, LT fades exceeding 20 dB might be encountered 10% of the time. Koparkar and Rastogi 31 showed that either the pre-midnight or the post-midnight scintillation events are most prevalent during the equinox months with a mild minimum during January and a deep minimum around JuneAugust. This result is similar to that at Calcutta18
•
Sushil et ae2 showed that pre-midnight scintillation is generally intense and fast, while post-midnight scintillations are weak and slow. They also showed that scintillation index expressed as peak-to-peak amplitude fluctuation in dB shows that in equinox and winter scintillation is strong at ;about 10 dB during most part of the night, while in rsummer it is less than 5 dB throughout the night. In the present study, we found that higher amplitude fluctuation level can be observed only in equinox and winter, while in summer month S4 is less than 0.4.
:r •
Acknowledgements The authors wish to acknowledge the financial
support from Indo-Russian Programme (ILTP) from Department of Science and Technology, New Delhi. (No. NP-29/JC-11). We are also thankful to Dr Pian Totarong, Military Research and Development Centre, Bangkok, Thailand, for providing us the GPS data and for his constant and useful help in data analysis.
References 1 Briggs B H, Ionospheric irregularities and radio scintillation,
Contemp Phys (UK), 16 (1975) 469. 2 Dabas R S, Banarjee P K, Bhattacharya S, Reddy B M &
Singh J, Gigahertz scintillation observations at 22.0°N magnetic latitude in the Indian zone, Radio Sci (USA), 26 (1991) 759.
3 Basu S, Mackenzie E & Basu S, Ionospheric constraints on VHF/UHF communications links during solar maximum and minimum period, Radio Sci (USA), 23 (3) (1988) 363.
4 Aarons J, Global morphology of ionospheric scintillation, Proc IEEE (USA), 70(4) (1982) 360.
5 Aarons J, The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence, Space Sci Rev (USA), 63 (1993) 209.
7 Aarons J, Global positioning system phase fluctuations at auroral latitudes, J Geophys Res (USA) , 102 (1997) 17.
8 Basu S, Weber E J, Groves P, Sultan P, Kuenzler H, Ning P, McNeil W J, Moonana D W & Kendra M J, National Space Weather Program: Ionospheric Scintillation and System Support, Proc. 1996 Ionospheric Effects Symp.(Alexandria), VA(1996).
9 Pi X, Manucci A J, Lindqwister U J & Ho C M, Monitoring of global ionospheric irregularities using worldwide GPS network, Geophys Res Lett (USA), 24 (1997) 2283.
10 Basu S, Mackenzie E, Basu Su, Costa E, Fougere P F, Carlson H C & Whitney H E, 250 MHzJGHz scintillation parameters in the equatorial, polar and auroral environments, IEEE J Selected Area in Communications (USA) , SAC-5 (1987) 102.
11 Taur R R, Ionospheric scintillations at 4 and 6 GHz, COMSATTech. Rev (USA) , 3 (1973) 145.
12 Mullen J P, Mackenzie E & Basu S, UHF/GHz scintillation observed at Ascension Island from 1980 through 1982, Radio Sci (USA), 20 (1985) 357.
13 Dasgupta A, Aarons A, Klobuchar J A, Basu S & Bushby A, Ionospheric electron content depletions associated with amplitude scintillations in the equatorial region, J Geophys Res (USA), 9 (1982) 147.
14 Basu S & Basu S, Equatorial scintillations: Advances since ISEA-6, J Atmos Terr Phys (UK), 47 (1985) 753.
15 Valladares C E, Hanson W B, McClure 1 P & Cragin B L, Bottomside sinusoidal irregularities in the equatorial F-region, J Geophys Res (USA), 88 (1983) 8085.
16 Rastogi R G, Koparkar P V & Pathan B M, Nighttime radio wave scintillations at equatorial stations in Indian and American zones, Geomagn Geoelect (Japan), 42 (1990) 1.
17 Pathan B M, Koparkar P V, Rastogi R G & Rao D R K, Dynamics of ionospheric irregularities producing VHF radio wave scintillation, Ann Geophys (France), 9 (1991) 126.
18 DasGupta A, Maitra A & Basu S, Occurrence of nighttime VHF scintillation near the equatorial anomaly crest in the Indian Sector, Radio Sci (USA), 16 (1981) 1455.
19 Dungey 1 W, Convective diffusion in the equatorial F-region, J Atmos Terr Phys (UK), 9 (1956) 304.
20 Tsunoda R T, Magnetic field-aligned characteristics of plasma bubbles in the nighttime equatorial ionosphere, J Atmos Terr Phys (UK), 42 (1980) 743.
21 Tsunoda R T, Time evolution and dynamics of equatorial back scatter plumes, J Geophys Res (USA), 86 (1981) 139.
22 Abdu M A, Bittencourt 1 A & Batista I S, Magnetic declination control of the equatorial F-region dynamo electric field development and spread-F, J Geophys Res (USA), 86 (1981) 11443.
DUBEY et al.; AMPLITUDE & PHASE SCINTILLATION AT GPS FREQUENCY 407
23 Kelley M C, Equatorial spread-F: Recent results and outstanding problems, J Atmos Terr Phys (UK), 47 (1985) 745.
24 Sultan P J, Linear theory and modeling of the RayleighTaylor instability leading to the occurrence of equatorial spread F, J Geophys Res (USA), 101 (1996) 26875.
25 Abdu M A, Outstanding problems in the equatorial ionosphere~thermosphere electrodynamics relevant to spread F, J Atmos Terr Phys (UK), 47 (2001),745.
27 Doherty P H, Delay S H, Cesar E & Klobuchar J A, Ionospheric Scintillation effects in the equatorial and auroral regions, ION GPS, 19-22 Sep, Salt Lake City, UT (2000).
28 Kumar S & Gwal A K, VHF ionospheric scintillations near the equatorial anomaly crest: solar and magnetic activity effects, J Annas Solar-Terr Phys (UK), 62 (2000) 157.
29 Aarons J & Basu S, lonosphnic Amplitude and Phase Fluctuations at the GPS Frequendes, ION 94 (Utha), (J 994) 1569.
30 Klobuchar J, Ionospheric Effects on GPS, Global Positioning System: Theory and Application, Parkinson B and J Spilker, New York, 1 (1996) 485.
31 Koparkar P V & R G Rastogi, VHF scintillation at Bombay, J Atmos Terr Phys (UK), 62 (1985) 157.
32 Sushi! K, Singh A K, Chauhan P, Gwal A K, Singh B & Singh R P, Multistation analysis of VHF radio wave scintillations at low latitudes, Indian J Radio Space Phys, 22 (1993) 267.
