AN APPROACH TO MITIGATE IONOSPHERIC SCINTILLATION EFFECTS ON GNSS RELATIVE POSITIONING: CASE STUDY IN NORTHERN EUROPE

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1 II Simpósio Brasileiro de Geomática Presidente Prudente - SP, -7 de julho de 7 ISSN 98-6, p. 3-8 N PPROCH TO MITIGTE IONOSPHERIC SCINTILLTION EFFECTS ON GNSS RELTIVE POSITIONING: CSE STUDY IN NORTHERN EUROPE MÁRCIO QUINO JOÃO FRNCISCO GLER MONICO LN DODSON HROLDO NTONIO MRQUES GIORGIN DE FRNCESCHI 3 LUCILL LFONSI 3 VINCENZO ROMNO 3 MRCUS NDREOTTI University of Nottingham Institute of Engineering Surveying and Space Geodesy University Park - Nottingham, UK {marcio.aquino, Sao Paulo State University - Unesp Faculty of Science and Technology Department of Cartography, Presidente Prudente - SP galera@fct.unesp.br, haroldoho@gmail.com 3 National Institute for Geophysics and Volcanology - INGV Rome, Italy {defranceschi, lucilla.alfonsi, romano}@ingv.it Geospatial Research Centre Ltd. Christchurch, New Zealand {marcus.andreotti@grcnz.com} BSTRCT - The effect of the ionosphere on the signals of Global Navigation Satellite Systems (GNSS, such as the Global Positioning System (GPS and the proposed European Galileo, is dependent on the ionospheric electron density, given by its Total Electron Content (TEC. Ionospheric time-varying density irregularities may cause scintillations, which are fluctuations in phase and amplitude of the signals. Scintillations occur more often at equatorial and high latitudes. They can degrade navigation and positioning accuracy and may cause loss of signal tracking, disrupting safety-critical applications, such as marine navigation and civil aviation. This paper addresses the results of initial research carried out that is relevant to GNSS users if they are to counter ionospheric scintillations, i.e mitigate their effects. We used the variance of the output error of the GPS receiver DLL (Delay Locked Loop and PLL (Phase Locked Loop, respectively, to modify the least squares stochastic model applied by an ordinary receiver to compute position. These errors were modeled according to Conker et al. (3, as a function of the S (amplitude scintillation and φ (phase scintillation indices measured by the GISTM (GPS Ionospheric Scintillation and TEC Monitor receivers. n improvement of up to 3% in relative positioning accuracy was achieved with this technique when applied to baselines of different lengths at high latitudes in Northern Europe. INTRODUCTION t high latitudes the effects of ionospheric scintillations on GNSS users are exacerbated during solar maximum, when geomagnetic storms may occur in M. quino et al. connection with enhanced solar activity. Irregularities in the ionosphere that cause scintillations tend to form mostly within the auroral oval, which expands and contracts due to varied geomagnetic conditions. Scintillation is expected to occur during periods when the

2 II Simpósio Brasileiro de Geomática Presidente Prudente - SP, -7 de julho de 7 signal intercepts the auroral oval region. This region is where aurora borealis or northern lights are observed and is the region containing the footprints of the magnetic field lines. This region naturally expands equatorwards during night time and if a solar coronal mass ejection (CME occurs, whose particles collide with the Earth, the Earth's magnetic field becomes more and more compressed, expanding the auroral oval over lower latitudes. This condition correlates with high values of the planetary geomagnetic index, Kp (RODRIGUES et al.,. Figure shows the average equatorward boundary of the midnight aurora in the Northern hemisphere for varying values of Kp. Figure - average equatorward boundary of the midnight aurora (from are measured (based on L GPS frequency raw data sampled at a Hz rate. The GISTM logs and outputs statistics of phase and amplitude scintillation (computed over 6 seconds from the Hz data. In the GISTM amplitude scintillation is measured by the widely used S index, which is a normalised standard deviation of signal intensity. mplitude scintillation affects the signal level and can cause loss of satellite lock. Phase scintillation causes the signal carriers to change by several cycles in short periods of time and cycle slips may therefore occur. Phase scintillation is measured by the φ index, which is the standard deviation of the carrier phase measurements computed by the GISTM over, 3,, 3 and 6 seconds (VN DIERENDONCK,. The 6 seconds version of the phase scintillation index, also known as Phi6, was used in the experiments described in this paper. Figure shows the locations of the 8 GISTM receivers used in the analyses presented in this paper. The ability of such a network to assess the effects of ionospheric scintillation on GPS positioning accuracy has been demonstrated in several papers, e.g. quino et al. (, 6a, lfonsi et al. (6. lso, comprehensive statistical studies aiming to characterize scintillation occurrence and potential impact to users in the region, based on this network, have been published in recent years, e.g. Rodrigues et al. (. Current research to counter the effects of scintillations has concentrated on characterizing, modeling and monitoring their occurrence. s part of this effort the Italian National Institute for Geophysics and Volcanology (INGV has kept GPS Ionospheric Scintillation and TEC Monitor (GISTM receivers permanently deployed at high latitudes, in the frame of the ISCCO (Ionospheric Scintillation rctic Campaign Coordinated Observations project (DE FRNCESCHI; LFONSI; ROMNO, 6. In a separate effort the Institute of Engineering Surveying and Space Geodesy (IESSG had deployed a network of GISTM receivers during part of the latest solar maximum (between June and December 3, covering geographic latitudes from ~3 N to ~7 N. That network was later decommissioned, but one GISTM was kept in Nottingham and another installed in Dourbes, Belgium, in collaboration with the Royal Meteorological Institute. This paper discusses results of analyses based on data collected by these GISTM receivers, with the aim of assessing potential techniques that could be used to mitigate ionospheric scintillation effects on GNSS users. NETWORK OF GPS SCINTILLTION ND TEC MONITOR RECEIVERS The GISTM is based on the dual frequency OEM Novtel card. It has however a stable ovenized crystal oscillator and tracks with a wider bandwidth so that all spectral components of amplitude and phase scintillation Figure - GISTM network used in the analyses 3 MITIGTION OF EFFECTS Our mitigation approach considers that, during the occurrence of scintillation, pseudorange and carrier phase measurements made by a GPS receiver to different satellites are degraded differently, depending on how each link is affected by the irregularities in the ionosphere. The degree of degradation of the pseudorange and the carrier phase in our experiments is given by the variances of the output error of the GPS receiver DLL (Delay Locked Loop and PLL (Phase Locked Loop respectively. The PLL minimizes the error between the input phase and its estimated phase output, which feeds the receiver processor. The loop will remain in lock or not depending on the magnitude of this error. The models of Conker et al. (3 calculate the variance of the output phase M. quino et al.

3 II Simpósio Brasileiro de Geomática Presidente Prudente - SP, -7 de julho de 7 tracking error (also referred to as tracking jitter for two common types of PLL (a third order L carrier PLL and a second order L-aided L carrier PLL, the latter also referred to as L semicodeless. We used these models to compute the tracking errors, which are functions of the receiver hardware parameters and, respectively of the S amplitude scintillation index and the φ phase scintillation index. For full details of these models the reader is referred to Conker et al (3. Here we provide only a brief description of them. The model of the L C/ code (C DLL tracking jitter variance is given by (in code chips: Bnd + ( / / ( ( η c n L C S L τ ( ( c / n ( S ( L Where : M. quino et al. L C / Bn is the one-sided noise bandwidth d is the correlator spacing (c/n L-C/ is the fractional form of signal-to-noise density ratio, equal to.c/n η is the predetection integration time, equal to.s S (L <.77 Similarly the model of the L semicodeless P code (P DLL is represented as follows: Bn + ( / ( ( η c n L P S L τ ( ( c / n ( S ( L LP The model for the L carrier PLL accounts for the effects of scintillation on the input phase and computes the tracking error variance at the output of the PLL ( φ as: where, + + φ s φ φ s φ T (3 φ T and φ osc φ osc are respectively the error variance components relating to the phase scintillation, the thermal noise and the oscillator noise (assumed as. radians in the receiver used in our experiments. mplitude scintillation is modeled as an increase in the thermal noise, related to the decrease in the received signal power: Bn + η ( c / n L C / ( S ( L φ ( T ( c / n ( S ( L Where, again: L C / B n is the L third-order PLL one-sided bandwidth, equal to Hz (c/n L-C/ is the fractional form of signal-to-noise density ratio, equal to.c/n η is the predetection integration time, equal to.s S (L <.77 Phase scintillation is modeled by: Where: φ s kf p n πt [k + p] π sin( k T is the spectral strength of the phase noise at Hz p is the spectral slope of the phase PSD k is the order of the PLL f n is the loop natural frequency Our proposed mitigation technique is based on the establishment of weights given by the inverse of the variance of these tracking errors. These weights are then applied to a modified least squares stochastic model used in the position computation. 3. Modified Stochastic Model To explain the method we take as an example the typical GPS pseudorange point positioning case, for which the usual least squares model, for a receiver at station observing satellites i, j, k and l, considers a weight matrix of the observations W, of the form: i / W / j / k / l where it is usually assumed there is no correlation between observations and all weights are the same, i.e.: i j k l (7 which ultimately simplifies the weight matrix to: where I is the identity matrix. ( I ( (6 W / (8

4 II Simpósio Brasileiro de Geomática Presidente Prudente - SP, -7 de julho de 7 Clearly, in a double difference pseudorange solution as the observable involves two satellites and two stations, correlation between observations must be accounted for. However assuming all measurements to be of the same weight is still usual practice, which simplifies the stochastic model. For instance, in the case of four satellites being observed, the double difference weight matrix W dd assumes the form: M. quino et al. W dd / (9 In our experiments we used UNESP (São Paulo State University s in-house GPS double difference software (GPSeq to compare the accuracy on baseline computation based on a conventional equal weights solution against a mitigated solution. This software uses double differences of C (L C/ code and P (L P code pseudoranges, as well as (in another version L and L carrier phase data, on an epoch by epoch solution. It assigns relative weights between the different observables, which however are the same for all satellites at all epochs. s explained above the mitigated solution is obtained by modifying the stochastic model with the introduction of satellite and epoch specific weights based on the inverse of the variances of the output error of the GPS receiver DLL and PLL (of L and L, calculated by the models of Conker et al. (3. The UNESP software was therefore modified to include this option. s the receivers used in the experiments were the GISTMs, the S and φ indices required for the computations were readily available. lthough the main purpose of these receivers is not their use in geodetic applications (i.e. accurate position estimation, several previous experiments involving pseudorange and carrier phase processing with GSV data resulted in accuracies compatible with geodetic grade receivers (e.g. quino et al, 6a. 3. Experiments and Results Initial experiments were carried out by applying the technique on a C and P pseudoranges only solution (QUINO, et al., 6b. Baselines of varying lengths were processed and under different geomagnetic conditions. Each baseline was processed for a period of hours, on an epoch by epoch solution of minute using GPSeq. s the geomagnetic conditions and ionospheric variability were different for each of the days it is not possible to compare results between baselines. Therefore each baseline solution should be analysed individually. Table gives a description of these baselines and the corresponding geomagnetic conditions given by the maximum value reached by the geomagnetic index Kp on each day. The location of the stations is seen in figure. Table Summary of experimental setup Baseline Length Date Maximum Kp Nya/Nya km 3// 6 Lyb/Nya km //6 Nott/Dour km 8/7/6 6 Bron/Hamm 7km /6/ Table gives a summary of the results when a standard solution is performed, against the proposed mitigated solution. We analysed in particular the error in height, as this is known to be the component most affected by the ionosphere. It is evident from table that, in the case of the pseudorange solution the method correctly assigns relative weights to the different observations and that under these particular scintillation conditions this leads to an improved least squares solution. Table - Summary of results RMS height Baseline error length without mitigation RMS height error with mitigation % Improvement km.m.3m km.8m.8m 3 km.m.6m 7 7km 6.3m.3m 7 Figure 3 shows the time series of the epoch by epoch solutions for the baseline of km, between stations Lyb (in Longyeabyen and Nya (in Ny lesund. The top plot is the non-mitigated solution and the bottom plot is the solution when the proposed mitigation technique is applied. height error (m - - rms.8 m rms.8 m km baseline, Dec 6, non-mitigated solution x mitigated solution Time - minutes on hours Figure 3 time series of height error for station Nya when baseline Lyb/Nya is processed. Only an initial experiment was carried out involving carrier phase data processing, using GPSeq. The carrier phase version of this software provides an epoch by epoch solution based on both pseudoranges and phase measurements on the L and L GPS frequencies. We analysed hour of data from baseline Lyb/Nya (km, between : and 3: UT on the December 6. During that period of time strong GPS

5 II Simpósio Brasileiro de Geomática Presidente Prudente - SP, -7 de julho de 7 ionospheric phase scintillation was observed by the GISTMs in the region. Figure shows the values of Phi6 as measured at station Nya. The time series of the height errors for the non-mitigated and mitigated solutions are shown in figures and 6 respectively. Figures and, when analysed in conjunction, show the correlation of the greater height errors with the occurrences of high values of Phi6, indicating the negative effect of phase scintillation on the solution. GPSeq uses the LMBD method (Teunissen, 996 to resolve the carrier phase ambiguities and at certain epochs during the period of high scintillation occurrence it fixed these ambiguities wrongly. Clearly, given the length of the baseline, fixing the ambiguities to their correct values would be very unlikely and the occurrence of scintillation somehow led to unreliable epochwise solutions (figure. However, when using the software version with the modified stochastic model according to our approach, at all epochs the float ambiguities solution prevails, providing a more accurate solution (figure 6.. Phi6 measured at station Nya, Dec 6, -3UT height error (m rms.87m LYB-NY baseline, -3UT, Dec 6, mitigated solution phase and code solution Minutes on the Hour (-3UT, Dec6 Figure 6 phase and code mitigated solution It must be noted that, in the carrier phase case, to use the PLL models of Conker et al (3, it was necessary to retrieve the spectral parameters p and T (equation from high rate (Hz phase scintillation data, which was available for both stations from the ISCCO project. This Hz raw scintillation data allowed the direct computation of T and p through FFT (Fast Fourier Transform spectral lines (Van Dierendonck, CONCLUSION Phi6 (radians Time - -3UT of December 6 Figure Phi6 time series measured at station Nya, December 6, -3UT height error (m rms.87 m LYB-NY baseline, -3UT, Dec 6, no weighting epoch by epoch solution (phase and code Minutes on the Hour (-3UT, Dec6 Figure phase and code non-mitigated solution Ionospheric scintillations are fluctuations in phase and amplitude of the signals from GNSS satellites. They are caused by time-varying density irregularities in the ionosphere, which occur more often at equatorial and high latitudes, in particular during periods of high solar flux. They can degrade navigation and positioning accuracy and may cause loss of signal tracking, disrupting safetycritical applications, such as marine navigation and civil aviation. Degradation of accuracy due to the occurrence of scintillation can also affect carrier phase based applications and in particular disrupt the resolution of the ambiguities inherent to the carrier phase observable. This paper describes a technique that was successfully used to counter the effects of scintillations on GNSS positioning. The proposed approach is based on data obtained from GPS scintillation monitor receivers and suitable receiver tracking models. Results on that front indicate potential for the development and implementation of algorithms that could be embedded in a scintillation monitoring network with the aim to mitigate these effects. The experiments presented in this paper and corresponding results, although encouraging, are considered of a preliminary nature. Plans are in place to undertake further investigations in this area. KNOWLEDGEMENTS The data used in this research was obtained through funding from the Engineering and Physical Sciences Research Council (EPSRC in the UK and from the ISCCO Project, supported by the Italian Program of ntarctic Researches (PNR M. quino et al.

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