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ESWW 5 Some ionospheric effects on ground based radar Y. Béniguel, J.-P. Adam

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ESWW 5 Ionospheric effects Total Electron Content (TEC) dependent effects (litterature review). Backscattering by electron density irregularities (litterature review). Scintillations (IEEA field of research).

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ESWW 5 TEC Group delay Dispersion Faraday rotation Direction of arrival

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ESWW 5 Medium’s Characterisation Electron Density TEC map

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

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ESWW 5 False targets Source : Stephen Quigley, “Space Weather System - Impact Products - SEEFS Overview”

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ESWW 5 Scintillations Temporal fluctuations of the trans-ionospheric signal : –Amplitude –Phase –Polarization –Direction of arrival

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ESWW 5 SATCOM AURORAL IRREGULARITIES GPS PLASMA BUBBLES GPS SATCOM MAGNETIC EQUATOR DAYNIGHT EQUATORIAL F LAYER ANOMALIES SBR POLAR CAP PATCHES Geographical occurrence of scintillations

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ESWW 5 Scintillations Parameters S4 and 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

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ESWW 5 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

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ESWW 5 Climatology

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ESWW 5 GISM to model the scintillations Phase Screen Technique radar calculated by FFT The screen size is chosen in relation to the medium coherence length X target

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ESWW 5 Amplitude (assuming an m-Nakagami distribution) : Rq : S4²=1/m for one way propagation Phase variance doubled Monostatic Radar : two ways effects

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ESWW 5 Medium’s Characterisation S4 map / Phase map Fluctuations of the electron density Mostly affects the equatorial regions -20° ML < < 20° ML

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ESWW 5 S4 animated map

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ESWW 5 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)

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ESWW 5 Irregularities Spectrum & Intensity 3 parameters to define the spectrum : The slope p 2 < p < 5 The cut off frequency = f ( 1 / L 0 ) The strength : usually 1 Hz value S4 = 0.25 5 dB ptp S4 = 0.53 13 dB ptp S4 = 0.66 20 dB ptp

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ESWW 5 Intensity Scintillations C / N0 drops of the fade depth level

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ESWW 5 Amplitude scintillation Source : Knepp, « Altair VHF /UHF Observations of Multipath and Backscatter Enhancement », IEEE-AP, 1991 Power RCS fast fades

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ESWW 5 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

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ESWW 5 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

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ESWW 5 Doppler and range spreads Phase fluctuations create Doppler spread Doppler range Doppler spread range spread Multipath inside the medium create range spread

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ESWW 5 L 0 = 2500 m. L 0 = 500 m. Doppler noise vs inhomogeneities average size Slope p = 3

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ESWW 5 Range delays due to scintillation ALTAIR Radar measurements at Kwajalein Island (4° North L) Source : D. Knepp IEEE-AP

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ESWW 5 Source : P. Cannon, N. Rogers, Qinetiq, Nottingham wkshp, Feb 2008 Scattering function as compared to measurements

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ESWW 5 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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