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Some ionospheric effects on ground based radar

Some ionospheric effects on ground based radar. Y. Béniguel, J.-P. Adam. Total Electron Content (TEC) dependent effects (litterature review). Backscattering by electron density irregularities (litterature review). Scintillations (IEEA field of research). Ionospheric effects. Group delay

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Some ionospheric effects on ground based radar

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  1. Some ionospheric effects on ground based radar Y. Béniguel, J.-P. Adam

  2. Total Electron Content (TEC) dependent effects (litterature review). Backscattering by electron density irregularities (litterature review). Scintillations (IEEA field of research). Ionospheric effects

  3. Group delay Dispersion Faraday rotation Direction of arrival TEC

  4. Medium’s Characterisation Electron Density TEC map

  5. Frequently at high latitude : Radar Auroral Clutter Possible impact : false targets Backscattering by electron density irregularities

  6. Source : Stephen Quigley, “Space Weather System - Impact Products - SEEFS Overview” False targets

  7. Temporal fluctuations of the trans-ionospheric signal : Amplitude Phase Polarization Direction of arrival Scintillations

  8. POLAR CAP PATCHES AURORAL IRREGULARITIES SATCOM GPS PLASMA BUBBLES EQUATORIAL F LAYER ANOMALIES DAY NIGHT MAGNETIC EQUATOR SBR GPS SATCOM Geographical occurrence of scintillations

  9. Scintillations Parameters S4 and sF S4 and  are statistical variables computed over a “reasonable” time period that satisfies both good statistics and stationarity, as follows “Reasonable Time” depends primarily on the effective velocity of the Line of sight raypath; varies from 10 to 100 seconds; the phase is derived from detrended time series These quantities depend on the density fluctuations in the medium

  10. GPS to monitor the Ionospheric Scintillations • GPS provides a convenient and cost effective way to monitor the ionospheric scintillations : many transmitting satellites + commercial receivers available. PRIS scintillation measurement campaign (ESTEC project) • S4 measured at GPS frequency (L-band) can be extrapolated to other frequencies : f-1.5 dependency

  11. Climatology

  12. GISM to model the scintillations Phase Screen Technique target X The screen size is chosen in relation to the medium coherence length calculated by FFT radar

  13. Amplitude (assuming an m-Nakagami distribution) : Rq : S4²=1/m for one way propagation Phase variance doubled Monostatic Radar : two ways effects

  14. Medium’s Characterisation Fluctuations of the electron density S4 map / Phase map Mostly affects the equatorial regions -20° ML < < 20° ML

  15. S4 animated map

  16. Effects on Radar Signal Processing Intensity scintillations  C / N0 drops Phase scintillations  increases the Doppler noise Angular and range errors  increases the ambiguity Medium’s coherent time drops  limit the integration time ( pb for low RCS targets)

  17. Irregularities Spectrum & Intensity 3 parameters to define the spectrum : S4 = 0.25  5 dB ptp S4 = 0.53  13 dB ptp S4 = 0.66  20 dB ptp The slope p 2 < p < 5 The cut off frequency = f ( 1 / L0) The strength : usually 1 Hz value

  18. Intensity Scintillations C / N0 drops of the fade depth level

  19. Amplitude scintillation Power RCS fast fades Source : Knepp, « Altair VHF /UHF Observations of Multipath and Backscatter Enhancement », IEEE-AP, 1991

  20. Angular Error (RMS) • The RMS angular error ( > 1° at 150 MHz) will increase the ambiguity and degrade the radar performances • The angular error is deduced from the phase autocorrelation function

  21. Coherence Time • The coherence time limits the integration time and will degrade the radar performances for low RCS targets • The coherence time decreases when increasing S4

  22. range spread range Doppler spread Doppler Doppler and range spreads • Phase fluctuations create Doppler spread • Multipath inside the medium create range spread

  23. Doppler noise vs inhomogeneities average size Slope p = 3 L0 = 2500 m. L0 = 500 m.

  24. Range delays due to scintillation ALTAIR Radar measurements at Kwajalein Island (4° North L) Source : D. Knepp IEEE-AP

  25. Scattering function as compared to measurements Source : P. Cannon, N. Rogers, Qinetiq, Nottingham wkshp, Feb 2008

  26. Conclusions • Ionosphere may limit significantly the radar performances especially for targets with low RCS • C / N0 may drop as much as 30 dB • The medium’s coherence time decreases when increasing scintillations (S4) and limit consequently the integration time • All the effects decrease roughly as f-1.5 but are very significant in the VHF and UHF frequency bands

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