Hal J. Strangeways

Propagation model for signal fluctuations on transionospheric radio links

The complex phase method has been further extended to the problem of electromagnetic (EM) field scintillations on Earth-satellite GPS paths of propagation. The numerical and analytic technique based on the method has been developed to characterize the transionospheric channel of propagation. The effects of additional range errors due to the ionospheric electron density fluctuations in space and time have been studied taking into account the ray bending due to the inhomogeneous background ionosphere and the diffraction on local random ionospheric inhomogeneities. In the method developed, the impact of the Earths magnetic field is accounted for by the anisotropic spatial spectrum of the ionospheric turbulence with different outer scales along and across the magnetic field lines. The variances of the EM field phase (yielding range errors) and level (log amplitude) fluctuations have been calculated for different models of the background ionospheres characterized by different height electron density profiles and total electron content. The conditions of the saturated regime of propagation, which will likely result in the degradation of a GPS navigation system, have been discussed. In addition, the scattering function of the GPS transionospheric channel of propagation has been constructed and simulated for a wideband signal.

On determining spectral parameters, tracking jitter, and GPS positioning improvement by scintillation mitigation

A method of determining spectral parameters p (slope of the phase PSD) andT (phase PSD at 1 Hz) and hence tracking error variance in a GPS receiver PLL fromjust amplitude and phase scintillation indices and an estimated value of the Fresnelfrequency has been previously presented. Here this method is validated using 50 HzGPS phase and amplitude data from high latitude receivers in northern Norway andSvalbard. This has been done both using (1) a Fresnel frequency estimated usingthe amplitude PSD (in order to check the accuracy of the method) and (2) a constantassumed value of Fresnel frequency for the data set, convenient for the situation whencontemporaneous phase PSDs are not available. Both of the spectral parameters(p, T) calculated using this method are in quite good agreement with those obtainedby direct measurements of the phase spectrum as are tracking jitter variances determinedfor GPS receiver PLLs using these values. For the Svalbard data set, a significantdifference in the scintillation level observed on the paths from different satellitesreceived simultaneously was noted. Then, it is shown that the accuracy of relative GPSpositioning can be improved by use of the tracking jitter variance in weighting themeasurements from each satellite used in the positioning estimation. This has significantadvantages for scintillation mitigation, particularly since the method can be accomplishedutilizing only time domain measurements thus obviating the need for the phase PSDsin order to extract the spectral parameters required for tracking jitter determination.

Comparison of 4 methods for transionospheric scintillation evaluation

A comparison is made between 4 different methods of scintillation determination (the phase screen method, the Hybrid method, geometrical optics and the Rytov method) for transionospheric paths at GNSS frequencies. Calculations are made for both phase and amplitude scintillation indices. The methods shows good agreement in the calculations of the scintillation variation with the variance of the irregularity electron density, spectral index, outer scale, path elevation and other parameters but there are some quantitative differences. The effect of varying the phase screen height, irregularity velocity and a lower cut-off frequency are also investigated.

WBMod assisted PLL GPS software receiver for mitigating scintillation affect in high latitude region

Ionospheric scintillation occurs for transionospheric radio waves propagating through random ionospheric irregularities, which affect the phase and/or amplitude observations made by the receiver. Generally, the scintillation induces excess carrier phase jitter in the phase lock loop (PLL) of the GPS receiver, and strong scintillation can cause a conventional PLL (ATAN [arctangent method], constant bandwidth Bn = 10 Hz) to lose phase lock resulting in no GNSS signal available at that time from the satellite path(s) affected. A PLL with a larger bandwidth is one solution to mitigate this but at the expense of extra phase noise, and this may not be an optimal solution during weak scintillation conditions. This study uses a novel WBMod (Wide Band Modeling) assisted PLL for robustness of availability of GPS services with lower introduction of extra phase noise. At the initial stage, an optimal PLL bandwidth is predicted using WBMod to stabilize the PLL during strong phase scintillation. A FAB (Fast Adaptive Bandwidth) PLL is used to minimize the phase error. To investigate this approach, a realistic scintillated signal is produced using 50 Hz raw GPS signal observations (carrier phase and intensity) collected at Yellowknife (Yell, 64.48˚ N, −114.52˚ E) in a Matlab-based GPS software receiver.

Regionally based alarm index to mitigate ionospheric scintillation effects for GNSS receivers

An approach to mitigate the effect of ionospheric scintillation on GNSS (Global Navigation Satellite System) users in the European region using TEC (total electron content) at 1 Hz rate is presented. The TEC in the study is derived using raw GPS (Global Positioning System) observations obtained from the EUREF networks. The study also presents derivation of a geographic mesh-map warning of the expected standard deviation of phase jitter in receiver carrier tracking loops, information which would help to mitigate scintillation effects in GPS software receivers.

GPS L1 phase scintillation using wavelet analysis at high latitude

Phase scintillation, e.g. as observed from GPS satellites by ground receivers, is generally measured as the standard deviation of the random fluctuations of the phase received over a fixed period of time which is ofteny taken to be 60s. The measured phase scintillation can be an important tool in understanding the ionospheric turbulence producing it, and consequently its effect on GPS positioning. Therefore, it is important to develop reliable ways of quantifying it. A new approach employing a wavelet analysis is implemented in this work to investigate GPS carrier phase fluctuations for different scintillation conditions using GPS data received at high latitudes where the scintillation effect is particularly marked. The phase scintillation obtained using wavelet analysis is also compared with that derived from the standard PSD technique using FFTs. It is concluded that this wavelet approach appears to be a promising method for recording fast variations of phase due to diffraction by ionospheric irregularities. Furthermore, the wavelet analysis, because it can better characterize conditions of non-stationary, can lead to a better understanding of these effects on phase lock loss in GPS receiver PLLs and hence can aid the design of GPS receivers that are more robust to scintillation effects.

Mapping of ionospheric irregularity and scintillation spectral indices for Northern Europe

In situ satellite observations have previously been employed to map the global morphology of electron density irregularity parameters which define scintillation. In particular the power exponent of the inverse spatial spectrum of the irregularities pi can be determined which will yield the slope p of the psd of the resultant scintillation from p=pi −1. This also means that the inverse spatial spectrum of the irregularities can be determined from the measured slope of the resultant phase scintillation (or amplitude scintillation above the Fresnel frequency) by measuring the slope of its PSD (on log log axes). The in situ measurement has the advantage of good temporal and spatial coverage but the disadvantage that a satellite in orbit at ionosphere altitudes is required. In this paper we present a method of determining the value of p and hence also pi from just scintillation indices (S4 and σ⊘) thus obviating the need for in-situ measurements or the necessity of deriving the fading frequency spectrum from FFTs of high (e.g 50 Hz) rate data. With the addition of an extensive data set from a number of ground-based scintillation monitoring stations this provides a way of easily investigating the variation of these spectral and irregularity parameters with location, time of day, season, Kp, solar activity etc.

Software-based receiver approach for acquiring GPS signals using block repetition method

The Global positioning system (GPS) uses long pseudorandom code sequences at L-band frequencies to provide navigation services to civilian and military users. However, in case of weak signals a long exhaustive search is required at the receiver for signal acquisition. A new acquisition method named Repetitive Block Acquisition (RBA) has been proposed to improve the detection performance and to speed up the acquisition process. The detection performance and computational complexity in terms FFT and IFFT operations of the proposed method is analyzed and is compared with the already existing Zero padding (ZP) and improved zero padding (IZP) methods. It is shown in this paper that the proposed acquisition method provide better detection performance when selecting long coherent lengths for signal acquisition by reducing the computational load of the receiver.

Characterization of ionospheric scintillation at high latitude in the European region

The GPS signal propagating through the ionosphere is affected by rapid and irregular fading which is known as ionospheric scintillation. The scattering or diffraction phenomena due to ionospheric irregularities are responsible for the occurrence of this scintillation on signals received on the ground. The percentage of occurrences of scintillation can be high at high latitudes. In this region, the ionospheric irregularities which result in scintillation are generally produced by an instability caused by the presence of an electric field together with a gradient in electron density and also with energetic particle precipitation. It is well known that severe ionospheric scintillation degrades the reliability of navigation applications using GPS technology. Therefore, further understanding of ionospheric scintillation and its characteristics at high latitude is important. This work studies the characteristics of high latitude ionospheric scintillation over the European region during the solar maximum period and utilizes an extensive data set of GPS observations made between 50˚ N and 75˚N latitude for year 2003 in Northern Europe. It investigates the percentage occurrence of scintillation at auroral and polar regions associated with aurora activity and changes in the interplanetary magnetic field (IMF). The experimental analysis shows that, at high latitudes during nighttime, electron precipitation near the auroral oval boundary extends equatorward causing phase scintillation which is quantified in terms of its percentage of its occurrence. By contrast, in the polar region, scintillation activity is driven by the direct interaction of solar flares into the Earths atmosphere. The results obtained enhance understanding of the different mechanisms responsible for scintillation at high latitude over European region and will be utilised in the construction of a scintillation prediction tool for this region.