Scintillation Monitor Research Articles

Analysis of the Characteristics of Low-Latitude GPS Amplitude Scintillation Measured During Solar Maximum Conditions and Implications for Receiver Performance

Ionospheric scintillations are fluctuations in the phase and/or amplitude of trans-ionospheric radio signals caused by electron density irregularities in the ionosphere. A better understanding of the scintillation pattern is important to make a better assessment of GPS receiver performance, for instance. Additionally, scintillation can be used as a tool for remote sensing of ionospheric irregularities. Therefore, the study of ionospheric scintillation has both scientific as well as technological implications. In the past few years, there has been a significant advance in the methods for analysis of scintillation and in our understanding of the impact of scintillation on GPS receiver performance. In this work, we revisit some of the existing methods of analysis of scintillation, propose an alternative approach, and apply these techniques in a comprehensive study of the characteristics of amplitude scintillation. This comprehensive study made use of 32 days of high-rate (50 Hz) measurements made by a GPS-based scintillation monitor located in Sao Jose dos Campos, Brazil (23.2°S, 45.9°W, −17.5° dip latitude) near the Equatorial Anomaly during high solar flux conditions. The variability of the decorrelation time (τ0) of scintillation patterns is presented as a function of scintillation severity index (S4). We found that the values of τ0 tend to decrease with the increase of S4, confirming the results of previous studies. In addition, we found that, at least for the measurements made during this campaign, averaged values of τ0 (for fixed S4 index values) did not vary much as a function of local time. Our results also indicate a significant impact of τ0 in the GPS carrier loop performance for S4 ≥ 0.7. An alternative way to compute the probability of cycle slip that takes into account the fading duration time is also presented. The results of this approach show a 38% probability of cycle slips during strong scintillation scenarios (S4 close to 1 and τ0 near 0.2 s). Finally, we present results of an analysis of the individual amplitude fades observed in our set of measurements. This analysis suggests that users operating GPS receivers with C/N0 thresholds around 30 dB-Hz and above can be affected significantly by low-latitude scintillation.

Characteristics of intense space weather events as observed from a low latitude station during solar minimum

[1] Using a dual-frequency high-resolution software-based GPS receiver, TEC and phase have been monitored from Calcutta, India situated near the northern crest of the Equatorial Ionization Anomaly for studying some Space Weather events during 2008–2010. Data from a dual-frequency Ionospheric TEC and Scintillation Monitor operational at this station under the international SCINDA program of the U.S. Air Force have also been used. This paper presents two cases of intense Space Weather events occurring in the equatorial latitudes under magnetically quiet conditions during the abnormally prolonged minimum of solar cycle 24. High values of S4 with maximum ∼0.8 were noted on GPS links located almost due south of Calcutta (22.58°N, 88.38°E geographic; magnetic dip: 32°N) when the look angles of the satellites are more-or-less aligned with the axis of the anisotropic field-aligned irregularities over the magnetic equator. Associated bite-outs in TEC of amplitude 40 units were recorded in the local post-sunset hours. Well-defined patches of phase scintillations and associated cycle slips were identified. On these days, higher values of ambient ionization were noted and the diurnal maximum of the electrojet strength was found to be delayed followed by a significant rise of the F region with a high upward drift velocity over the magnetic equator around sunset indicated by ionosonde. Measurements of in situ ion density using LEO DMSP corroborate the F region height rise. Presence of irregularities in ionization density distributions around 450km was found from C/NOFS measurements.

Equatorial ionospheric zonal drift by monitoring local GPS reference networks

[1] The propagation of electromagnetic waves through the turbulent ionosphere produces scintillations through diffraction, and understanding the physical nature of scintillations is important for engineers and technologists as well as for scientists. In recent years, the establishment of the Global Positioning System (GPS) provided a new technique that can be used to study ionospheric scintillations. The usual way of doing that is the deployment of GPS receivers closely spaced in east-west magnetic direction and then estimating the zonal drift velocities based on the signal power observations. One of the weaknesses of this method is that high-rate sampling such as 20 Hz is required for close-spaced stations and generally no such data are available for studying ionospheric scintillation in the past years. In this research work, a scintillation monitoring method based on slant TEC (STEC) observations of local GPS Continuously Operating Reference Station (CORS) network is proposed. First, the past research works on the equatorial ionospheric drift velocities are summarized. Then, by comparing the scintillation pattern of the signal power and STEC observations of California local GPS reference network, we find that the STEC is a good choice for estimating the ionospheric zonal drift velocity. Then it is illustrated how to calculate the ionospheric scintillation velocity based on STEC. Finally, the proposed method is applied to Hong Kong GPS reference network and several cases of the calculated ionospheric zonal velocities are given.

Relationship between strong range spreadFand ionospheric scintillations observed in Hainan from 2003 to 2007

Data from a DPS-4 Digisonde and an ionospheric scintillation monitor, both located at the low-latitude station Hainan (109.1 degrees E, 19.5 degrees N; dip latitude 9 degrees N), were analyzed to study the strong range spread F (SSF) and its correlation with ionospheric scintillations observed in the period of declining solar cycle 23 from 2003 to 2007. The results show that the maximum and minimum of the occurrence of SSF appeared in nearly the same months as those of the GPS L band scintillations. The variations in SSF occurrence were also similar to those of the scintillations. From 2003 to 2007, both the SSF and the scintillation occurrences decreased from the high solar activity year to the low solar activity year. The correlation coefficient between the occurrences of the SSF and the GPS L band scintillation was as high as 0.93, suggesting associated mechanisms producing SSF and scintillations. Electron density depletions extending from the bottomside to the topside ionosphere are the likely cause explaining the high correlation.

Equatorial scintillation calculations based on coherent scatter radar and C/NOFS data

During its transit through a region of equatorial ionospheric irregularities, sensors on board the Communication/Navigation Outage Forecasting System (C/NOFS) satellite provide a one‐dimensional description of the medium, which can be extended to two dimensions if the structures are assumed to be elongated in the direction of the magnetic field lines. The C/NOFS scintillation calculation approach assumes that the medium is equivalent to a diffracting screen with random phase fluctuations that are proportional to the irregularities in the total electron content, specified through the product of the directly measured electron density by an estimated extent of the irregularity layer along the raypaths. Within the international collaborative effort anticipated by the C/NOFS Science Definition Team, the present work takes the vertical structure of the irregularities into more detailed consideration, which could lead to improved predictions of scintillation. Initially, it describes a flexible model for the power spectral density of the equatorial ionospheric irregularities, estimates its shape parameters from C/NOFS in situ data and uses the signal‐to‐noise ratio S/N measurements by the São Luís coherent scatter radar to estimate the mean square electron density fluctuation 〈ΔN2〉 within the corresponding sampled volume. Next, it presents an algorithm for the wave propagation through a three‐dimensional irregularity layer which considers the variations of 〈ΔN2〉 along the propagation paths according to observations by the radar. Data corresponding to several range‐time‐intensity maps from the radar is used to predict time variations of the scintillation index S4 at the L1 Global Positioning System (GPS) frequency (1575.42 MHz). The results from the scintillation calculations are compared with corresponding measurements by the colocated São Luís GPS scintillation monitor for an assessment of the prediction capability of the present formulation.

Preliminary Statistic Analysis of L-band Ionospheric Scintillation Over Nanjing Region in China

利用南京地区2008年11月至2009年10月电离层闪烁监测数据, 统计分析了该地区一年间L波段电离层幅度闪烁发生率的逐日变化、逐月变化、地方时变化和空间分布等特征. 统计结果表明, 在此期间, 南京地区L波段电离层幅度闪烁活动比较平静, 主要以0.1<<I>S</I><SUB>4</SUB><0.2的闪烁为主, <I>S</I><SUB>4</SUB>>0.2的闪烁很少发生.不同强度幅度闪烁表现出一致的时间变化和空间分布特征, 2008年11月为最小,2009年6月、8至10月闪烁发生率都有明显的增强, 在10月达到最大, 在8月和10月的<I>S</I><SUB>4</SUB>>0.2的闪烁主要出现于正午到日落前这段时间; 对于闪烁的空间分布, 单站一年数据统计显示, 测站北向闪烁的发生频率高于南向, 但该统计结果需要更多的数据样本进行进一步的验证.

Interplanetary and Ionosphere Scintillation Monitoring of Radio Sources Ensemble at the Solar Activity Minimum

Results are presented of twenty-four-hour interplanetary and ionosphere scintillation monitoring obtained in Pushchino Radioastronomy Observatory during three observation series: between 18 November and 30 December 2006; between 11 June and 18 June 2007; between 20 October and 4 November 2008. Observations were carried out simultaneously in 16 beams of the radio telescope BSA LPI (Big Scanning Array of Lebedev Physical Institute) at the frequency 111 MHz. All the sources with scintillating flux greater than 0.2 Jy were recorded in the range of declination from +3° to +12°40 by observations in the years 2006, 2008 and from 21° to 28° in the year 2007. The values of radio source flux fluctuation temporal structure functions were measured at the temporal lags of 0.1 s, 1 s and 10 s, which characterize noise, interplanetary scintillations and ionosphere scintillations, respectively. About 1000 scintillating radio sources were observed every day. The number of sources N (σ IPS≥σ IPS,0) with scintillating flux σ IPS greater than the given value σ IPS,0 in the sky area with sizes 8° in declination and 0.5 hours in right ascension was used as the parameter describing turbulent interplanetary plasma. This parameter is shown to be proportional to the mean scintillation index of the radio source statistical ensemble in the given sky area. Similarly, the parameter N (σ IONS≥σ IONS,0) was introduced for a description of the turbulent ionosphere. The mean twenty-four-hour temporal dependence of the scintillation index is found. Weak day-to-day variations of the scintillation index were observed. In general, interplanetary plasma, as well as the ionosphere, was in a quiet state during the observation periods. Unusually a weak dependence of scintillation index on the position of the sources relative to the Sun was observed in the year 2008 at very deep solar activity minimum. Such a weak dependence is explained by a strong elongation of the turbulence level spatial distribution along the solar equatorial plane during a very deep minimum of solar activity.

Optimum parameter for estimating phase fluctuations on transionospheric signals at high latitudes

Transionospheric radio signals may experience fluctuations in their amplitude and phase due to irregularity in the spatial electron density distribution, referred to as scintillation. Ionospheric scintillation is responsible for transionospheric signal degradation that can affect the performance of satellite based navigation systems. Usually, the scintillation activity is measured by means of indices such as the normalised standard deviation of the received intensity S 4 and the standard deviation of the received phase σ ϕ typically calculated over 1 min of data. Data from a GPS scintillation monitor based on 50 Hz measurements recorded at Dirigibile Italia Station (Ny-Alesund, Svalbard), in the frame of the ISACCO project ( De Franceschi et al., 2006) are used to investigate possible adoption of an alternative parameter for the estimate of phase fluctuations: i.e., the standard deviation of the phase rate of change S ϕ . This parameter is shown to better correlate with S 4 being much less detrending dependent than σ ϕ . The couple ( S 4, S ϕ ) should be then considered a more physical proxy of radio scintillation than the couple ( S 4, σ ϕ ).

Comparison of ionospheric scintillation models with experimental data for satellite navigation applications

A comparison between two of the most used scintillation models and experimental data is presented. The experimental data have been derived from a GPS scintillation monitor developed at Cornell University and placed in Tucuman (Argentina), under the peak of the anomaly. The models used (GISM and WBMOD) have been run for the geophysical conditions corresponding to the measurements. The comparison is done by subdividing the information on the basis of an ionospheric grid of 5° ×5° surface square boxes. The comparison has been performed for several local times, from 18 LT until 04 LT. Here, only a few cases of particular interest are shown. The goal is to understand if the models are able to forecast actual scintillation morphology (from the satellite navigation systems point of view) and if they could be used to yield an estimate of scintillation effects on satellite navigation systems.

Ionospheric scintillation monitoring and modelling

This paper presents a review of the ionospheric scintillation monitoring and modelling by the European groups involved in COST 296. Several of these groups have organized scintillation measurement campaigns at low and high latitudes. Some characteristic results obtained from the measured data are presented. The paper also addresses the modeling activities: four models, based on phase screen techniques, with different options and application domains are detailed. Finally some new trends for research topics are given. This includes the wavelet analysis, the high latitudes analysis, the construction of scintillation maps and the mitigation techniques.

Correlations between Strong Range Spread-F and GPS L-Band Scintillations Observed in Hainan in 2004

Data from the DPS-4 digisonde and the GPS L-band ionospheric scintillation monitor are employed to study the correlations between strong range spread-F (SSF) and GPS L-band scintillations observed in the ionosphere over Hainan Island, China (19.5°N, 109.1°E geogr., dip lat. 9°N) in 2004. The SSF in the ionogram is different from the general range spread-F because it extends in frequency well beyond FoF2 and makes FoF2 difficult to be determined. The observations show that the SSF phenomenon is frequently accompanied by the occurrence of GPS L-band scintillations. The SSF and GPS L-band scintillations occur frequently in the equinoctial months (March, April, September, and October), but rarely in the winter (January, February, November, and December) and summer (May–August); especially, occurrence variations of the SSF and GPS L-band scintillations nearly have a same trend. The SSF and scintillations may be associated with the occurrence of topside plasma bubbles and could be explained by the generalized Rayleigh–Taylor instability.

Space Weather and the Global Positioning System

Space Weather and the Global Positioning System

Monitoring of interplanetary and ionosphere scintillations at frequency 110 MHz

Abstract Large Phased Array of the P.N. Lebedev Physical Institute is the most sensitive in the world at the metre waveband. It allows simultaneous observation of two independent diagrams each consisting of 16 beams. We are planning to introduce a program of monitoring interplanetary and ionospheric scintillation using observations of radio sources by one multi-beam diagram. The methods of observations and data reduction are discussed. We present the first results of seven day test observations for a strip of the sky in a region with declinations between 28° and 32° by using an eight-beam diagram.

Pulsar Coherent De-dispersion Experiment at Urumqi Observatory

A Pulsar coherent de-dispersion experiment has been carried out using the 25-m Nanshan radio telescope at Urumqi Observatory. It uses a dual polarization receiver operating at 18 cm and a VLBI back-end: Mark5A, the minimum sampling time is 5 ns. The data processing system is based on a C program on Linux and a 4-node Beowulf cluster. A high quality data acquisition system and a cluster with more processors are needed to build an on-line pulsar coherent de-dispersion system in future. The main directions for the instrument are studies of pulsar timing, scintillation monitoring, etc.

ISACCO: an Italian project to monitor the high latitudes ionosphere by means of GPS receivers

As the high latitude ionosphere is directly connected with outer space by means of the field line reconnection of the geomagnetic field through the magnetopause, it is highly sensitive to the enhancement of the electromagnetic radiation and energetic particles coming from the Sun. Under such conditions the ionosphere may become highly turbulent showing the presence of small-scale (from centimetres to meters) structures or irregularities imbedded in the large-scale (tens of kilometres) ambient ionosphere. These irregularities can produce short-term phase and amplitude fluctuations in the carrier frequency of the radio waves which pass through them, commonly called ionospheric amplitude and phase scintillations (see, e.g., Morrissey et al. 2004, and references therein). The high latitude ionosphere encounters significant fading, with the most intense fading depths in the polar cap regions and less intense fading in the auroral regions. Severe amplitude fading and strong phase scintillation affect the reliability of GPS navigational systems and satellite communications. As the scarceness of a continuous and systematic monitoring of ionospheric scintillations over polar and auroral regions, the deployment of network(s) of GPS receivers, opportunely configured to observe the ionosphere under quiet and stormy conditions, could represent an important achievement for both space weather purposes and scientific aims. In this paper, a general overview on ISACCO (Ionospheric Scintillations Arctic Campaign Coordinated Observation) is given. ISACCO is an Italian project to monitor ionospheric scintillations at polar regions by means of modified GPS receivers. After some historical and technical notes on the project, the paper presents examples of some of our current investigations based on the data acquired during the almost 3 years of the ISACCO project.

GPS Observations of Ionospheric Effects of the Major Storm of Nov. 7–10, 2004

AbstractThe TEC and amplitude scintillation measurements have been carried out at Wuhan (114.36°E, 30.53°N, magnetic latitude 19.4°) since 2003 using a GPS ionospheric total electron content (TEC) and scintillation monitor to investigate the ionospheric irregularities and TEC disturbances at the region of the equatorial anomaly. In this paper ionospheric effects of the magnetic storm of November, 2004 are analyzed by using GPS observations. Results of data analysis show that the ionospheric storm during this magnetic storm mainly exhibits the long duration positive storm phase around Wuhan, and the positive deviations of TEC occur during two main phases of the storm on November 8 and 10 with the largest deviation of 50 TECU. Another primary effect of the storm on the ionosphere is the remarkably enhanced activity and intensity of amplitude scintillations during the later main phase. The maximum value of the S4 index exceeded 1.0. There occurred the plasma bubbles and TEC fluctuations initiated from the equatorial region at the same time when strong amplitude scintillations occurred. A comparison between amplitude scintillations and TEC fluctuations was made. The statistic analysis shows there is a remarkable positive correlation between standard deviation of the change rate of TEC, the ROTI index, and the S4 index with correlation coefficient of 0.97.

Amplitude and phase scintillation study at Chiang Rai, Thailand

Ionospheric scintillation is a rapid variation of amplitude and phase in radio signals caused by irregularities in the ionosphere. We have studied the effect of ionospheric scintillations on Global Positioning System (GPS) signals from the low latitude station at Chiang Rai, Thailand, and also studied the occurrence of scintillation for geomagnetic-quiet and -disturbed conditions. Amplitude and phase scintillation are investigated by using the single-frequency GPS Ionospheric Scintillation Monitor (GISM) at Chiang Rai (lat. 19.57°N, Ion. 99.52°E). This system is capable of tracking up to 11 GPS satellites at LI frequency of 1575.42 MHz. The purpose of the GISM receiver is to automatically record scintillation parameters of amplitude and phase at a 50 Hz rate averaged over 60 s. The result shows that the amplitude scintillation can occur with or without phase scintillation but phase scintillation is always accompanied by amplitude scintillation.

Ionospheric Scintillation Monitoring and Preliminary Statistic Analysis Over Hainan Region

为开展赤道区的电离层闪烁形态特性及相关物理过程的研究,空间中心海南台站建立了一套GPS电离层闪烁监测系统.该系统是利用Plessey GPS Builder-2系统开发的,对软件的源码进行了修改,使其能以高采样率(50/s)同时并行记录11个通道GPS信号强度数据.对2003年7-12月间L-波段电离层闪烁事件的初步统计分析结果表明,电离层闪烁主要发生在日落后到午夜附近,其中9-11月较7-8月闪烁发生和结束的时间明显提前;电离层闪烁发生的频率和强度在9-11月较其他月份明显增强,其中10月达到最大;电离层闪烁的逐日变化具有很强的随机性,闪烁的发生在秋分附近9月底到10月中旬的磁静日期间达到最大;太阳和地磁活动的增强通常会抑制电离层闪烁的发生,这种情形在秋分附近尤为明显.

Estimating the amplitude scintillation index from sparsely sampled phase screen data

Phase screen techniques are commonly used to model scintillation of radio signals passing through a disturbed ionosphere, but observational phase or in situ density from both archival and real‐time sources is often sampled at rates well below the resolution desired for input to such models. Previous phase screen resolution criteria do not address the computation of the amplitude scintillation index (S4), which is widely used in scintillation monitoring systems, for practical applications that rely on coarsely sampled phase data. We investigate the accuracy of S4 estimates from sparsely populated one‐dimensional phase screen models through systematic removal of samples from high‐resolution discrete phase screen models in novel numerical experiments. We also provide analytic approximations for the weak‐scatter case and study the effects of antialias filtering. This “subsampling” analysis shows that 2–5 samples per Fresnel radius (rF) are usually sufficient to compute S4 to within 90% of its true value, depending on the form of the phase spectral density function (SDF) and the strength of scattering. We make initial application of the subsampling techniques to data‐based phase screens for studying equatorial ionospheric scintillation with generally satisfactory results. Finally, we outline the use of the techniques developed in this paper to other practical problems, including SDF determination and computing irradiance patterns.

Seasonal modulation of GPS performance due to equatorial scintillation

Evening scintillation is an aspect of space weather occurring primarily in the equatorial region due to scattering of satellite signals by ionospheric F‐region irregularities. In order to quantify scintillation‐related performance degradation in GPS (Global Positioning System), we operated single frequency (L1) Ionospheric Scintillation Monitors (ISM), sampling at 50 Hz, at 6 sites in the South East Asia/Oceania region during the most recent solar maximum period 1998–2002. Scintillation indices, horizontal position, receiver lock time and dilution of precision were recorded every minute. Data reveal that individual one‐minute positional errors can increase from a few meters to 30 m during scintillation, whilst statistics at the hourly upper decile level reveal an underlying seasonal modulation from about 5 m to 15 m. Error maxima occur during the equinoxes and are more pronounced at the equatorial anomaly crests than at the magnetic equator, as expected. Such errors may concern users of systems that are dependent on GPS.