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ESWW 5 Some ionospheric effects on ground based radar Y. Bniguel, J.-P. Adam Slide 2 ESWW 5 Ionospheric effects Total Electron Content (TEC) dependent effects (litterature review). Backscattering by electron density irregularities (litterature review). Scintillations (IEEA field of research). Slide 3 ESWW 5 TEC Group delay Dispersion Faraday rotation Direction of arrival Slide 4 ESWW 5 Mediums Characterisation Electron Density TEC map Slide 5 ESWW 5 Backscattering by electron density irregularities Frequently at high latitude : Radar Auroral Clutter Possible impact : false targets Slide 6 ESWW 5 False targets Source : Stephen Quigley, Space Weather System - Impact Products - SEEFS Overview Slide 7 ESWW 5 Scintillations Temporal fluctuations of the trans-ionospheric signal : Amplitude Phase Polarization Direction of arrival Slide 8 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 Slide 9 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 Slide 10 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 Slide 11 ESWW 5 Climatology Slide 12 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 Slide 13 ESWW 5 Amplitude (assuming an m-Nakagami distribution) : Rq : S4=1/m for one way propagation Phase variance doubled Monostatic Radar : two ways effects Slide 14 ESWW 5 Mediums Characterisation S4 map / Phase map Fluctuations of the electron density Mostly affects the equatorial regions -20 ML < < 20 ML Slide 15 ESWW 5 S4 animated map Slide 16 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 Mediums coherent time drops limit the integration time ( pb for low RCS targets) Slide 17 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 Slide 18 ESWW 5 Intensity Scintillations C / N0 drops of the fade depth level Slide 19 ESWW 5 Amplitude scintillation Source : Knepp, Altair VHF /UHF Observations of Multipath and Backscatter Enhancement , IEEE-AP, 1991 Power RCS fast fades Slide 20 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 Slide 21 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 Slide 22 ESWW 5 Doppler and range spreads Phase fluctuations create Doppler spread Doppler range Doppler spread range spread Multipath inside the medium create range spread Slide 23 ESWW 5 L 0 = 2500 m. L 0 = 500 m. Doppler noise vs inhomogeneities average size Slope p = 3 Slide 24 ESWW 5 Range delays due to scintillation ALTAIR Radar measurements at Kwajalein Island (4 North L) Source : D. Knepp IEEE-AP Slide 25 ESWW 5 Source : P. Cannon, N. Rogers, Qinetiq, Nottingham wkshp, Feb 2008 Scattering function as compared to measurements Slide 26 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 mediums 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