Global Positioning System Scintillation Research Articles

Dependencies of GPS Scintillation Indices on the Ionospheric Plasma Drift and Rate of Change of TEC Around the Dawn Sector of the Polar Ionosphere

AbstractThe dependencies of global positioning system (GPS) scintillation indices on ionospheric plasma flow and the rate of change of total electron content (TEC) around the dawn sector for the first time of the polar ionosphere are investigated. The phase scintillation index (σφ) derived from GPS measurements of the Canadian High Arctic Ionospheric Network (CHAIN) shows linear dependencies on both the plasma drift speed measured by the SuperDARN radar and on the rate of change of TEC estimated from the GPS receivers of CHAIN. However, the amplitude scintillation index (S4) does not show any dependence on the plasma flow or the rate of change of TEC. These results further support Wang et al. (2018), https://doi.org/10.1002/2017JA024805 at the noon sector. The dependence of the phase scintillation index on the plasma flow further evidences that the standard phase scintillation index is dominated by refractive variations due to the use of a fixed cut‐off frequency of 0.1 Hz while detrending the phase observable. The dependence of the phase scintillation index on the rate of change of TEC consolidates the dominance of refractive variations inside.

Inferring the Horizontal Speed of an Ionospheric Irregularity From a Single GPS Scintillation Receiver at High Latitudes

AbstractWe propose a new technique to infer the horizontal speed of ionospheric irregularity at high latitudes from the refractive components of the global positioning system (GPS) L1 (1575.42 MHz) and L2 (1227.60 MHz) carrier phases using only a single ground‐based receiver that can log measurements at 50–100 Hz. The test data at Jang Bogo Station (geographic: 74.6°S and 164.2°E; geomagnetic: 77.2°S) in Antarctica and at Sachs Harbor (geographic: 71.9°N and 234.7°E; geomagnetic: 75.6°N) in the Arctic were analyzed for a one‐year period in 2019. The results were compared with data from ground‐based radar observations, the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) in Antarctica and the Canadian Advanced Digital Ionosonde (CADI) in the Arctic, which measures the ion drift velocity at the apex. Although it is difficult to directly compare GPS and radar measurements due to the steep gradient of fast plasma motion in narrow regions at high latitudes, the probability density of the ionospheric irregularity speeds from the two different instruments were consistent with the correlation coefficients of 0.81 and 0.85 in the Southern and Northern hemispheres.

Extreme Low‐Latitude Total Electron Content Enhancement and Global Positioning System Scintillation at Dawn

AbstractWe report on an extreme ionospheric plasma density enhancement and Global Positioning System (GPS) scintillation at dawn, observed within the expanding equatorial ionization anomaly (EIA). The total electron content (TEC) in central America reached 50 TECu at sunrise, the value almost twice as high as the normal afternoon peak. The enhanced EIA expanded poleward and westward from just below 20° magnetic latitude (MLAT) to beyond 30° MLAT at sunrise. The chief ramification of the enhanced EIA was strong GPS scintillation which was observed poleward of 30° northern MLAT and lasted until 8:00 local time. In total, the amplitude scintillation and phase fluctuations lasted for ∼5 h at latitudes north of 20°MLAT in central America.

Influence of different types of ionospheric disturbances on GPS signals at polar latitudes

Abstract. The comparative research of the influence of different types of auroral particle precipitation and polar cap patches (PCPs) on the global positioning system (GPS) signals disturbances in the polar ionosphere was done. For this purpose, we use the GPS scintillation receivers at Ny-Ålesund and Skibotn, operated by the University of Oslo. The presence of the auroral particle precipitation and polar cap patches was determined by using data from the EISCAT 42m radar on Svalbard. The optical aurora observations in 557.7 and 630.0 nm spectrum lines on Svalbard were used as well for the detection of ionospheric disturbances. The cusp identification was done with using SuperDARN (Hankasalmi) data. We consider events when the simultaneous EISCAT 42m and GPS data were available for the years 2010–2017, and in this paper we present, in detail, typical examples describing the overall picture, and we present the statistics for 120 events. We considered the dayside/cusp precipitation, substorm precipitation, daytime and nighttime PCPs, and precipitation associated with the interplanetary shock wave arrival. We demonstrate that substorm-associated precipitation (even without PCPs) can lead to a strong GPS phase (σϕ) scintillations up to ∼ 1.5–3 radians, which is much stronger than those usually produced by other types of considered ionosphere disturbances. The value of the substorm-phase scintillations in general correlate with the value of the geomagnetic field disturbance. But sometimes even a small geomagnetic substorm, when combined with the PCPs, produces quite strong phase scintillations. Cusp phase scintillations are lower than dayside PCPs scintillations. PCPs can lead to stronger ROT (rate of total electron content) variations than other types of ionosphere disturbances. So our observations suggest that the substorms and PCPs, being different types of the high-latitude disturbances, lead to the development of different types and scales of ionospheric irregularities.

Ionospheric irregularities and scintillations: a direct comparison of in situ density observations with ground-based L-band receivers

Ionospheric irregularities can affect satellite communication and navigation by causing scintillations of radio signals. The scintillations are routinely measured using ground-based networks of receivers. This study presents observations of ionospheric irregularities by Langmuir probes on the Swarm satellites. They are compared with amplitude scintillation events recorded by the Global Positioning System-Scintillation Network and Decision Aid (GPS-SCINDA) receiver installed in Mbarara (Lat: 0.6^{circ }hbox {S}, Lon: 30.8^{circ }hbox {E} , Mag. lat: 10.2^{circ }hbox {S}). The study covers the years from 2014 to 2018 when both data sets were available. It was found that the ground-based amplitude scintillations were enhanced when Swarm registered ionospheric irregularities for a large number of passes. The number of matching observations was greater for Swarm A and C which orbited at lower altitudes compared to Swarm B. However, some counterexamples, i.e., cases when in situ electron density fluctuations were not associated with any observed L-band amplitude scintillation and vice versa, were also found. Therefore, mismatches between observed irregularity structures and scintillations can occur just over a few minutes and within distances of a few tens of kilometers. The amplitude scintillation strength, characterized by the S4 index was estimated from the electron density data using the well-known phase screen model for weak scattering. The derived amplitude scintillation was on average lower for Swarm B than for A and C and less in accordance with the observed range. Irregularities at an altitude of about 450 km contribute strongly to scintillations in the L-band, while irregularities at about 510-km altitude contribute significantly less. We infer that in situ density fluctuations observed on passes over or near Mbarara may be used to indicate the risk that ionospheric radio wave scintillations occur at that site.

Geomagnetically Conjugate Observations of Equatorial Plasma Irregularities From Swarm Constellation and Ground‐Based GPS Stations

AbstractThe near‐polar orbit satellites of Swarm mission provide a good opportunity to investigate the conjugacy of equatorial plasma irregularities (EPIs) since their trajectories at low latitudes are basically aligned with fixed geographical longitude. However, the Swarm in situ electron density occasionally shows EPIs at only one hemisphere at this longitude. In this study, we provide detailed analysis of such EPI events from the in situ electron densities and onboard global positioning system (GPS) measurements of Swarm low pair satellites, and simultaneous GPS data from two geomagnetically conjugate ground stations at the Africa longitudes. The result indicates that when Swam in situ electron density sometime shows EPIs at only one hemisphere, the GPS scintillations are still observed from the Swarm onboard receiver and by the two conjugate ground stations. It implies that the EPIs should generally elongate along the geomagnetic flux tube. More than two‐year statistic results show that the onset time of scintillation in the northern station is on average 16 and 18 min earlier than that in the southern station for September equinox and December solstice in 2015, while for March equinox in 2016 the onset time of scintillation of northern station is about 11 min later than that of southern station, which indicates the asymmetry features of EPIs along the flux tube. Further analysis of nearly three‐year GPS data from two conjugate stations at the Asia longitudes, we find that during solar maximum years the local sunset time plays an important role for causing the difference of onset time of scintillation between two conjugate stations.

GPS Scintillations and Losses of Signal Lock at High Latitudes During the 2015 St. Patrick's Day Storm

AbstractWe investigate the Global Positioning System (GPS) amplitude and phase scintillations during a severe geomagnetic storm on 17 March 2015. The auroral oval expanded significantly due to a strongly southward interplanetary magnetic field (Bz was −25 nT). When the auroral oval was over Skibotn in northern Norway, significant enhancements in total electron content (TEC) fluctuations, amplitude, and phase scintillation were observed. The strongest amplitude and phase scintillations were observed when a TEC blob propagated across the field of view. Strong amplitude and phase scintillations were observed near the edges of the TEC blob. The European Incoherent SCATter ultrahigh frequency radar observed significant enhancement of electron density (from 0.8 × 1011 to 1.6 × 1011 m−3) near the edge of the TEC blob in the F2 region, while the E region was only slightly enhanced. This indicates that the plasma processes and instability modes, which accounted for the strong GPS scintillations, should involve the F2 region ionosphere. We also analyzed the tracking performance of the GPS receiver during strong ionospheric scintillation condition. While the receiver maintained tracking of the GPS L1 signal, the strong amplitude scintillation resulted in a power fade up to 12 dB‐Hz. Losses of lock occurred in the GPS L2 band. Both the power fade and rapid phase fluctuation should contribute to losses of lock.

Observational study of ionospheric irregularities and GPS scintillations associated with the 2012 tropical cyclone Tembin passing Hong Kong

AbstractThis study presents the ionospheric responses observed in Hong Kong to a Typhoon, namely, Tembin, from the aspects of the occurrence of ionospheric irregularities and scintillations, using Global Positioning System (GPS) observations from a ground‐based GPS scintillation monitoring station in Hong Kong and from GPS receivers on board the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites. The ionospheric irregularities and scintillations are characterized by the rate of total electron content variation index (ROTI) and the amplitude scintillation index S4, respectively. The typhoon Tembin formed over the western North Pacific during 18–30 August 2012 and approached Hong Kong during 24–27 August 2012 with the closest distance 290 km from Hong Kong at around 17 universal time (UT) on 25 August 2012. The ground‐based observations indicate that in the nighttime period of 20:00–02:00 local time (LT = UT + 8 h) on 26 August when Tembin passed closely to Hong Kong, the ionospheric irregularities and scintillations of GPS signals were observed in the south of Hong Kong, over the area of 13°N ~ 23°N in latitude and 110°E ~ 120°E in longitude. From the COSMIC observations, it shows that the number of radio occultation scintillation events peaks on 26 August 2012 during the passage of Tembin. Without the presence of strong geomagnetic or solar activity, it is suspected that gravity waves might be generated in the lower atmosphere and likely seed the formation of ionospheric plasma irregularities. This work for the first time from Hong Kong observes the sign of coupling between the lower atmosphere and ionosphere in a tropical cyclone event, combining both ground‐ and space‐based GPS observation data.

GPS scintillations in the high latitudes during periods of dayside and nightside reconnection

AbstractWe use an automated procedure to identify periods of enhanced dayside reconnection followed by enhanced nightside reconnection in measurements of the polar cap size by the Active Magnetosphere and Planetary Electrodynamics Response Experiment between January 2010 and December 2012; we find 490 such events. We investigate the dynamics of the spatial distributions of the total electron content (TEC) and phase scintillations of Global Positioning System (GPS) signals across the northern polar region and here report three important findings: (1) While a TEC enhancement (due to polar cap patches) propagates across the polar cap during these events, this enhancement is not associated with significant GPS phase scintillations. (2) Instead, a significant impact on GPS signal quality is first found when the TEC enhancements cross the nightside auroral boundary. (3) In combination with upward field‐aligned currents, these TEC enhancements cause the strongest GPS phase scintillations. We conclude that polar cap patches are not, as previously thought, a space weather threat inside the polar cap but instead reveal their biggest impact once they reach the nightside auroral oval, in particular when combined with upward field‐aligned currents.

A comparative study of GPS ionospheric scintillations and ionogram spread F over Sanya

Abstract. We analyze the data recorded during December 2011–November 2012 by a digital ionosonde and a GPS (Global Positioning System) scintillation and (total electron content) TEC receiver collocated at Sanya (109.6° E, 18.3° N; dip lat. 12.8° N), a low-latitude station in the Chinese longitude sector, to carry out a comparative study of ionospheric scintillations and spread F. A good consistency between the temporal variations of GPS scintillation (represented by the S4 index) and of ionogram spread F (represented by the QF index) is found in the pre-midnight period during equinox. However in the post-midnight period during equinox and in the period from post-sunset to pre-sunrise during June solstice, moderate spread F is seen without concurrent GPS scintillation. The possible cause responsible for the difference between post-midnight GPS scintillation and spread F during equinox could be due to the decaying of 400 m scale irregularities associated with equatorial spread F. Regarding the irregularities producing moderate QF and low S4 indices during June solstice, we suggest that the frequently observed sporadic E (Es) layer and the medium-scale traveling ionospheric disturbances (MSTIDs) over Sanya could play important roles in triggering the June solstitial spread-F events.

Climatology of GPS phase scintillation at northern high latitudes for the period from 2008 to 2013

Abstract. Global positioning system scintillation and total electron content (TEC) data have been collected by ten specialized GPS Ionospheric Scintillation and TEC Monitors (GISTMs) of the Canadian High Arctic Ionospheric Network (CHAIN). The phase scintillation index σΦ is obtained from the phase of the L1 signal sampled at 50 Hz. Maps of phase scintillation occurrence as a function of the altitude-adjusted corrected geomagnetic (AACGM) latitude and magnetic local time (MLT) are computed for the period from 2008 to 2013. Enhanced phase scintillation is collocated with regions that are known as ionospheric signatures of the coupling between the solar wind and magnetosphere. The phase scintillation mainly occurs on the dayside in the cusp where ionospheric irregularities convect at high speed, in the nightside auroral oval where energetic particle precipitation causes field-aligned irregularities with steep electron density gradients and in the polar cap where electron density patches that are formed from a tongue of ionization. Dependences of scintillation occurrence on season, solar and geomagnetic activity, and the interplanetary magnetic field (IMF) orientation are investigated. The auroral phase scintillation shows semiannual variation with equinoctial maxima known to be associated with auroras, while in the cusp and polar cap the scintillation occurrence is highest in the autumn and winter months and lowest in summer. With rising solar and geomagnetic activity from the solar minimum to solar maximum, yearly maps of mean phase scintillation occurrence show gradual increase and expansion of enhanced scintillation regions both poleward and equatorward from the statistical auroral oval. The dependence of scintillation occurrence on the IMF orientation is dominated by increased scintillation in the cusp, expanded auroral oval and at subauroral latitudes for strongly southward IMF. In the polar cap, the IMF BY polarity controls dawn–dusk asymmetries in scintillation occurrence collocated with a tongue of ionization for southward IMF and with sun-aligned arcs for northward IMF. In investigating the shape of scintillation-causing irregularities, the distributions of scintillation occurrence as a function of "off-meridian" and "off-shell" angles that are computed for the receiver–satellite ray at the ionospheric pierce point are found to suggest predominantly field-aligned irregularities in the auroral oval and L-shell-aligned irregularities in the cusp.

A Study Of Ionospheric GPS Scintillation During Solar Maximum at UTeM Station

Wireless signals propagated along global positioning system (GPS) channel are affected by ionospheric electron density irregularities such that GPS signals may experience amplitude and phase fluctuations. The global navigation satellite system (GNSS), ionospheric scintillation, and total electron content (TEC) monitor (GISTM) receiver has been installed at UTeM, Malaysia (2.3139°N, 102.3183°E) for monitoring ionospheric scintillation at several frequencies. In this paper, the GPS ionospheric scintillations are concerned for the dual frequency L1 (fL1 = 1.57542 GHz) and L2C (fL2= 1.2276 GHz). Ionospheric scintillation data has been collected during solar maximum cycle 2013-2014 for six months October 2013–March 2014. Solar activities significantly impact the ionospheric GPS scintillation, especially in the equatorial region where Malaysia is located. The GPS link is analyzed to investigate how the scintillation increases during the solar maximum cycle. When the sun flux is maximum, the total of electrons is increased in the ionospheric layer and the scintillation values gradually become high. The ionospheric amplitude/phase scintillation, carrier-to-noise (C/No) ratio, and availability of GPS satellites are reported in the proposed experimental GPS model. Consequently, for Malaysia, typical threshold received C/No ratio is 43 dB-Hz, implying that C/No ratio should be greater than 43 dB-Hz to receive good signals at the GPS receiver.

Statistical characteristics of low-latitude ionospheric scintillation over China

The Global Positioning System (GPS) L-band ionospheric scintillation produced by electron density irregularities in the ionospheric E- and F-regions, is mainly a low- and high-latitude phenomenon. In this study, the statistical behavior of GPS ionospheric scintillation over a Chinese low-latitude station Sanya (18.3°N, 109.6°E; dip lat: 12.8°N) has been investigated. A detailed study on the seasonal and solar activity dependence of scintillation occurrence during July 2004–December 2012 show that the amplitude scintillation pattern, with a maximum occurrence during equinox of solar maximum, agrees with plasma bubble observations by in situ satellites in this longitude. A few daytime periodic scintillation events are found during June solstice months of solar minimum. Interestingly, a significant equinoctial asymmetry of scintillation onset time is found in 2011–2012. The initiation of scintillation during September–October is on average earlier than that of March–April about 25min. Meanwhile, the zonal drifts of irregularities estimated using two spatially separated GPS receivers over Sanya show a similar behavior during the two equinoxes, slowly decreasing from 150m/s at post-sunset to 50m/s near midnight. The possible mechanisms responsible for the occurrence characteristics of GPS scintillation over Sanya, and relevant aspects of the zonal drifts of the irregularities are discussed.

Ionospheric irregularities during a substorm event: Observations of ULF pulsations and GPS scintillations

Plasma instability in the ionosphere is often observed as disturbances and distortions of the amplitude and phase of the radio signals, which are known as ionospheric scintillations. High-latitude ionospheric plasma, closely connected to the solar wind and magnetospheric dynamics, produces very dynamic and short-lived Global Positioning System (GPS) scintillations, making it challenging to characterize them. It is observed that scintillations in the high-latitude ionosphere occur frequently during geomagnetic storms and substorms. In addition, it is well known that Ultra Low Frequency (ULF) pulsations (Pi2 and Pi1B) are closely associated with substorm activity. This study reports simultaneous observations of Pi2 and Pi1B pulsations and GPS phase scintillations during a substorm using a newly designed Autonomous Adaptive Low-Power Instrument Platform (AAL-PIP) installed at the South Pole. The magnetic field and GPS data from the instruments appear to be associated in terms of their temporal and spectral features. Moreover, the scintillation events were observed near the auroral latitudes where Pi1B pulsations are commonly detected. The temporal, spectral and spatial association between the scintillation and geomagnetic pulsation events suggests that the magnetic field perturbations and enhanced electric fields caused by substorm currents could contribute to the creation of plasma instability in the high-latitude ionosphere, leading to GPS scintillations.

Observations of GPS scintillation during an isolated auroral substorm

This paper reports simultaneous observations of ionospheric scintillation during an auroral substorm that were made using an all-sky full-color digital single-lens reflex (DSLR) camera (ASC) and a Global Positioning System (GPS) ionospheric scintillation and total electron content monitor (GISTM) in Tromsø (69.60 N, 19.20 E), Norway. On the night of November 19, 2009, a small substorm occurred in northern Scandinavia. The ASC captured its temporal evolution from the beginning of the growth phase to the end of the recovery phase. The amplitude scintillation, as monitored by the S4 index from the GISTM, did not increase in any substorm phase. By contrast, phase scintillation, as measured by the σ φ index, occurred when discrete auroral arcs appeared on the GPS signal path. In particular, the phase scintillation was significantly enhanced for a few minutes immediately after the onset of the expansion phase. During this period, bright and discrete auroral forms covered the entire sky, which implies that structured precipitation on the scale of a few kilometers to a few tens of kilometers dominated the electron density distribution in the E region. Such inhomogeneous ionization structures probably produced significant changes in the refractive index and eventually resulted in the enhancement of the phase scintillation.

GPS phase scintillation associated with optical auroral emissions: First statistical results from the geographic South Pole

Ionospheric irregularities affect the propagation of Global Navigation Satellite System (GNSS) signals, causing radio scintillation. Particle precipitation from the magnetosphere into the ionosphere, following solar activity, is an important production mechanism for ionospheric irregularities. Particle precipitation also causes the aurorae. However, the correlation of aurorae and GNSS scintillation events is not well established in literature. This study examines optical auroral events during 2010–2011 and reports spatial and temporal correlations with Global Positioning System (GPS) L1 phase fluctuations using instrumentation located at South Pole Station. An all‐sky imager provides a measure of optical emission intensities ([OI] 557.7 nm and 630.0 nm) at auroral latitudes during the winter months. A collocated GPS antenna and scintillation receiver facilitates superimposition of auroral images and GPS signal measurements. Correlation statistics are produced by tracking emission intensities and GPS L1 σφ indices at E and F‐region heights. This is the first time that multi‐wavelength auroral images have been compared with scintillation measurements in this way. Correlation levels of up to 74% are observed during 2–3 hour periods of discrete arc structuring. Analysis revealed that higher values of emission intensity corresponded with elevated levels of σφ. The study has yielded the first statistical evidence supporting the previously assumed relationship between the aurorae and GPS signal propagation. The probability of scintillation‐induced GPS outages is of interest for commercial and safety‐critical operations at high latitudes. Results in this paper indicate that image databases of optical auroral emissions could be used to assess the likelihood of multiple satellite scintillation activity.

Total electron content of the ionosphere at two stations in East Africa during the 24–25 October 2011 geomagnetic storm

The equatorial ionosphere has been known to become highly disturbed and thus rendering space-based navigation unreliable during space weather events, such as geomagnetic storms. Modern navigation systems, such as the Global Positioning System (GPS) use radio-wave signals that reflect from or propagate through the ionosphere as a means of determining range or distance. Such systems are vulnerable to effects caused by geomagnetic storms, and their performance can be severely degraded. This paper analyses total electron content (TEC) and the corresponding GPS scintillations using two GPS SCINDA receivers located at Makerere University, Uganda (Lat: 0.3o N; Lon: 32.5o E) and at the University of Nairobi, Kenya (Lat: 1.3o S; Lon: 36.8o E), both in East Africa. The analysis shows that the scintillations actually correspond to plasma bubbles. The occurrence of plasma bubbles at one station was correlated with those at the other station by using observations from the same satellite. It was noted that some bubbles develop at one station and presumably “die off” before reaching the other station. The paper also discusses the effects of the geomagnetic storm of the 24–25 October 2011 on the ionospheric TEC at the two East African stations. Reductions in the diurnal TEC at the two stations during the period of the storm were observed and the TEC depletions observed during that period showed much deeper depletions than on the non-storm days. The effects during the storm have been attributed to the uplift of the ionospheric plasma, which was then transported away from this region by diffusion along magnetic field lines.

Initial GPS scintillation results from CASES receiver at South Pole, Antarctica

Connected Autonomous Space Environment Sensor (CASES) Global Positioning System (GPS) software‐defined receivers developed for ionospheric scintillation studies have been deployed on Autonomous Adaptive Low‐Power Instrument Platforms (AAL‐PIP) at South Pole, Antarctica. In this paper, we describe the AAL‐PIP experimental setup focusing on CASES. We explain in detail the method developed for analyzing CASES data, and report initial AAL‐PIP CASES results. Furthermore, we compare the CASES measurements with those from a modified Novatel GSV4004 GPS Ionospheric Scintillations and TEC Monitor (GISTM) receiver at the South Pole. CASES receivers have been successfully deployed and reliably operated in equatorial and midlatitude regions. Four of these GPS receivers, for the first time, are deployed in high‐latitude regions as a part of the National Science Foundation (NSF) funded project of deploying space science instrument platforms, AAL‐PIPs, in Antarctica since December 2010–2011. We present initial scintillation results recorded by a CASES receiver at South Pole during the storm on 24 January 2012 along with AAL‐PIP magnetometer observations. We have deduced that the CASES receiver scintillation observations agree with those from the Novatel GPS scintillation receiver. Since this is the first time a CASES receiver has been deployed to operate in a high latitude, low temperature, and low humidity environment, we consider this comparison a demonstration of its reliable operation as a science‐grade scintillation receiver in such conditions. We plan to study high latitude ionospheric irregularities by using observations from CASES and other ancillary instruments from Antarctica coupled with physical parameters derived from models.

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

[1] 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 Sao Luis 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 Sao Luis GPS scintillation monitor for an assessment of the prediction capability of the present formulation.

First evidence of anisotropy of GPS phase slips caused by the mid-latitude field-aligned ionospheric irregularities

The mid-latitude field-aligned irregularity (FAI) along the magnetic field line is a common phenomenon in the ionosphere. However, few data reveal the field-aligned ionospheric irregularities. They are insufficient to identify FAIs effects so far, particularly effect on global positioning system (GPS) signals. In this paper, the mid-latitude FAIs by line-of-sight angular scanning relative to the local magnetic field vector are investigated using the denser GPS network observations in Japan. It has been the first found that total GPS L2 phase slips over Japan, during the recovery phase of the 12 Feb 2000 geomagnetic storm were caused by GPS signal scattering on FAIs both for the lines-of-sight aligned to the magnetic field line (the field of aligned scattering, FALS) and across the magnetic field line (the field of across scattering, FACS). The FALS results are also in a good agreement with the data of the magnetic field orientation control of GPS occultation observations of equatorial scintillation during thorough low earth orbit (LEO) satellites measurements, e.g. Challenging Minisatellite Payload (CHAMP) and Satellite de Aplicaciones Cientificas-C (SAC-C). The role of large-angle scattering almost along the normal to the magnetic field line in GPS scintillation is determined by attenuation of the irregularity anisotropy factor as compared with the other factors.