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Some ionospheric effects on ground based radar
Y. Béniguel, J.-P. Adam
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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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TEC
• Group delay
• Dispersion
• Faraday rotation
• Direction of arrival
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Medium’s CharacterisationElectron Density
TEC map
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Backscattering by electron density irregularities
• Frequently at high latitude : Radar Auroral Clutter
• Possible impact : false targets
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False targets
Source : Stephen Quigley, “Space Weather System - Impact Products - SEEFS Overview”
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Scintillations
• Temporal fluctuations of the trans-ionospheric signal :
– Amplitude– Phase– Polarization– Direction of arrival
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SATCOM
AURORAL IRREGULARITIES
GPS
PLASMA BUBBLES
GPS SATCOM
MAGNETICEQUATOR
DAY NIGHT
EQUATORIAL F LAYERANOMALIES
SBR
POLAR CAPPATCHES
Geographical occurrence of scintillations
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Scintillations ParametersS4 and
S4 and are statistical variables computed over a “reasonable” time
period that satisfies both good statistics and stationarity, as follows
I
IstdS
)(4 var
“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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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.
offline on line
Javad receiver GSV receiver
Kiruna
Can. Islands*
Vietman
Cayenne
Bandung
Douala
São Paulo
Ndjamena*
offline on line
Javad receiver GSV receiver
Kiruna
Can. Islands*
Vietman
Cayenne
Bandung
Douala
São Paulo
Ndjamena*
Kiruna
Can. Islands*
Vietman
Cayenne
Bandung
DoualaNdjamena*
offline on line
Javad receiver GSV receiver
Kiruna
Can. Islands*
Vietman
Cayenne
Bandung
Douala
São Paulo
Ndjamena*
Kiruna
Can. Islands*
Vietman
Cayenne
Bandung
Douala
São Paulo
Ndjamena*
offline on line
Javad receiver GSV receiver
Kiruna
Can. Islands*
Vietman
Cayenne
Bandung
Douala
São Paulo
Ndjamena*
Kiruna
Can. Islands*
Vietman
Cayenne
Bandung
DoualaNdjamena*
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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Climatology
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GISM to model the scintillations Phase Screen Technique
dK )x K j k 2
z Kj ( exp ) 0 , K ( E e ) z x, ( E
2 zk j
radarcalculated by FFT
The screen size is chosen in relation to the medium coherence length
X target
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• 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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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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S4 animated map
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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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1 103
0.01 0.1 1 10 1001 10
6
1 105
1 104
1 103
0.01
0.1
1
10
100
1 103
PhaseAmplitude
Power densities
frequency (Hz)
Irregularities Spectrum & Intensity
3 parameters to define the spectrum :
The slope p 2 < p < 5
The cut off frequency = f ( 1 / L0)
The strength : usually 1 Hz value
-15
-10
-5
0
5
0 5 10 15
S4 = 0.66
S4 = 0.53
S4 = 0.26
dB
time (s.)
S4 = 0.25 5 dB ptpS4 = 0.53 13 dB ptpS4 = 0.66 20 dB ptp
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Intensity Scintillations
C / N0 drops of the fade depth level
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Amplitude scintillation
Source : Knepp, « Altair VHF /UHF Observations of Multipath and Backscatter Enhancement », IEEE-AP, 1991
Power RCS fast fades
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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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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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Doppler and range spreads
•Phase fluctuations create Doppler spread
Doppler
rang
eDopplerspread
ran
ge
spre
ad
•Multipath inside the medium create range spread
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L0 = 2500 m. L0 = 500 m.
Doppler noise vs inhomogeneities average size
Slope p = 3
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Range delays due to scintillation
ALTAIR Radar measurements at Kwajalein Island (4° North L)
Source : D. Knepp IEEE-AP
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Source : P. Cannon, N. Rogers, Qinetiq, Nottingham wkshp, Feb 2008
Scattering function as compared to measurements
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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