Transcript 2-Bordoni_IGARSS11_A..

Ambiguity Suppression by Azimuth Phase
Coding in Multichannel SAR Systems
F. Bordoni, M. Younis, G. Krieger
DLR - Institut für Hochfrequenztechnik und Radarsysteme
IGARSS 2011, 24-29 July, Vancouver,Folie
Canada
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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
o
APC performance versus system parameters
o
Example: two multichannel systems for high resolution wide swath imaging
Conclusions
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Introduction
Current spaceborne SAR systems limitation:
trade-off spatial resolution v.s. swath width
Research in two main directions:
New, more flexible SAR systems
- Multichannel systems
-Digital Beamforming (DBF) on receive
- Multichannel processing
Processing methods for
removing the ambiguities
APC
- low implementation complexity
- effectiveness for
point and distributed ambiguities
APC is conceived for conventional SAR systems:
 APC in multichannel systems based on DBF on receive?
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Review of the APC Technique
APC is a technique for range ambiguity suppression, conceived for
conventional (1 Tx and 1 Rx) SAR systems [Dall, Kusk 2004]
[Dal04] J. Dall, A. Kusk, “Azimuth Phase Coding for Range Ambiguity Suppression in SAR”, IGARSS 2004.
APC is based on three main steps:
1) Azimuth, i.e. pulse to pulse, phase modulation on Tx
APC modulation phase
mod (l )
Tx pulse number
2) Azimuth phase demodulation on Rx
APC demodulation phase
dem (n)  mod (n  m) @ round-trip delay
APC residual phase
res (n, k , M )
azimuth sample number, order of range ambiguity,
APC shift-factor
3) Azimuth filtering over the processing bandwidth
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APC residual phase  Doppler shift
Time domain: linear phase
Frequency domain: Doppler shift
xk ,apc (n)  xk (n)  exp jres (n, k , M )
X k ,apc ( f )  X k ( f  f )
2
kn
M
order of range ambiguity (0 useful signal)
 PRF 
f  f (k , M )  mod k

 M  PRF / 2
res (n, k , M ) 
x
+
0
1
x
…
2
k
M
…
k
x
M
=
…
1
n
(t  n / PRF )
Az. FILTER
x
1
f (2, M)
k=2
k=
…
+
2
M
0
k
=
k=1
2
2
Az. FILTER
res
k=0
f (1, M)
0
PRF
M
2
PRF
M
f
Bp
PRF
M=2  maximum Doppler shift of the 1st order range ambiguity
 Larger oversampling PRF / Bp  Larger ambiguity suppression
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Application to Multichannel Systems
Multichannel SAR system: 1 transmitter, N receivers
1
N
2
N Rx az. signals sampled at PRF
PRF << Bp
mod (l, M )
dem (n, M )
dem (n, M )
APC residual phase: res (n, k, M )
X k ,apc ( f )  X k  f  f (k ) 
X kr,apc ( f )
MULTICHANNEL PROCESSING
reconstructed multichannel signal sampled at PRFeff =N PRF:
X kr,apc ( f )
mc mc
APC residual phase: res (n , k , M )
 The behavior of the APC changes when applied to a multichannel system
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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
(uniform PRF*)
mc
res
N M 2
2
x x

,
,
0
x x
0
PRF
x k= 1, 3, …
x x
nmc
2
4
t  n
mc
/( N  PRF )

0
f
Bp
PRF
PRFeff = 2 PRF
mc

2
n 

k, M ) 
k  int 
 The residual phase a “stair” shape (<≠> Doppler shift):

M
N




r
r
mc
 The ambiguity spectrum: X k ,apc ( f )  X k ( f )  FT exp  jres (n, k , M ) 


mc mc
res
(n ,


*PRF matched to the antenna length and No. of apertures > regular sampling in azimuth results
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Figure of Merit
Measurement of the ambiguity suppression induced by APC
APC Gain:
Computed on the SAR signal after multichannel processing
PSD (Power Spectral Density) range ambiguity of 1st order
if APC is not applied
 Bp / 2

2
r


X
(
f
)
df
1
 


B
/
2
p

Gapc  
 Bp / 2

2
r

X
( f ) df 
1,apc

processed bandwidth 


B
/
2
 p

r
X 1r ( f )  X 0,apc
(f)
useful signal after multichannel
reconstruction (neglect. elev.)
PSD range ambiguity of 1st order if APC is applied
 Note: the Gapc depends on the azimuth pattern shape
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APC Performance Analysis
Reference Multichannel Planar Systems
Parameter
System #
1
2
3(Ref.)
Orbit height [km]
4
520
Carrier frequency [GHz]
9.600
Rx antenna total length [m]
3
6
12
Tx antenna length [m]
(and Rx subapert. length)
24
3
Bp
No. of az. Rx channels
1
2
4
8
PRF [Hz]
5068
2534
1267
633.5
(uniform)
PRFeff [Hz]
The systems have the
same azimuth patterns
PRFeff
5068
Processing bandwidth 2316 Hz ≤ Bp ≤ 4168 Hz
Investigation:
Behavior of APC versus the number of Rx channels, N
Effect of the Doppler oversampling   N  PRF Bp
 The effect of the pattern shape is not evident
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Numerical Results: Gapc
APC Gain v.s. oversampling factor
N=1
N=2
N=4
N=8
For the considered systems, for M=2:
 0.1dB ≤ Gapc ≤ 3.13dB
 for a given N, the Gapc increases with the oversampling factor, 
 the Gapc decreases for increasing number of channels, N
 the sensitivity of Gapc to  decreases with increasing N
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Numerical Results: PSD v.s. N
Normalized PSD 1st range ambiguity after multichannel reconstruction
N = 1, 2, 8
without APC
The thickness of the curves is a
fast variation of the spectrum,
due to aliasing
Bp
with APC
N=1
Bp
N=2
Bp
N=8
Bp
 larger N, the upper profile PSD with or without APC are similar and Gapc reduces
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HRWS SAR Multichannel Systems
HRWS (High-Resolution Wide-Swath) SAR System
promoted by the German Aerospace Centre (DLR)
conceived to obtain high resolution and wide swaths
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
(1 m resolution, 70 km swath width in stripmap mode)
Different Rx azimuth patterns & multichannel reconstruction
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Peculiarities HRWS Systems
Planar system
Reflector system
Bp
Bp
Be
PRF
Be  Bp / N
N  PRF
The pattern of each Rx channel covers Bp
The pattern of each Rx channel covers 1/N of Bp
Multichannel processing: Multi-Aperture Reconstr.
Multichannel processing: Spectral decomposition
The patters do not change along the swath
The patters change along the swath
 Evidence of the dependence of the APC performance on the pattern shape
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Numerical Results: Planar HRWS System
Normalized PSD 1st range ambiguity used to compute the Gapc
(after multichannel reconstruction)
without APC
with APC
Bp
Bp
For M=2, Gapc = 0.69 dB
The high number of channels (7) and the small oversampling (1.96) associated low Gapc
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Numerical Results: Reflector HRWS System
Normalized PSD 1st range ambiguity used to compute the Gapc
(before multichannel reconstruction, single Rx channel)
without APC
with APC
Be
Be
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
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Conclusions
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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  = 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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