Global Positioning System Scintillation Research Articles

The Remote Equatorial Nighttime Observatory of Ionospheric Regions Project and the International Heliospherical Year

We describe a new suite of instruments planned for deployment to Cape Verde as part of the International Heliospherical Year. The Remote Equatorial Nighttime Observatory of Ionospheric Regions (RENOIR) project consists of a bistatic Fabry–Perot interferometer system, an all-sky imaging system, a dual-frequency Global Positioning System (GPS) receiver, and an array of single-frequency GPS scintillation monitors. This instrumentation will allow for studying the low-latitude thermosphere/ionosphere (TI) system in great detail. Investigations to be conducted using this instrumentation while in Cape Verde include studying equatorial irregularity processes, the effects of neutral winds and gravity waves on irregularity development, the midnight temperature maximum, and ion-neutral coupling in the nighttime TI system. Initial observations from the RENOIR instrumentation during pre-deployment testing at the Urbana Atmospheric Observatory are presented, as is the deployment scenario for the project in Cape Verde.

GPS scintillation over the European Arctic during the November 2004 storms

Small-scale irregularities in the background electron density of the ionosphere can cause rapid fluctuations in the amplitude and phase of radio signals passing through it. These rapid fluctuations are known as scintillation and can cause a Global Positioning System (GPS) receiver to lose lock on a signal. This could compromise the integrity of a safety of life system based on GPS, operating in auroral regions. In this paper, the relationship between the loss of lock on GPS signals and ionospheric scintillation in auroral regions is explored. The period from 8 to 14 November 2004 is selected for this study, as it includes both geomagnetically quiet and disturbed conditions. Phase and amplitude scintillation are measured by GPS receivers located at three sites in Northern Scandinavia, and correlated with losses of signal lock in receivers at varying distances from the scintillation receivers. Local multi-path effects are screened out by rejection of low-elevation data from the analysis. The results indicate that losses of lock are more closely related to rapid fluctuations in the phase rather than the amplitude of the received signal. This supports the idea, suggested by Humphreys et al. (2005) (performance of GPS carrier tracking loops during ionospheric scintillations. Proceedings Internationsl Ionospheric Effects Symposium 3–5 May 2005), that a wide loop bandwidth may be preferred for receivers operating at auroral latitudes. Evidence from the Imaging Riometer for Ionospheric Studies (IRIS) appears to suggest that, for this particular storm, precipitation of particles in the D/E regions may be the mechanism that drives the rapid phase fluctuations in the signal.

Scattering height estimation using scintillating Wide Area Augmentation System/Satellite Based Augmentation System and GPS satellite signals

An experiment to measure equatorial amplitude scintillations on the geostationary Wide Area Augmentation System (WAAS) Satellite Based Augmentation System (SBAS) signal was conducted in Cachoeira Paulista (22.70°S, 45.01°W geographic coordinates; −17.74°N, 21.74°E geomagnetic coordinates), Brazil from December 2003 through February 2004. The purpose of this paper is to estimate the scattering height of the irregularities using the WAAS signal scintillations as compared to nearby Global Positioning System (GPS) signal scintillations. Estimating the scattering height is important because the calculated zonal drift velocity of the irregularities using the measured scintillation pattern velocity on the ground is height dependent. Accurate height estimation is also required if one wishes to develop phase screen scintillation models. The difference in the pattern velocities is due to the different signal puncture point velocities with respect to the ionospheric drift. Two east‐west receivers are used to measure the scintillation pattern drift velocity and to compare the results of the geostationary WAAS satellite signal to that of a GPS satellite signal, which has a nonzero ionospheric signal puncture point velocity. By varying the assumed scattering height for the measurements from the nearby GPS signal, the zonal velocity measurements from the GPS scintillations can be matched to those of the WAAS scintillations, and a scattering height estimate can be made. When the puncture points have minimal separation, the inferred ionospheric irregularity zonal velocities should be equal. On the two nights for which data are available, scattering height estimates of 669 ± 209 km for the first night and 388 ± 139 km for the second night were obtained. On the second night, the reported mean hmF2 as calculated using a collocated Digisonde was 385 ± 17 km over the same period as the GPS/WAAS scattering height estimate. The geometry of this experiment was not optimal, but analysis demonstrates that future experiments could be substantially more accurate.

GPS scintillation channel model for the disturbed low‐latitude ionosphere

A Global Positioning System (GPS) scintillation channel model for the disturbed low‐latitude ionosphere is developed by combining first‐principles physics‐based simulated low‐latitude ionospheric irregularities together with an electromagnetic wave propagation model. Using this combined model, basic channel parameters, that is, phase variance, decorrelation length, decorrelation time, and coherence bandwidth, are computed and are favorably compared with recent ground‐based low‐latitude GPS experimental observations.

Perils of the GPS phase scintillation index (σϕ)

The phase scintillation index (σϕ), equal to the standard deviation of measured phase, is often used to characterize Global Positioning System (GPS) observations in ionospheric environments that may be scintillated. Since σϕ is dominated by large‐scale fluctuations, questions of usage and interpretation exist as will be illustrated here. Beyond traditional concerns with detrending, multipath and receiver phase noise, there are at least two issues to be considered. The first is the marginal suitability of σϕ to characterize a power law phase screen with a poorly defined low‐frequency component (e.g., outer scale). Second, observed σϕ parameters may not be relevant to GPS receiver tracking impacts. These arguments are outlined here in greater detail and are illustrated with simple one‐dimensional phase screen propagation modeling results. The conclusion is that GPS σϕ values depend critically on the circumstances of measurement and are difficult to compare among observations without additional knowledge, particularly of relative ionospheric drift and irregularity orientation, that may not be available from an isolated GPS receiver. The development of suitable alternative measures requires careful consideration of the elements of GPS scintillation and its impacts. The broader GPS scintillation community should take an active role in developing suitable replacement measures for σϕ.

Equatorial anomaly effects on GPS scintillations in brazil

In a collaborative study, INPE and Cornell University have installed several Global Positioning System (GPS) based scintillation monitors over the Brazilian territory in order to study L Band scintillation. These scintillation monitors were developed by Cornell University to measure the amplitude scintillation observed at L1 (1.575 GHz) GPS signal and are sensitive to ionospheric irregularities of about 400 meters scale size. This paper describes some characteristics of the intensity of scintillations observed at three observation sites in Brazil: (1) São Luís (2.33 ° S, 44 ° W, dip latitude 1.3 ° S), located at magnetic equator, (2) São José dos Campos (23.21 ° S, 45.86 ° W, dip latitude 17.8 ° S), located under the equatorial anomaly peak and (3) Cuiabá (15.33 ° S, 56.46 ° W, dip latitude 6.1 ° S), an intermediate observation site located in between the magnetic equator and the equatorial anomaly peak. Analysis of data from January to March of 2000 showed that the occurrence percentage as well as the magnitude of the L Band scintillation increase with latitude from the magnetic equator to the equatorial anomaly crest as previously reported by Basu et al. (1988). Strong scintillation with S 4 index exceeding 0.5 only has been observed under equatorial anomaly peak while at magnetic equator scintillation intensity (S 4 index) did not exceed 0.3. Such studies from the network of stations set up by INPE and Cornell University in Brazil, where the effect of large declination controls the ESF statistics, will be very useful for developing a regional scintillation model for use in IRI.

Development and use of a GPS ionospheric scintillation monitor

Global Positioning System (GPS) satellite signals provide convenient radio beacons for ionospheric studies. Among other propagation phenomena, the ionosphere affects GPS signal propagation through amplitude scintillations that develop after radio waves propagate through ionospheric electron density irregularities. This paper outlines the design, testing, and operation of a specialized GPS receiver to monitor L-band amplitude scintillations: the Cornell scintillation monitor. The Cornell scintillation monitor consists of a commercial GPS receiver development kit with its software modified to log signal strength from up to 12 channels at a high data rate (50 samples/s). Other features of the receiver include the optional assignment of a channel to monitor the receiver noise level in the absence of signal tracking and the means to synchronize measurements between nearby independent receivers to perform drift measurements and correlation studies. The Cornell scintillation monitor provides characterization of the operational L-band scintillation environment and additionally permits study of the multipath environment of a static antenna, GPS scintillation monitors can provide information about the state of ionospheric irregularities for pure research purposes as well. Their strength lies in the fact that they are inexpensive and compact and therefore can he readily proliferated. Even a single scintillation monitor can supplement radar spatial coverage of irregularities in a limited way because it monitors several satellite lines of sight simultaneously.

Nighttime equatorial ionosphere: GPS scintillations and differential carrier phase fluctuations

The presence of scintillation‐producing irregularities in the nighttime equatorial ionosphere, in the path of Global Positioning System (GPS) signals received at an equatorial station, causes dual‐frequency measurements of the differential carrier phase of GPS L1 and L2 signals to have a contribution from phase scintillations on the two signals. Dual‐frequency data for fluctuations in the total electron content (TEC) along the path of GPS signals to the equatorial station Ancon (1.5° dip), sampled at a rate of 1 Hz, are used to separate this contribution from the slower TEC variations. Rapid fluctuations in the differential carrier phase, usually on timescales < 100 s, which result from diffraction, are seen to follow the pattern of intensity scintillations on the L1 signal. Intensity scintillations are also related to the variations in TEC which arise from density fluctuations associated with ionospheric irregularities. An approximate version of the transport‐of‐intensity equation, based on a phase screen description of the irregularities, suggests that a quantitative measure of intensity scintillations may be provided by the derivative of rate of change of TEC index (DROTI), obtained from the second derivative of TEC. This equation also yields the dependence of the scaling factor between DROTI and S4 on the Fresnel frequency. Comparison of DROTI computed from relative TEC data to corresponding S4 indices indicates that there may be lesser uncertainity in a quantitative relation between the two than between the index ROTI, introduced in recent years, and S4. Power spectral analysis of TEC fluctuations and simultaneous intensity scintillations on L1 signal, recorded at Ancon, does not indicate any simple dependence of the scaling factor between DROTI and S4 on the spectral characteristics.

Characteristics of plasma structuring in the cusp/cleft region at Svalbard

Satellite scintillation, all‐sky optical imager, and digisonde observations were coordinated during a cusp campaign conducted at Ny Alesund, Svalbard (78.9°N, 11.8°E 75.7°N corrected geomagnetic latitude, over the period January 4–15, 1997. This paper is focused on a study of the distribution and dynamics of mesoscale (tens of kilometers to tens of meters) electron density irregularities in the dayside auroral region. This study has been performed at Ny Alesund, Svalbard, by measuring the effects of these irregularities on the amplitude scintillation of 250‐MHz transmissions from a quasi‐stationary polar satellite as well as the amplitude and phase scintillation of 1.6‐GHz signals from Global Positioning System (GPS) satellites. These GPS scintillation measurements were augmented by the use of dual‐frequency (1.2 and 1.6 GHz) GPS phase data acquired at the same station by the Jet Propulsion Laboratory for the International GPS Geodynamic Service (IGS). The continuous 250‐MHz scintillation observations explored the daytime auroral ionosphere 2° poleward of Ny Alesund and showed that the scintillation spectra are often broad, as may be expected for irregularities in a turbulent flow region. Such irregularity dynamics were detected poleward of the nominal cusp region over the interval of 0600–1500 magnetic local time. The period of observations included the magnetic storm of January 10–11, 1997, when GPS observations of the IGS detected polar cap patches with total electron contents of 3×1016 m−2 and large‐scale (tens of kilometers) phase variations at the GPS frequency of 1.6 GHz that corresponded to temporal gradients of 2×1016 m−2 min−1. However, amplitude scintillations at the GPS frequency of 1.6 GHz could not be detected in association with these large‐scale phase variations, indicating that the irregularities with wavelengths less than the Fresnel dimension of 400 m were below the detectable limit. This is shown to be consistent in the context of enhanced ionospheric convection determined by digisonde and scintillation spectra.