74 lines
1.7 KiB
Plaintext
74 lines
1.7 KiB
Plaintext
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subroutine xcor(ss,ipk,nsteps,nsym,lag1,lag2,ccf,ccf0,lagpk,flip,fdot,nrobust)
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! Computes ccf of a row of ss and the pseudo-random array pr. Returns
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! peak of the CCF and the lag at which peak occurs. For JT65, the
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! CCF peak may be either positive or negative, with negative implying
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! the "OOO" message.
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use jt65_mod
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parameter (NHMAX=3413) !Max length of power spectra
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parameter (NSMAX=322) !Max number of half-symbol steps
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real ss(NSMAX,NHMAX) !2d spectrum, stepped by half-symbols
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real a(NSMAX)
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real ccf(-11:540)
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data lagmin/0/ !Silence g77 warning
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save
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df=12000.0/8192.
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dtstep=0.5/df
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fac=dtstep/(60.0*df)
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do j=1,nsteps
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ii=nint((j-nsteps/2)*fdot*fac)+ipk
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if( (ii.ge.1) .and. (ii.le.NHMAX) ) then
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a(j)=ss(j,ii)
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endif
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enddo
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if(nrobust.eq.1) then
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! use robust correlation estimator to mitigate AGC attack spikes at beginning
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! this reduces the number of spurious candidates overall
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call pctile(a,nsteps,50,xmed)
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do j=1,nsteps
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if( a(j).ge.xmed ) then
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a(j)=1
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else
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a(j)=-1
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endif
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enddo
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endif
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ccfmax=0.
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ccfmin=0.
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do lag=lag1,lag2
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x=0.
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do i=1,nsym
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j=2*i-1+lag
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if(j.ge.1 .and. j.le.nsteps) x=x+a(j)*pr(i)
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enddo
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ccf(lag)=2*x !The 2 is for plotting scale
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if(ccf(lag).gt.ccfmax) then
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ccfmax=ccf(lag)
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lagpk=lag
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endif
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if(ccf(lag).lt.ccfmin) then
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ccfmin=ccf(lag)
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lagmin=lag
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endif
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enddo
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ccf0=ccfmax
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flip=1.0
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if(-ccfmin.gt.ccfmax) then
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do lag=lag1,lag2
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ccf(lag)=-ccf(lag)
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enddo
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lagpk=lagmin
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ccf0=-ccfmin
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flip=-1.0
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endif
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return
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end subroutine xcor
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