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subroutine synciscat(cdat,npts,nh,npct,s0,jsym,df,ntol,NFreeze, &
MouseDF,mousebutton,mode4,nafc,psavg,xsync,sig,ndf0,msglen, &
ipk,jpk,idf,df1)
! Synchronize an ISCAT signal
! cdat() is the downsampled analytic signal.
! Sample rate = fsample = BW = 11025 * (9/32) = 3100.78125 Hz
! npts, nsps, etc., are all reduced by 9/32
parameter (NMAX=30*3101)
parameter (NSZ=4*1400)
complex cdat(NMAX)
complex c(288)
real s0(288,NSZ)
real fs0(288,96) !108 = 96 + 3*4
real savg(288)
real sref(288)
real psavg(72) !Average spectrum of whole file
integer icos(4)
data icos/0,1,3,2/
data nsync/4/,nlen/2/,ndat/18/
! Silence compiler warnings:
sigbest=-20.0
ndf0best=0
msglenbest=0
ipkbest=0
jpkbest=0
ipk2=0
idfbest=mousebutton
fsample=3100.78125 !New sample rate
nsps=144/mode4
nsym=npts/nsps - 1
nblk=nsync+nlen+ndat
nfft=2*nsps !FFTs at twice the symbol length,
kstep=nsps/4 ! stepped by 1/4 symbol
df=fsample/nfft
fac=1.0/1000.0 !Somewhat arbitrary
savg=0.
s0=0.
ia=1-kstep
do j=1,4*nsym !Compute symbol spectra
ia=ia+kstep
ib=ia+nsps-1
if(ib.gt.npts) exit
c(1:nsps)=fac*cdat(ia:ib)
c(nsps+1:nfft)=0.
call four2a(c,nfft,1,-1,1)
do i=1,nfft
s0(i,j)=real(c(i))**2 + aimag(c(i))**2
savg(i)=savg(i) + s0(i,j) !Accumulate avg spectrum
enddo
i0=40
enddo
jsym=4*nsym
savg=savg/jsym
do i=1,71 !Compute spectrum in dB, for plot
if(mode4.eq.1) then
psavg(i)=2*db(savg(4*i)+savg(4*i-1)+savg(4*i-2)+savg(4*i-3)) + 1.0
else
psavg(i)=2*db(savg(2*i)+savg(2*i-1)) + 7.0
endif
enddo
do i=nh+1,nfft-nh
call pctile(savg(i-nh),2*nh+1,npct,sref(i))
enddo
sref(1:nh)=sref(nh+11)
sref(nfft-nh+1:nfft)=sref(nfft-nh)
do i=1,nfft !Normalize the symbol spectra
fac=1.0/sref(i)
if(i.lt.11) fac=1.0/savg(11)
do j=1,jsym
s0(i,j)=fac*s0(i,j)
enddo
enddo
nfold=jsym/96
jb=96*nfold
ttot=npts/fsample !Length of record (s)
df1=df/ttot !Step size for f1=fdot
idf1=-25.0/df1
idf2=5.0/df1
if(nafc.eq.0) then
idf1=0
idf2=0
else if(mod(-idf1,2).eq.1) then
idf1=idf1-1
endif
xsyncbest=0.
do idf=idf1,idf2 !Loop over fdot
fs0=0.
do j=1,jb !Fold s0 into fs0, modulo 4*nblk
k=mod(j-1,4*nblk)+1
ii=nint(idf*float(j-jb/2)/float(jb))
ia=max(1-ii,1)
ib=min(nfft-ii,nfft)
do i=ia,ib
fs0(i,k)=fs0(i,k) + s0(i+ii,j)
enddo
enddo
ref=nfold*4
i0=27
if(mode4.eq.1) i0=95
ia=i0-nint(ntol/df)
ib=i0+nint(ntol/df)
if(ia.lt.1) ia=1
if(ib.gt.nfft-3) ib=nfft-3
smax=0.
ipk=1
jpk=1
do j=0,4*nblk-1 !Find sync pattern: lags 0-95
do i=ia,ib !Search specified freq range
ss=0.
do n=1,4 !Sum over 4 sync tones
k=j+4*n-3
if(k.gt.96) k=k-96
ss=ss + fs0(i+2*icos(n),k)
enddo
if(ss.gt.smax) then
smax=ss
ipk=i !Frequency offset, DF
jpk=j+1 !Time offset, DT
endif
enddo
enddo
xsync=smax/ref - 1.0
if(nfold.lt.26) xsync=xsync * sqrt(nfold/26.0)
xsync=xsync-0.5 !Empirical
sig=db(smax/ref - 1.0) - 15.0
if(mode4.eq.1) sig=sig-5.0
! if(sig.lt.-20 .or. xsync.lt.1.0) sig=-20.0
! if(sig.lt.-20) sig=-20.0
ndf0=nint(df*(ipk-i0))
smax=0.
ja=jpk+16
if(ja.gt.4*nblk) ja=ja-4*nblk
jj=jpk+20
if(jj.gt.4*nblk) jj=jj-4*nblk
do i=ipk,ipk+60,2 !Find User's message length
ss=fs0(i,ja) + fs0(i+10,jj)
if(ss.gt.smax) then
smax=ss
ipk2=i
endif
enddo
msglen=(ipk2-ipk)/2
if(msglen.lt.2 .or. msglen.gt.29) cycle
if(xsync.ge.xsyncbest) then
xsyncbest=xsync
sigbest=sig
ndf0best=ndf0
msglenbest=msglen
ipkbest=ipk
jpkbest=jpk
idfbest=idf
endif
enddo
xsync=xsyncbest
sig=sigbest
ndf0=ndf0best
msglen=msglenbest
ipk=ipkbest
jpk=jpkbest
idf=idfbest
if(nafc.eq.0) idf=0
return
end subroutine synciscat
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