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Binary phase shift keying on orthogonal carriers for multi-channel CO2 absorption measurements in the presence of thin clouds

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Abstract

A new modulation technique for Continuous Wave (CW) Lidar is presented based on Binary Phase Shift Keying (BPSK) using orthogonal carriers closely spaced in frequency, modulated by Maximum Length (ML) sequences, which have a theoretical autocorrelation function with no sidelobes. This makes it possible to conduct multi-channel atmospheric differential absorption measurements in the presence of thin clouds without the need for further processing to remove errors caused by sidelobe interference while sharing the same modulation bandwidth. Flight tests were performed and data were collected using both BPSK and linear swept frequency modulation. This research shows there is minimal or no sidelobe interference in the presence of thin clouds for BPSK compared to linear swept frequency with significant sidelobe levels. Comparisons between of CO2 optical depth Signal to Noise (SNR) between the BPSK and linear swept frequency cases indicate a 21% drop in SNR for BPSK experimentally using the instrument under consideration.

© 2014 Optical Society of America

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Figures (4)

Fig. 1
Fig. 1 Baseline instrument block diagram. Here the TIA is the transimpendence amplifier and DAQ is the data acquisition unit..
Fig. 2
Fig. 2 Autocorrelation function using the BPSK reference waveform represented by Eq. (9) is completely lacking in sidelobes or other artifacts, which is the key advantage for using BPSK modulation.
Fig. 3
Fig. 3 Comparison of range profiles from aircraft measurement for swept frequency linear scale (a) and log scale (c) vs. BPSK linear scale (b) and log scale (d) through clouds shows dramatic reduction in side lobe level. Measurements for each case were taken over different altitudes and flight legs.
Fig. 4
Fig. 4 Optical depth measurement for clear sky case for linear swept frequency (a) vs BPSK (b) at 11.4 km altitude using 203200 point frames. Each used the exact same center frequencies for each channel. Optical depth SNR is about 267 for linear swept frequency vs 212 for BPSK over water using 0.1 sec averages. The difference is mainly due to more of the BPSK signal being filtered out because of its wider total bandwidth.

Tables (1)

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Table 1 Modulation parameters used in flight tests. The frequencies f1-f4 represent the center modulation frequency of each channel.

Equations (10)

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Λ off =1+m ξ off ( t ), Λ on =1+m ξ on ( t ),
P on,k R ( t )= K k r 2   P on,k T ¯ exp( 2S 0 r β( r' )dr' )exp( 2τ )exp( 2τ' )( 1+m ξ on ( t2r/c ) ), P off,k R ( t )= K k r 2   P off,k T ¯ exp( 2S 0 r β( r' )dr' )exp( 2τ )( 1+m ξ off ( t2r/c ) ),
s( t )= k [ C 1k m ξ on ( t2 r k /c )+ C 2k m ξ off ( t2 r k /c ) ] ,
C 1k = K k ' r 2   P on T ¯ exp( 2S 0 r k β( r' )dr' )exp( 2 τ k )exp( 2 τ k ' ), C 2k = K k ' r 2   P off T ¯ exp( 2S 0 r k β( r' )dr' )exp( 2 τ k ),
τ g '= 1 2 ln( C 2g P on T ¯ C 1g P off T ¯ ) 1 2 ln( P off, g R ¯ P on T ¯ P on, g R ¯ P off T ¯ ),
ξ on ( n )=( 2Z( n )1 )cos( 2πn  f on / f s ), ξ off ( n )=( 2Z( n )1 )cos( 2πn  f off / f s ),
f 01 = n 1 2PT , f 02 = n 2 2PT ,..., f 0K = n K 2PT ,
Γ on ( n )=( 2Z( n )1 )exp( 2πin f on / f s ), Γ off ( n )=( 2Z( n )1 )exp( 2πin f off / f s ).
Γ ' on ( n )=Z( n )exp( 2πin f on / f s ),Γ ' off ( n )=Z( n )exp( 2πin f off / f s ).
R( ref,data )= 1 N m=0 N1 re f * ( m )  data( m+n )     =DF T 1 ( DF T * ( re f * )DFT( data ) ),
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