Eurico R. de Paula

Fading timescales associated with GPS signals and potential consequences

The effect of equatorial ionospheric scintillations on the operation of GPS receivers is investigated, with special attention given to the effect of scintillation timescales on the code division multiple access (CDMA) protocol used by GPS. We begin by examining the timescales of scintillation fades modeled as a horizontally drifting pattern whose timescales are determined by the Fresnel length and the drift speed. The model is tested by comparing the speed, determined by dividing the Fresnel length by the autocorrelation time (width), with the speed estimated using spaced receivers, and the two independent estimates of speed are shown to possess a linear relationship. Next we show that the scintillation pattern drift speed is given by the difference of the ionospheric drift and the speed of the GPS signal F region puncture point. When the ionosphere and GPS signal puncture point speeds match, the fade timescales lengthen. Additionally, if the fade depth is adequate, during periods of longer fade times the loss of receiver lock on GPS signals is more likely, as shown in several examples; that is, both larger fade depths and longer fade timescales are required to produce loss of tracking. We conclude by demonstrating that speed matching or resonance between the ionosphere and receiver is most likely when the receiver is moving from west to east at speeds of 40–100 m/s (144–360 km/h). This is in the range of typical aircraft speeds.

Global Positioning System measurements of the ionospheric zonal apparent velocity at Cachoeira Paulista in Brazil

Ionospheric irregularities and their zonal apparent drift were studied using Global Positioning System (GPS) measurements at Cachoeira Paulista (22.41°S, 45.00°W, −26° dip angle) in Brazil during November 6–19, 1998. Radio scintillations at the GPS L1 frequency (1.575 GHz) were monitored using four GPS receivers spaced geomagnetically east–west and north–south. Total electron content (TEC) was measured through the ionospheric advance of the GPS L1 and L2 (1.227 GHz) phases. Strong amplitude scintillations coincided with TEC fluctuations associated with spread F bubbles elongated along the magnetic field. Movement of the Presnel-scale (400 m) ionospheric irregularity layers caused the scintillation to drift, and their zonal apparent drift velocities were measured using a cross-correlation technique. Our measurements show that the apparent eastward velocity varies from 200 m/s to 150 m/s at 2000 LT, and then it decreases to 100–50 m/s at midnight. On a magnetically disturbed day, reversal of the zonal apparent drift was observed just after midnight, and the apparent westward velocities observed at early in the morning showed large variations with location in the sky. From the receivers spaced in the geomagnetic north–south direction we measured near-zero time shifts, from which we conclude that the correlation length of several-hundred-meter-scale irregularities is much larger than 70-m separation between the north and south receivers.

Incoherent scatter radar, ionosonde,and satellite measurements of equatorial F region vertical plasma drifts in the evening sector

Studies of equatorial F region evening vertical plasma drifts using different measurement techniques have produced conflicting results. We examine the relationship of incoherent scatter radar and ionosonde drift observations over the Peruvian equatorial region, and AE-E satellite drifts for different geophysical conditions. Our data show that there is large day-to-day variability on the ratios of radar and ionosonde drifts, but on the average the measurements from these two techniques are in fair agreement during low and moderate solar flux conditions. For high solar activity, however, the Jicamarca evening drifts during equinox and December solstice are significantly larger than the ionosonde drifts. These results can be explained by the different height ranges of the radar and ionosonde measurements, and the increase of the upward drift velocity with height below the F region peak. This altitudinal variation is related to the longitudinal gradient of the zonal plasma drifts as a result of the curl-free electric field condition. Our results also indicate that during equinox the increase of the vertical prereversal velocity enhancement with solar activity is largely longitude independent.

Extended ionospheric amplitude scintillation model for GPS receivers

Ionospheric scintillation is a phenomenon that occurs after sunset, especially in the low-latitude region, affecting radio signals that propagate through the ionosphere. Depending on geophysical conditions, ionospheric scintillation may cause availability and precision problems to Global Navigation Satellite System users. The present work is concerned with the development of an extended model for describing the effects of the amplitude ionospheric scintillation on GPS receivers. Using the α-μ probabilistic model, introduced by previous authors in different contexts, the variance of GPS receiver tracking loop error may be estimated more realistically. The proposed model is developed with basis on the α-μ parameters and also considering correlation between amplitude and phase scintillation. Its results are interpreted to explain how a receiver may experience different error values under the influence of ionospheric conditions leading to a fixed scintillation level S4. The model is applied to a large experimental data set obtained at Sao Jose dos Campos, Brazil, near the peak of the equatorial anomaly during high solar flux conditions, between December 2001 and January 2002. The results from the proposed model show that depending on the α-μ pair, moderate scintillation (0.5 ≤ S4 ≤ 0.7) may be an issue for the receiver performance. When S4 > 0.7, the results indicate that the effects of scintillation are serious, leading to a reduction in the receiver availability for providing positioning solutions in approximately 50% of the cases.

On the second order statistics for GPS ionospheric scintillation modeling

Equatorial ionospheric scintillation is a phenomenon that occurs frequently, typically during nighttime, affecting radio signals that propagate through the ionosphere. Depending on the temporal and spatial distribution, ionospheric scintillation can represent a problem in the availability and precision for the Global Navigation Satellite Systems users. This work is concerned with the statistical evaluation of the amplitude ionospheric scintillation fading events, namely, level crossing rate (LCR) and average fading duration (AFD). Using α-μ model, the LCR and AFD are validated against experimental data obtained in Sao Jose dos Campos (23.1°S; 45.8°W; dip latitude 17.3°S), Brazil, a station located near the southern crest of the ionospheric equatorial ionization anomaly. The amplitude scintillation data were collected between December 2001 and January 2002, a period of high solar flux conditions. The obtained results with the proposed model fitted quite well with the experimental data and performed better when compared to the widely used Nakagami-m model. Additionally, this work discusses the estimation of α and μ parameters, and the best fading coefficients found in this analysis are related to scintillation severity. Finally, for theoretical situations in which no set of experimental data are available, this work also presents parameterized equations to describe these fading statistics properly.

Study of ionospheric irregularities during intense magnetic storms

The effects of two intense magnetic storms over ionospheric irregularities were analyzed using GPS signal scintillation data from the stations of Sao Luis (2.57oS, 44.21oW, dip latitude 1.73oS) in the equatorial region, Sao Jose dos Campos (23.07oS, 45.86oW, dip latitude 18.01oS) and Cachoeira Paulista (22.57oS, 45.07oW, dip latitude 18.12oS) both under the Equatorial Ionization Anomaly (EIA), and Sao Martinho da Serra (29.28oS, 53.82oW, dip latitude 18.57oS), located in the South of Brazil. Total Electron Content (TEC) data for Sao Luis and Sao Jose dos Campos, were also analyzed. The analyzed storms occurred on October 28-31, 2003 and on November 7-11, 2004. Both storm periods presented two main phases. In the nights of 29/30 and 30/31 of October, during the two storm main phase, it was observed that TEC over Sao Jose dos Campos reached higher values than the TEC for the magnetically quiet day of October 10, due to the effect of eastward electric field prompt penetration to magnetic equator that intensified the EIA. Compared to a quiet day (Oct 10), scintillation in the GPS signal amplitude due to ionospheric irregularity, quantified by the scintillation index S4, was stronger for Cachoeira Paulista (under EIA) during the night of 30/31but not for the night of 29/30 and for Sao Martinho da Serra was stronger during the nights of 29/30 and 30/31. Scintillation for the nights of 29/30 and 30/31 at these two stations lasted longer than on October 10, reaching the post midnight time sector. During the November 7-11 storm, TEC kept the behavior of a quiet day except during days 10 and 11 (up to 9 UT), when a large TEC decrease was observed. The GPS scintillation, compared to the quiet day November 19, was larger at the equatorial station of Sao Luis during the nights of 7/8 and 8/9 and it was completely inhibited for the Sao Luis and Sao Jose dos Campos stations during the nights of 9/10 and 10/11, probably due to action of westward disturbance dynamo electric field penetration to equator.

The Acoustic Gravity Wave Induced Disturbances in the Equatorial Ionosphere

The role of acoustic gravity waves (AGWs) to excite atmospheric and ionospheric disturbances is examined in this work. These waves are launched in the atmosphere by tropospheric thermal sources and convective activity. An alternative fully time-spatial dependent nonlinear wave equation of acoustic gravity wave is derived and solved numerically using implicit finite-difference scheme. Their propagation in the atmosphere through mesopause thermal duct and lower thermosphere density duct, the role of nonlinear viscous effect to limit the amplitude of these waves in the density duct and to allow them to escape to higher altitude where they attain large amplitude in the bottomside F region Ionosphere, and the role of the mean zonal wind to reduce their amplitude are investigated in present study. To study AGW induced disturbances in the equatorial Ionosphere, the AGW model is coupled with hydromagnetic equations in Ionosphere. This coupling is explored in the context of the collisional interchange instability (CII) in the F region leading to the formation of equatorial F region plasma bubbles. To do so, AGW model is coupled with the CII model and simultaneously solved numerically. The possible role of the AGW to act as a seeding perturbation for equatorial plasma bubbles under varying nature of mean zonal wind and tropospheric thermal source are also investigated.

Validation of the - Model of the Power Spectral Density of GPS Ionospheric Amplitude Scintillation

The - model has become widely used in statistical analyses of radio channels, due to the flexibility provided by its two degrees of freedom. Among several applications, it has been used in the characterization of low-latitude amplitude scintillation, which frequently occurs during the nighttime of particular seasons of high solar flux years, affecting radio signals that propagate through the ionosphere. Depending on temporal and spatial distributions, ionospheric scintillation may cause availability and precision problems to users of global navigation satellite systems. The present work initially stresses the importance of the flexibility provided by - model in comparison with the limitations of a single-parameter distribution for the representation of first-order statistics of amplitude scintillation. Next, it focuses on the statistical evaluation of the power spectral density of ionospheric amplitude scintillation. The formulation based on the - model is developed and validated using experimental data obtained in Sao Jose dos Campos (23.1°S; 45.8°W; dip latitude 17.3°S), Brazil, located near the southern crest of the ionospheric equatorial ionization anomaly. These data were collected between December 2001 and January 2002, a period of high solar flux conditions. The results show that the proposed model fits power spectral densities estimated from field data quite well.

GPS availability and positioning issues when the signal paths are aligned with ionospheric plasma bubbles

The propagation paths of signals through equatorial ionospheric irregularities are analyzed by evaluating their effects on Global Navigation Satellite System (GNSS) positioning and availability. Based on observations during 32 days by a scintillation monitor at São José dos Campos, Brazil, it was noted that there is a dominance of enhanced scintillation events for Global Positioning System (GPS) ray paths aligned with the azimuth angle of 345° (geographic northwest). This azimuth corresponds to the magnetic meridian that has a large westward declination angle in the region (21.4ºW). Such results suggest that the enhanced scintillation events were associated with GPS signals that propagated through plasma bubbles aligned along the direction of the magnetic field. It will be shown that, under this alignment condition, the longer propagation path length through plasma bubbles can result in more severe scintillation cases and more losses of signal lock, as supported by proposed statistics of bit error probability and mean time between cycle slips. Additionally, large precise positioning errors are also related to these events, as demonstrated by precise point positioning experiments.

Mesosphere–Ionosphere Coupling Processes Observed in the F Layer Bottom-Side Oscillation

During the Spread FEx campaign, under the NASA Living with a star (ILWS) program which was carried out in the South American Magnetic Equatorial region from September to November 2005, we observed formation of the bottom-type spread F and simultaneous occurrence of mesospheric gravity wave events. The events were monitored by the ionosonde, coherent radar and airglow OI 630.0 nm and OH imager. It is found that the bottom-type scattering layer has a wave form generated most probably by local gravity waves. Reverse ray-tracing of the observed gravity waves indicate their possible sources in the troposphere or thermosphere. Forward ray-tracing indicates their penetration into the ionosphere. The present work summarizes the observational evidence and results of the data analysis and discusses the mesosphere–ionosphere coupling processes.