ionospheric hf radars pasha ponomarenko. outline conventional radars vs ionospheric radars...
TRANSCRIPT
Ionospheric HF radars
Pasha Ponomarenko
Outline
• Conventional radars vs ionospheric radars
• Collective scatter processes
• Aspect angle effects
• HF propagation features
• SuperDARN
Radars• RAdio Detection And Ranging
• Remote sensing tool based on EM wave emission and its reflection/scatter from a target
Main target parameters:
• range – time of flight
• direction – antenna directivity
• velocity – Doppler shift
Antenna b
eam
echo
Radar Ground
signal
2
tcr
Distance
“Normal” radars
target
The target is usually smaller than the spatial pulse length.
Doppler velocity
f
fcV D
D 2Dft,
Antenna b
eam
Radar
Ionospherewith irregularities
Ground
signal
echo
~300
km
Ionospheric radars
1, eee NNNn
Refractive index fluctuations
The scattering volume is larger than the spatial pulse length.
eNn
n
nn
nnR
22
12
12
2
k
λ
Collective scatter theory
λ/2
irregularities
k
λ
Constructive interference(Bragg condition)
2
l
Spatial array with l = λ/2
Ionospheric turbulenceContinuous spatial spectrum of irregularities
lk
kkS p
2
)(
ln S
ln k
Bragg scatter still works!
ln S
ln k
k
λ
k
λ
λ/2
Constructive interference condition(Bragg condition)
2
l
Anisotropic turbulence
ll ||
E and F region irregularities are aligned with the background magnetic field
B
l
l
B
Dipole antenna with l >> λ
2|| l
Aspect conditions
ψ
ψ
B
phase front
HF Propagation
Ionosphericscatter
Ground scatterGround scatter
Radar
eNN Nf
f
fn ,1
20
22 n <1
n = 1
h
xHF: fN ~ f0
10-20 MHz
SuperDARNSuper Dual Auroral Radar Network
Dual Radar Network
Things to remeber:
• Backscatter signals are produced by plasma structures with Bragg scale sizes, l = λ/2
• Most of the backscatter power comes form areas where the radio wave propagates orthogonally to the geomagnetic field, k B0
• HF signals are capable of over-the-horizon propagation due to consecutive reflections from the ionosphere and the ground