2-bordoni_igarss11_apc.ppt
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Folie 1
Ambiguity Suppression by Azimuth Phase Coding in Multichannel SAR Systems
DLR - Institut für Hochfrequenztechnik und Radarsysteme
F. Bordoni, M. Younis, G. Krieger
IGARSS 2011, 24-29 July, Vancouver, Canada
Microwaves and Radar Institute 2
Outline
o Introduction
o APC (Azimuth Phase Coding) technique
o APC in multichannel SAR (Synthetic Aperture Radar) systems
o Figure of merit
o Numerical analysis
o APC performance versus system parameters
o Example: two multichannel systems for high resolution wide swath imaging
o Conclusions
Microwaves and Radar Institute 3
Introduction
Current spaceborne SAR systems limitation: trade-off spatial resolution v.s. swath width
Research in two main directions:
Processing methods for removing the ambiguities
APC- low implementation complexity
- effectiveness for point and distributed ambiguities
New, more flexible SAR systems- Multichannel systems-Digital Beamforming (DBF) on receive- Multichannel processing
APC is conceived for conventional SAR systems:
APC in multichannel systems based on DBF on receive?
Microwaves and Radar Institute 4
Review of the APC TechniqueAPC is a technique for range ambiguity suppression, conceived for
conventional (1 Tx and 1 Rx) SAR systems [Dall, Kusk 2004]
mod ( )lϕ
mod( ) ( )dem n n mϕ ϕ= −
azimuth sample number, order of range ambiguity, APC shift-factor
APC residual phase
[Dal04] J. Dall, A. Kusk, “Azimuth Phase Coding for Range Ambiguity Suppression in SAR”, IGARSS 2004.
@ round-trip delay
( , , )res n k Mϕ
APC modulation phase
Tx pulse number
3) Azimuth filtering over the processing bandwidth
APC demodulation phase
APC is based on three main steps:
1) Azimuth, i.e. pulse to pulse, phase modulation on Tx
2) Azimuth phase demodulation on Rx
Microwaves and Radar Institute 5
APC residual phase ⇔ Doppler shift
order of range ambiguity (0 useful signal)
M=2 maximum Doppler shift of the 1st order range ambiguity Larger oversampling Larger ambiguity suppression / pPRF B
{ }, ( ) ( ) exp ( , , )k apc k resx n x n j n k Mϕ= ⋅
Time domain: linear phase Frequency domain: Doppler shift
, ( ) ( )k apc kX f X f f= − ∆
2( , , )res n k M k n
M
πϕ =
resϕ
1 M…
2π
x0
0
( / )
n
t n PRF=
x
x
x
+
+
2
M
π
…
k = 1
k =
2
k = 1
2k
M
π
… …
resϕ
1 M…
2π
x0
0
( / )
n
t n PRF=
x
x
x
+
+
2
M
π
…
k = 1
k =
2
k = 1
2k
M
π
… …
f0
PRF
Bp
k =
0
k =
1
k =
2
∆f (1, M)
PRF
M2
PRF
M
∆f (2, M)
f0
PRF
Bp
k =
0
k =
1
k =
2
∆f (1, M)
PRF
M2
PRF
M
∆f (2, M)
Az.
FIL
TE
R
Az.
FIL
TE
R
/ 2
( , ) modPRF
PRFf f k M k
M ∆ = ∆ =
Microwaves and Radar Institute 6
Application to Multichannel Systems
mod ( , )l Mϕ
MULTICHANNEL PROCESSING
N Rx az. signals sampled at PRF
APC residual phase:
APC residual phase: ( , , )mc mcres n k Mϕ
( ), ( ) ( )k apc kX f X f f k= − ∆
1 2 N
( , )dem n Mϕ
( , , )res n k Mϕ
( , )dem n Mϕ
Multichannel SAR system: 1 transmitter, N receivers
The behavior of the APC changes when applied to a multichannel system
, ( )rk apcX freconstructed multichannel signal sampled at PRFeff =N PRF:
, ( )rk apcX f
PRF << Bp
Microwaves and Radar Institute 7
APC & Reconstructed Multichannel Signal The APC residual phase has no more a linear trend versus the azimuth sample
(pulse) number no shift of the Spectrum
The residual phase a “stair” shape (<≠> Doppler shift):
The ambiguity spectrum: { }, ( ) ( ) exp ( , , )r r mck apc k resX f X f FT j n k Mϕ = ⊗
N M 2= =mcresϕ
2
2π
00 ( )/( )
mc
mc
n
t n N PRF= ⋅
k= 1, 3, …x
xx4
π
xx
xx
mcresϕ
2
2π
00 ( )/( )
mc
mc
n
t n N PRF= ⋅
k= 1, 3, …x
xx xx4
π
xx xx
xx xx
f0
PRFeff = 2 PRF
Bp
PRF
f0
PRFeff = 2 PRF
Bp
PRF
2( , , ) int
mcmc mcres
nn k M k
M N
πϕ = ⋅
,
,
PRF
(uniform PRF*)
*PRF matched to the antenna length and No. of apertures > regular sampling in azimuth results
Microwaves and Radar Institute 8
Figure of MeritMeasurement of the ambiguity suppression induced by APC
p
p
p
p
B / 22r
1B / 2
apc B / 22r
1,apcB / 2
X ( f ) df
G
X ( f ) df
−
−
=
∫
∫
PSD (Power Spectral Density) range ambiguity of 1st order if APC is not applied
processed bandwidth
PSD range ambiguity of 1st order if APC is applied
APC Gain:
r r1 0,apcX ( f ) X ( f )=
useful signal after multichannel reconstruction (neglect. elev.)
Computed on the SAR signal after multichannel processing
Note: the Gapc depends on the azimuth pattern shape
Microwaves and Radar Institute 9
Parameter System #
1 2 3(Ref.) 4
Orbit height [km] 520
Carrier frequency [GHz] 9.600
Rx antenna total length [m] 3 6 12 24
Tx antenna length [m](and Rx subapert. length)
3
No. of az. Rx channels 1 2 4 8
PRF [Hz] (uniform) 5068 2534 1267 633.5
PRFeff [Hz] 5068
APC Performance Analysis
Reference Multichannel Planar Systems
PRFeff
Bp
PRFeff
Bp
The systems have the same azimuth patterns
Effect of the Doppler oversampling The effect of the pattern shape is not evident
pN PRF Bγ = ⋅Behavior of APC versus the number of Rx channels, N
Processing bandwidth 2316 Hz ≤ Bp ≤ 4168 Hz
Investigation:
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Numerical Results: Gapc
0.1dB ≤ Gapc ≤ 3.13dB
for a given N, the Gapc increases with the oversampling factor, γ
the Gapc decreases for increasing number of channels, N
the sensitivity of Gapc to γ decreases with increasing N
N=1
N=2
N=4
N=8
N=1
N=2
N=4
N=8
APC Gain v.s. oversampling factor
For the considered systems, for M=2:
Microwaves and Radar Institute 11
Numerical Results: PSD v.s. N
larger N, the upper profile PSD with or without APC are similar and Gapc reduces
Normalized PSD 1st range ambiguity after multichannel reconstruction
BpBp BpBpBpBpBpBp
N = 8
with APC
N = 2N = 1
without APC
BpBpBp
N = 1, 2, 8
The thickness of the curves is a fast variation of the spectrum,
due to aliasing
Microwaves and Radar Institute 12
HRWS (High-Resolution Wide-Swath) SAR Systempromoted by the German Aerospace Centre (DLR)conceived to obtain high resolution and wide swaths
(1 m resolution, 70 km swath width in stripmap mode)
Different Rx azimuth patterns & multichannel reconstruction
HRWS SAR Multichannel Systems
Parameter Planar Reflector
Orbit height [km] 520 745
Carrier frequency [GHz] 9.600 9.650
Tx/Rx antenna total length [m] 8.75
Paraboloid diameter (elev., az.) [m] 10, 12
Total number of feeds (elev., az.) 60, 10
No. of az. Rx channels 7 10
PRF [Hz] 1750 2792
Processed bandwidth [Hz] 6252 5946
Oversampling factor 1.960 4.696
Planar system: currently adopted design
Reflector system:alternative design option,
studied in DLR
Microwaves and Radar Institute 13
Peculiarities HRWS Systems
Reflector systemPlanar system
The pattern of each Rx channel covers Bp
Multichannel processing: Multi-Aperture Reconstr.
The patters do not change along the swath
PRF
Be
PRF
Be
e pB B / N=
pB
N PRF⋅
pB
The pattern of each Rx channel covers 1/N of Bp
Multichannel processing: Spectral decomposition
The patters change along the swath
Evidence of the dependence of the APC performance on the pattern shape
Microwaves and Radar Institute 14
Numerical Results: Planar HRWS System
For M=2, Gapc = 0.69 dB
The high number of channels (7) and the small oversampling (1.96) associated low Gapc
BpBpBp BpBpBp
with APCwithout APC
Normalized PSD 1st range ambiguity used to compute the Gapc
(after multichannel reconstruction)
Microwaves and Radar Institute 15
Numerical Results: Reflector HRWS System
For M=2, 3.2 dB ≤ Gapc ≤ 8.6 dB over the swath, depending on the azimuth pattern
The azimuth pattern strongly affects the APC performance
The reflector based system, characterized by a higher oversampling factor (4), takes better advantage from the application of APC
Normalized PSD 1st range ambiguity used to compute the Gapc
(before multichannel reconstruction, single Rx channel)
BeBeBe
BeBe
without APC with APC
Microwaves and Radar Institute 16
Conclusionso In multichannel systems, the APC effect is no more a frequency shift of the
range ambiguity.
o Also in multichannel systems, the APC allows for improved ambiguity suppression.
o The azimuth pattern strongly affects the APC performance.
o For a given azimuth pattern, the suppression is directly proportional to the oversampling factor and inversely proportional to the number of receive channels.
o In a conventional SAR system with γ = 2, the achievable suppression of each ambiguity of odd order is about 3 dB. In multichannel systems based on planar antenna architectures, the suppression is generally poorer.
Reflector based systems reach better performance, because of the higher oversampling.
o In the planar and reflector based HRWS systems the APC suppression is about 0.7 dB and between 3 and 8 dB, respectively.
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