Adam, Jean-Pierre

Beniguel, Y.

Bourdillon, Alain

Lassudrie-Duchesne, Patrick

HAL-Université de Bretagne Occidentale

Ionosphere scintillation effects on navigation systems

HAL Descartes

HAL-Univ-Nantes

HAL-Rennes 1

Yannick Béniguel

Jean-Pierre Adam

Alain Bourdillon

P. Lassudrie-Duchesne

Comptes Rendus Physique

C. R. Physique 12 (2011) 186–191Contents lists available at ScienceDirectComptes Rendus Physiquewww.sciencedirect.comPropagation and plasmas: new challenges, new applicationsIonosphere scintillation effects on navigation systemsEffets de scintillations ionosphèriques sur des systèmes de navigationYannick Béniguel a,∗, Jean-Pierre Adam a, Alain Bourdillon b, P. Lassudrie-Duchesne ca IEEA, 13, promenade Paul-Doumer, 92400 Courbevoie, Franceb IETR, University of Rennes 1, 2, rue du Thabor, 35065 Rennes cedex, Francec Telecom Bretagne, Technopôle Brest-Iroise, 29238 Brest, Francea r t i c l e i n f o a b s t r a c tArticle history:Available online 4 March 2011Keywords:Navigation systemsScintillationsPositioning errorsLoss of lockThis article deals with the impact of ionospheric electron density inhomogeneities onthe functionality of global navigation satellite systems emphasizing positioning errors.The scintillation characteristics of transmitted signals have been obtained using datagathered in measurement campaigns. The effects on a standard receiver are then presented.Positioning errors due to scintillations were shown to be greater than 10 meters in theworst case.© 2011 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.r é s u m éCet article a pour objet l’étude de l’impact de la propagation à travers l’ionosphère,et plus particulièrement à travers les inhomogénéités à l’intérieur du milieu, sur leserreurs observées par les systèmes de navigation par satellite, en particulier pour lepositionnement. Les modifications apportées sur les signaux transmis ont été évaluées àl’occasion de campagnes de mesures. L’analyse des effets sur un récepteur standard estensuite présentée. Les erreurs de positionnement liées aux scintillations sont supérieures àla dizaine de mètres dans les pires cas.© 2011 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.1. IntroductionAs a result of propagation through ionosphere electron density irregularities, transionospheric radio signals may expe-rience amplitude and phase fluctuations. In equatorial regions, these signal fluctuations specially occur during equinoxes,after sunset, and last a few hours. They are more intense in periods of high solar activity. These fluctuations result in signaldegradation from VHF up to C band. The corresponding errors are the most prominent errors for Global Navigation SatelliteSystems (GNSS).The signal fluctuations, referred as scintillations, are created by random fluctuations of the medium’s refractive index,which are caused by inhomogeneities inside the ionosphere. These inhomogeneities such as plasma turbulences or electrondensity bubbles develop under several plasma instability processes. These processes start after sunset when the sun ion-ization drops to zero, consequently at nighttime. To produce scintillation, the irregularities sizes should be below a typicaldimension (typically one km) such that the diffracting pattern is inside the first Fresnel zone. The Fresnel zone dimension* Corresponding author.E-mail address: beniguel@ieea.fr (Y. Béniguel).1631-0705/$ – see front matter © 2011 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.doi:10.1016/j.crhy.2011.01.005Y. Béniguel et al. / C. R. Physique 12 (2011) 186–191 187Fig. 1. Scintillation index measurement in Hué (Vietnam), year 2006.also depends on the distance from the Ionospheric Pierce Point (usually defined at about 350 km height) to the receiverand on the frequency.In Section 2, we present the signal scintillation characteristics, mainly the levels, the probabilities, the fades statisticsand the coherence properties. These characteristics have been obtained from various measurement campaigns, in particularin South America (Doherty et al. [1], De Paula et al. [2]), India (Chandra et al. [3]) and Japan (El-Arini et al. [4] and Otsukaet al. [5]). More recently, a campaign was conducted under an ESA/ESTEC initiative (Béniguel et al. [6]) and another oneis ongoing aiming to operate for the next peak of the solar cycle. A modeling activity has been conducted concurrently,also under ESA/ESTEC initiative (Béniguel [7]). The Global Ionospheric Scintillation Model (GISM) allows estimating thescintillation characteristics. It may also be used to assess the receiver performances in such an environment.Section 3 of this article presents the effects of scintillation on a satellite navigation system receiver. It is a follow onto a previous paper on the same topic (Lassudrie-Duchesne et al. [8]) with more insight on the receiver functioning. Boththe phase and intensity fluctuations have consequences on the system. In a high scintillation regime, the phase may exhibitcycle slips with consequences on the receiver phase loop. In addition, depending on the link elevation angle and as a result,on the signal to noise ratio, the scintillations may lead to loss of lock. The navigation equation is affected in all cases.Scintillation may lead to errors up to tens of meters in the worst case, making this problem a major issue for navigationsystems.2. Scintillation characteristicsFor a navigation system, the parameters we are interested in, as shown in the next section, are: the probabilities ofoccurrence of scintillations, the scintillation indices, the fades statistics, the spectrum characteristics and the evolution ofthese parameters with respect to the time. The scintillation indices allow characterizing the strength of turbulence, inparticular the S4 parameter which corresponds to the RMS intensity for a normalized intensity. S4 value is between 0 and 1.A value of 1 will correspond to about 35 dB peak to peak of intensity fluctuations. The time between fades and the fadesduration are key parameters to assess the capability of receivers to operate in such an environment.The Assessment of the scintillation parameters can be done either by modeling or by measurement and preferablymixing both approaches. This was done in the frame of several ESA/ESTEC contracts. A model named Global IonosphericScintillation Model (GISM) [7] was developed. It is continuously improved. Ionosphere scintillation measurements campaignsare still ongoing, mostly supported by ESA and EU. The results reported hereafter are taken from the PRIS measurementcampaign [6]. A number of receivers were deployed both at low and high latitudes, in particular in Vietnam, Indonesia,Guiana, Cameroon, Chad and Sweden. These receivers are dedicated receivers, operating at 50 Hz, to be compatible withthe scintillation spectrum as shown on Fig. 4. A data bank was constituted and the scintillation characteristics here abovementioned were derived from an extensive analysis using this data bank. Those results are used in the next section.Fig. 1 shows measurement results obtained at Hué (Vietnam) in 2006. There is clearly a seasonal dependency withmaximum at equinoxes. The peak value is on average 0.2 which is quite a low value but this is related to the fact that itcorresponds to the minimum of the solar cycle.Fig. 2 shows the correspondence between a Total Electron Content (TEC) map and a scintillation map. Those two mapswere obtained by modeling using NeQuick [9] model for the TEC and GISM [7] for scintillations. They correspond to verticallinks. The electron density is consequently integrated along a vertical at each grid point on the map to get the TEC. Slantobservations may however exhibit higher values. The propagation length inside the ionosphere would increase in that caseand by consequence the levels obtained.Fig. 2 was obtained with a solar radio flux at 10.7 cm set to 150. It corresponds to a high value. Universal time is 10 p.m.for the TEC map and 12 p.m. for the scintillation map. At this time the peak values for the TEC occur in the Pacific Ocean188 Y. Béniguel et al. / C. R. Physique 12 (2011) 186–191Fig. 2. TEC (left panel) and scintillation map (right panel) obtained by modeling.Fig. 3. Simplified GPS digital receiver channel.area. For the scintillations the time duration of the events is a few hours after sunset. This is what gives the model. Bothplots reproduce the same features regarding the peak values on both sides of the magnetic equator. The values decreaseincreasing the latitude. For scintillations the model calculates the effects at the equatorial regions. The high latitudes regionsare also concerned by this problem but this is not taken into account by the model. The TEC maximum is 80 TEC units whichis a significant value. It is directly linked to the solar flux value. The peak value for the intensity RMS (S4 parameter) is 0.7.Such a value corresponds to strong fluctuations. It is also linked to the electron density levels. Depending on the signal tonoise ratio, as detailed in Section 3.3, receivers may lose lock at this level.3. Scintillation errors3.1. GPS receiver architectureAny GPS receiver locking onto a GPS satellite signal has to do a two-dimensional search for the signal. The first dimensionis time. The GPS signal structure for each satellite consists of a 1023 bit long pseudo-random number (PRN) sequence sentat a rate of 1.023 megabits/sec, i.e. the code repeats every millisecond. To perform acquisition in this dimension, the receiverhas to set an internal clock to the correct slot among the 1023 possible time slots. This is done by trying all possible valuesof the time delay. Once the correct delay is found, it is tracked with a Delay Lock Loop (DLL).The second dimension is the frequency. The receiver must correct for inaccuracies in the apparent Doppler frequency.Once the carrier frequency is evaluated, it is tracked with a Phase Lock Loop (PLL). Fig. 3 shows an extremely simplifiedPLL/DLL architecture. A more precise description of the GPS signal processing can be found in Ward [10].3.2. Phase noise at receiver levelWhen the receiver is unable to track the carrier phase, the signal is lost. Loss of lock is directly related with PLL cycleslips. To evaluate the occurrence of cycle slips, the tracking error variance at the output of the PLL has to be considered.This variance is expressed as a sum of three terms, Conker et al. [11]:σ 2Φ = σ 2 + σ 2 + σ 2Φ,osc (1)ΦS ΦTY. Béniguel et al. / C. R. Physique 12 (2011) 186–191 189Fig. 4. Intensity time series (left panel) and corresponding spectrum (right panel).where σΦS is the phase scintillation variance, σΦT is the thermal noise variance and σΦ,osc is the receiver oscillator noise(0.122 rad) [11]. The phase variance scintillation at the output of the PLL is given by:σ 2ΦS =∞∫−∞∣∣1 − H( f )∣∣2SΦ( f )d f (2)where SΦ( f ) is the Power Spectral Density of phase scintillation.The phase scintillation spectrum is presented on Fig. 4. |1 − H( f )|2 is the closed loop transfer function of the PLL anddepends on k, the loop order, and fn , the loop natural frequency. Its expression is given by (3). Typical values are k = 3 andfn = 1.91 Hz.∣∣1 − H( f )∣∣2 = f 2kf 2k + f 2kn(3)When there is no scintillation, the standard thermal noise tracking error for the PLL is given by equation:σ 2ΦT =Bn(c/n0)[1 + 12η(c/n0)](4)where c/n0 is the signal to noise ratio, Bn is the receiver bandwidth, and η is the predetection time. For airborne GPSreceiver, Bn = 10 Hz and η = 10 ms. According to Conker et al. [11], in presence of scintillation characterized by its S4index, the variance of the thermal noise tracking error is given by:σ 2ΦT =Bn[1 + 12η(c/n0)(1−2S24)](c/n0)(1 − S24)(5)Amplitude scintillations will increase the S4 value and consequently the thermal noise. In addition, considering the fadesduration as compared to the integration time (cf. below Fig. 6), the signal to noise will drop by the fade depth.Eq. (5) requires the evaluation of the SNR which depends on the elevation angle. Eq. (5) can be used to compute the PLLtracking error variance. Fig. 5 is a comparison of the PLL standard deviation vs c/n0 for S4 = 0.7 and S4 = 0.5. Loss of lockis highly probable for values of the PLL standard deviation above the 15◦ threshold. Therefore a receiver is able to toleratescintillation conditions if the c/n0 is above a minimum value. This minimum is 26 dB for S4 = 0.5 and 32 dB for S4 = 0.7.3.3. Loss of lock probabilityThe thermal noise appears to be the essential contribution to the PLL tracking error. It is the unique S4 dependent termin (5). The importance of the S4 index is exhibited on Fig. 5. Fig. 6 shows statistics of fade duration for different levels ofS4. This fade duration always exceeds the pre-integration time.As a consequence it corresponds to a degradation of the c/n at receiver level:c/n = c/n0 + Is(in dB) (6)190 Y. Béniguel et al. / C. R. Physique 12 (2011) 186–191Fig. 5. PLL standard deviation vs c/n0.Fig. 6. Fades duration vs fade depth. Fig. 7. Probability of loss of lock.Fig. 8. Positioning error at Naha (Japan) under scintillation conditions, computed with GISM.or, with the fractional form:c/n = c/n0 ∗ Is (7)where Is is the scintillation intensity. Its mean value is 1 and it has a Nakagami distribution characterized by S4. Eq. (5) ismodified to take the fading into account:σ 2ΦT =Bn[1 + 1](8)(c/n0) Is 2η(c/n0) IsY. Béniguel et al. / C. R. Physique 12 (2011) 186–191 191This relation expresses the thermal noise as a decreasing function of the scintillation intensity. As a result, if σΦT is abovethe 15◦ threshold then Is is below a value computed using (8). As the Is distribution is known for a given S4, the probabilityof occurrence of “Is < threshold” can be evaluated. The result is the probability of Loss of Lock. Fig. 7 presents this proba-bility versus S4 for given values of the SNR. It can be noticed that links with high c/n0 are quite robust. On the contrary,links with low values of c/n0 are likely to be lost.3.4. Positioning errorsIn most cases, scintillations do not affect all visible satellites. If the number of visible satellites is above 4 then a standardreceiver should be able to provide navigation information. However, the number of satellites and their positions affect thepositioning precision. The Dilution Of Precision (DOP) is usually used to quantify this precision. The DOP is related to thegeometrical distribution of the visible constellation. The DOP is used to derive the positioning error (σp) from the UserEquivalent Range Error (UERE):σp = DOP × UERE (9)GISM was used to compute all scintillation parameters for each GPS satellite visible from Naha (Japan). The S4 parameterwas measured for all satellites in view. The tracking error was derived from these values using formulas presented inprevious sections, for typical receiver characteristics. Satellites with tracking errors above the 15◦ threshold were ignoredfor the DOP evaluation. The navigation solution was then calculated.Even if the signal transmitted from a GPS satellite is not lost, it can be degraded enough to alter the position precision.One of the DLL functions is the measurement of the delay between the code carried by the GPS signal and the receiverinternal clock. This delay is an estimation of the time needed by the GPS signal to reach the receiver. The receiver is thenable to compute the satellite to receiver distance. Errors in this estimation are collected in the UERE. To take the scintillationinto account, we have to consider the DLL tracking errors.The DLL can be studied like the PLL to evaluate its tracking error variance in degrees. Satellites with high DLL trackingerrors have also high PLL tracking errors and therefore they might be considered as lost and do not contribute to the UERE.The combination of both effects is presented on Fig. 8. Satellites with PLL tracking errors above 15◦ were consideredlost for the DOP calculation. All other links with visible satellites were used to compute a mean UERE contribution due toscintillation.4. ConclusionThe evaluation of scintillation errors due to propagation through the ionosphere, with the two aspects of characterizationof the propagation medium and modeling of the propagation impairments was presented in this paper. It was shown inparticular that positioning errors may reach more than 10 meters making this error the most important for navigationsystems.The characterization of signal scintillations benefits from many measurement campaigns. The number of scintillationsreceivers deployed for this has significantly increased, both at low and high latitudes. This allows, in particular an improvedaccuracy in the modeling, a better understanding of this issue and a support to the development of mitigation techniquesto minimize these errors.References[1] P.H. Doherty, S.H. Delay, C.E. Valladares, J. Klobuchar, Ionospheric scintillation effects in the equatorial and auroral regions, in: ION-GPS-2000, Salt LakeCity, Utah, September 19–22, 2000, pp. 662–671.[2] E. De Paula, P.H. Doherty, T. Dehel, Scintillation activity and recent measurements from the Brazilian region, in: SBAS-IONO Meeting, Maastricht, May2001.[3] H. Chandra, P.V.S. Ramarao, P.N. Vijay Kumar, B.M. Pathan, K.N. Iyer, A.K. Gwal, R.P. Singh, Singh Birbal, A. Dasgupta, VHF scintillations at low latitudesin India, in: Beacon Satellite Symposium, Boston College, Boston, 2007.[4] M.B. El-Arini, R.S. Conker, S.D. Ericson, K.W. Bean, F. Niles, K. Matsunaga, K. Hoshinoo, Analysis of the effects of ionospheric scintillation on GPS L2 inJapan, in: ION-GPS-2003, Portland, OR, September 2003.[5] Y. Otsuka, K. Shiokawa, T. Ogawa, Equatorial ionospheric scintillations and zonal irregularity drifts observed with closely-spaced GPS receivers inIndonesia, J. Meteorological Soc. Japan 84A (2006) 343–351.[6] Y. Béniguel, J.-P. Adam, N. Jakowski, T. Noack, V. Wilken, J.-J. Valette, M. Cueto, A. Bourdillon, P. Lassudrie-Duchesne, B. Arbesser-Rastburg, Analysis ofscintillation recorded during the PRIS measurement campaign, Radio Sci. 44 (2009), doi:10.1029/2008RS004090.[7] Y. Béniguel, GIM: a global ionospheric propagation model for scintillations of transmitted signals, Radio Sci. 37 (3) (2002) 1032,doi:10.1029/2000RS002393.[8] P. Lassudrie-Duchesne, Y. Béniguel, A. Bourdillon, R. Fleury, J.J. Valette, M. Le Huy, L. Tran Thi, Les effets de la scintillation ionosphérique sur le GPS, in:Revue de Navigation, Paris, 2010.[9] S.M. Radicella, The NeQuick model genesis, uses and evolution, Ann. Geophys. 52 (3–4) (July–August 2009).[10] P. Ward, Satellite signal acquisition and tracking, in: E.D. Kaplan (Ed.), Understanding GPS Principles and Applications, Artech House, Boston, 1996,pp. 119–208.[11] R.S. Conker, M.B. El-Arini, C.J. Hegarty, T. Hsiao, Modeling the effects of ionospheric scintillation on GPS/SBAS availability, Radio Sci. 38 (1) (2003),doi:10.1029/2000RS002604.

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Ionosphere scintillation effects on navigation systems

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