Ionospheric Scintillation of GNSS Signals: Impacts and Mitigation - PDF Document

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  1. Ionospheric Scintillation of GNSS Signals: Impacts and Mitigation Keith Groves and Charles Carrano keith.groves@bc.edu Workshop on the Applications of Global Navigation Satellite Systems Suva, Fiji 24-28 June 2019

  2. Outline • Motivation • Ionospheric effects on propagation • Characteristics of equatorial irregularities • Mitigation • Conclusions PRN 7 2

  3. GPS Positioning Errors During Solar Max Scintillation can cause rapid fluctuations in GPS position fix; Typical night from recent field experiments 3

  4. The Ionosphere is a Small Perturbation for GNSS  c n = = 10 f MHz v  p k  =  45 n = vacuum = 1 1575 f MHz 1 1 2 p f = − − 4 10 !!   1 2 p 2 5 n / f f 2 f n  1 Snell’s Law: Ionosphere Fo= 10 MHz 2  = sin( )  sin( ) n n  = 1 1 2 2 45.001 2 For the parameters shown at right, the change in angle is 0.001° (20 μrad)! Can you see it? (water) n = 1.33 2 Perturbation to index of refraction is very small, yet it is enough to cause serious propagation effects!

  5. Scintillation Physics Simple Picture r c N  = + / e tot 2 R c FROM SATELLITE d  2 f  = ( ) N N z dz tot e IONOSPHERE N ΔNe  =  2 − / tot fR c r c e f     Phase change due to ionized layer   VDRIFT Radio Wave Interference Pattern 5 TEC  EARTH’S SURFACE radians • Phase variations on wavefront cause diffraction pattern on ground • A phase changes of ~  radians (i.e., 0.6 TEC units) required for total destructive interference • But the variations must occur over limited spatial scale, the Fresnel zone, , ~ 400-500 meters for L1 and typical iono parameters 5

  6. GPS Signal Fluctuations Caused by Ionospheric Scintillation Intensity fluctuations reduce signal-to-noise in GNSS receiver S4: normalized st. dev. of intensity Phase variations stress GNSS signal tracking loops   : st. dev. of phase 6

  7. Effect of Electron Density on S4 Scintillation requires two physical ingredients: 1. Electron density 2. Irregularities • Significant relative density fluctuations will not cause scintillation if the background electron density is too low • NmF2 must exceed ~1e5/cc for VHF, ~1e6 for GNSS (~50 TEC units) Weak Scatter Approximation 7

  8. Implications for the Ionosphere Recall L1 we need ~0.6 TEC unit variations over spatial scales of a few 100 meters to achieve strong scintillation; lesser variations will cause correspondingly weaker intensity fluctuations • Solar max TEC ~ 50-100 – Small relative density fluctuations required (1-2%) • Solar min TEC ~ 1-5 (nighttime) – Large relative density fluctuations required (10-50%) • Consistent with expectations, GPS scintillations are generally weak during solar minimum • Scintillation impacts on GPS are limited to solar max periods (3-4 years around peak) 8

  9. Solar Flux, Density & S4 Solar flux determines electron density which determines S4 9

  10. Solar Flux & Positioning Errors Solar flux controls S4 which controls impact on GNSS performance 10

  11. Disturbed Ionospheric Regions and Systems Affected by Scintillation POLAR CAP PATCHES SATCOM AURORAL IRREGULARITIES GPS EQUATORIAL F LAYER ANOMALIES PLASMA BUBBLES DAY NIGHT MAGNETIC EQUATOR GPS SATCOM 11

  12. DMSP Satellite Observations: Bubble Detection 1999 - 2002 In situ irregularities detection statistics 800 km circular polar orbit 800 km Occurrence Climatology From Burke & Huang [2004]

  13. What Are Equatorial Dynamics? Formation of Anomaly Region • Presence of anomaly crests strengthens off-equator scintillations • State of anomaly formation is indicative of equatorial dynamics Anomaly crests are areas of maximum F-region ionization density off equator • Daytime eastward electric field (E) drives plasma “up” (E   B) • Plasma moves toward crests (g ,  P ) (View looking east) 13

  14. Why Do Disturbances Form? Unique Equatorial Magnetic Field Geometry Equatorial scintillation occurs because plasma disturbances form readily with horizontal magnetic field Magnetic (Dip) Equator Plasma moves easily along field lines, which act as conductors Horizontal field lines support plasma against gravity– unstable configuration E-region “shorts out” electrodynamic instability during the day • Unstable Plasma F Region • Magnetic Field Lines Daytime “Shorting” E Region • Earth 14

  15. What Is Instability Process? Basic Plasma Instability View along bottomside of ionosphere (E-W section, looking N from equator) Plasma supported by horizontal field lines against gravity is unstable • (a) Bottomside unstable to perturbations (density gradient against gravity) • (b) Analogy with fluid Rayleigh-Taylor instability • Perturbations start at large scales (100s km) • Cascade to smaller scales (200 km to 30 cm) (a) Heavy Fluid Light Fluid (b) from Kelley [1989] 15

  16. 3D Model Realizations of Bubbles Rino, et al., 2018 Full fluid treatment simulations at scintillation-scale spatial resolution (~500 m) • 16

  17. Determining Satellite Availability with S4 Performance will be receiver dependent 16 Mar 2002, ASI Example: blue = used in NAV red = not available (corresponds to spike in DOP) S4 UT (hrs) As expected, the probability that a satellite will be available decreases as scintillation intensity increases, but there is no simple S4 threshold for losing lock Availability Likelihood (%) Best metric might depend on receiver's “failure mode” • If phase fluctuations tend to break the phase lock loop (PLL), use σφ • Other parameters (e.g., decorrelation time) should also be considered S4 17

  18. Scintillation Occurrence & Apex Altitude Mod SSN High SSN Ground-based VHF measurements show that scintillation occurrence at Ascension Island (18°S maglat) reached 50-80% during the peak seasons between 2011-2015 • Assuming bubble height determines meridional extent, structures must rise to over 1000 km to reach Ascension, but only about 400 km to reach Cape Verde • 18

  19. Apex Height vs Latitude: South Pacific 1100 1000 900 Altitude (km) 800 ~Median solar max bubble height 700 600 ~Median solar min bubble height 500 400 300 Samoa Fiji 200 -20 -15 -10 -5 0 5 10 15 20 Magnetic Latitude (Glat – 2.5°) • Site (magnetic) latitude determines what activity will be visible as a function of solar flux • Implications for South Pacific Island Nations: − Further from the equator: Less frequent activity but intense − Closer to the equator: More frequent and more moderate 19

  20. South Pacific Magnetic Latitude Geography Magnetic Equator Solomons −15° MLat Samoa Vanuatu Tonga 20

  21. GPS Positioning Errors from Space Weather Magnetic Latitude Dependence • Night time positioning errors from 2013-2014 in South America • Largest errors occur 15-20 degrees from magnetic equator (~Fiji / Samoa) Equatorial Anomaly Structure Solar Cycle Intensity Fiji Magnetic Latitude VTEC Local Time Data Collection Period 21

  22. Assessing Impacts on GNSS Performance L-Band Impacts at Solar Maximum Multiple GNSS-ground links will be affected simultaneously Objective to produce multi-frequency GNSS position error maps Error Ionospheric Disturbance Visualization 15 Equatorial scintillation structures may routinely degrade optimal navigation solution geometry; potential impacts under investigation 10 5 0 At present, we don’t know threshold of pain for most GNSS receivers 22

  23. 6300 Å All-sky Imagery Scintillating GPS SATS GPS SATS Turbulent Depletions 23

  24. GNSS as a Mitigation Strategy Transition to true GNSS will improve performance, but maybe not in a linear way • Large sectors of the sky are often blocked, so DOP will still be impacted • Fewer periods when less than four satellites are available (inability to navigate) • 24

  25. Longitudinal Variability Examine 250 MHz scintillation observations from three separate longitude sectors in 2011 25

  26. Extreme Day-to-Day Variability ? Cuiaba, Brazil VHF 2011 • Occurrence dominated by seasonal factors • Increase in solar flux evident in last quarter of the year 26

  27. Scintillation Occurrence in W. Africa Cape Verde VHF 2011 • Response looks pretty similar to Cuiaba • Wet and Dry seasons 27

  28. Scintillation Occurrence in E. Africa Nairobi, Kenya, VHF 2011 • Region shows a lot of activity, much of it severe • Fundamental shift in local time of onset during June/July • Data appears to show more variability than American sector 28

  29. Kwajalein, R.M.I. Kwajalein Atoll VHF 2011 • Variability exists throughout the year, but average severity is markedly less than in Nairobi • Part of the difference in severity may be attributable to mag lat 29

  30. Christmas Island (Kiritimati) Christmas Island, Kiribati VHF 2011 • Overall pattern similar to Kwajalein • Decrease in severity may be magnetic latitude effect (1° vs 4°) 30

  31. Summary Relatively weak ionospheric interaction with L-band signals produces surprisingly strong propagation effects Numerous scintillation-induced GPS performance impacts have been observed and documented during solar maximum periods Strong scintillation requires the presence of small-scale irregularities and relatively high background densities Post-sunset cintillation occurrence at low-latitudes is common and more than 90% of the activity occurs during quiet solar periods Severity is greatest near the equatorial anomaly regions, but occurrence frequency maximizes near the magnetic equator S4 and sigma_phi are useful indices, but do not fully characterize the propagation environment and are inadequate to predict impacts on GNSS receivers Multi-constellation observations represent one of the best mitigation strategies for navigation outages but large-scale scintillation structures will still increase positioning errors, primarily through impacts on DOP • • • • • • • 31

  32. • High-rate GNSS observations from South Pacific Island Nations could dramatically improve our understanding of scintillation and equatorial anomaly dynamics in this longitude sector. Bula Bula Vinaka Vinaka! ! Questions? 32