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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