Study of amplitude and phase scintillation at GPS frequency

Transcription

1 Indian Journal of Radio & Space Physics Vol. 34, December 25, pp 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 26, India 2 Military Research and Development Centre, Bangkok, Thailand smitadubey@yahoo.com Received 3 September 24; revised 23 February 25; accepted 29 July 25 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, 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.4 in summer months. Keywords: Ionosphere, F-region, GPS frequency, GPS PACS No: 94.2.S; 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 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 this region, during the solar cycle maxima periods, amplitude fading at 1.5 GHz may exceed 2 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.

2 DUBEY et al.; AMPLITUDE & PHASE SCINTILLATION AT GPS FREQUENCY 43 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 N, long E), Thailand using an Ionospheric Scintillation Monitor (ISM) single frequency receiver configured to measure amplitude and phase scintillation at the L1= GHz from January 21 to December 21. The ISMs are based on Novate! GPS single frequency (Ll) receiver, which has been modified to process raw data, sampled at 5 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.5. In scintillation conditions, the value from.5 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:- 6: 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 3 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 (2-22 hrs LT) z i= 5 ::5 c;,e 4 ~ui I-(.) Zz ()w (1):: a=> ::> (.) 2 t-u ::JO ~ 1 QL-~~~~--~~~~--~~~ Jan. Mar. May. July Sep. Nov. Fig.!-Annual variation of percentage of occurrence of amplitude scintillation from January 21 to December 21

3 44 INDIAN J RADIO & SPACE PHYS, DECEMBER 25 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 (18-24 hrs L T) and post midnight (24-6 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 5 f= :5~...J - -UJ I- () ~z 4 u w (/):: o::j ::J () J- U 2 :JO ~ <! 1 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 18 hrs LT to 6 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.4 in the summer months, while in winter S4 index varies between.5 and.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 S4 Fig. 2- TIME, hrs LT Nocl;Jrnal variation of percentage of occurrence of amplitude scintillation for different seasons Strong Moderate Weak _Yery Weak.6 < S4::; 1.4 < S4::;.6.2 < S4::;.4.5 < S4 ::;.2.5 < (J l> ~ ::;.6.25 < (Jl>$ ::;.5.1 <al> ::;.25.5 < (Jl>$ ::;.1 z i= :5~...J - -W J-u ~z ()W (f):: o => ::>u J-u :JO ::2 <( I Jan. EIJ Premldnight - Postmidnight I rn II [. ND ll ND Mar. May. July Sep. Nov. Fig. 3-Monthly percentage of the occurrence of scintillation during pre-midnight and post-midnight 7 z 6 f= :5 ~ 5...J - - W 1- ~z 4 o w (J)C:: o=> :::> () 1-() ::J O ~ <( Mar. +:.:5<84<.2.:.2<54< 4 X :.4<84<.6 :.6<54 <1 May. July Sep. Nov. Fig. 4--Percentage of occurrence of S4 index

4 DUBEY et al.; AMPLITUDE & PHASE SCINTILLATION AT GPS FREQUENCY :.5SigJ!la6<.2 :.2 Sigma 6<.4 z 6 X:.4 Sigma6.<.6 Q ~ o :.6 Sigma6<1 1- ~-...J w 5...JU 1-w - z 4 :?;a: u o: (/)::::> wu cnu ~ I a. 1 Jan. Mar. May. July Sep. Nov. 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. 1 showed the fading characteristics of scintillation at different frequencies. Results show that strong scattering is observed at 244 and 257 MHz signal but at and 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 25 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 J i ciuring post sunset hours and several authors credit its generation to the generation of Rayleigh-Taylor (RT) plasma instability Several studies were made using radio signals transmitted by geostationary satellite, in order to understand the morphology of irregularities 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

5 46 INDIAN J RADIO & SPACE PHYS, DECEMBER 25 Gwaf 8 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 expected 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 (American Atlantic sector). They found that at solar maximum during 2-24 hrs, LT fades exceeding 2 db might be encountered 1% 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 June August. This result is similar to that at Calcutta 18 Sushil et ae 2 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 1 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.4. :r Acknowledgements The authors wish to acknowledge the financial support from Indo-Russian Programme (ILTP) from Department of Science and Technology, New Delhi. 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