2006-09-06 14:09:05 -04:00
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subroutine longx(dat,npts0,ps,DFTolerance,noffset,
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+ msg,msglen,bauderr)
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C Look for 441-baud modulation, synchronize to it, and decode message.
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C Longest allowed data analysis is 1 second.
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parameter (NMAX=11025)
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parameter (NDMAX=NMAX/25)
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real dat(npts0)
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real ps(128),psmo(20)
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integer DFTolerance
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real y1(NMAX)
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real y2(NMAX)
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real y3(NMAX)
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real y4(NMAX)
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real wgt(-2:2)
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integer dit(NDMAX)
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integer n4(0:2)
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character msg*40
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character c*48
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common/acom/a1,a2,a3,a4
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data c/' 123456789.,?/# $ABCD FGHIJKLMNOPQRSTUVWXY 0EZ '/
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data wgt/1.0,4.0,6.0,4.0,1.0/
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NSPD=25 !Change if FSK110 is implemented
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LTone=2
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NBaud=11025/NSPD
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npts=min(NMAX,npts0)
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df=11025.0/256.0
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smax=0.
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C Find the frequency offset of this ping.
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C NB: this might be improved by including a bandpass correction to ps.
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ia=nint((LTone*NBaud-DFTolerance)/df)
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ib=nint((LTone*NBaud+DFTolerance)/df)
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do i=ia,ib !Search for correct DF
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sum=0.
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do j=1,4 !Sum over the 4 tones
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m=nint((i*df+(j-1)*NBaud)/df)
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do k=-2,2 !Weighted averages over 5 bins
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sum=sum+wgt(k)*ps(m+k)
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enddo
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enddo
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k=i-ia+1
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psmo(k)=sum
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kpk=0
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if(sum.gt.smax) then
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smax=sum
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noffset=nint(i*df-LTone*NBaud)
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kpk=k
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endif
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enddo
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if(kpk.gt.1 .and. kpk.lt.20) then
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call peakup(psmo(kpk-1),psmo(kpk),psmo(kpk+1),dx)
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noffset=nint(noffset+dx*df)
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endif
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C Do square-law detection in each of four filters.
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f1=LTone*NBaud+noffset
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f2=(LTone+1)*NBaud+noffset
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f3=(LTone+2)*NBaud+noffset
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f4=(LTone+3)*NBaud+noffset
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call detect(dat,npts,f1,y1)
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call detect(dat,npts,f2,y2)
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call detect(dat,npts,f3,y3)
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call detect(dat,npts,f4,y4)
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C Bandpass correction:
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npts=npts-(NSPD-1)
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do i=1,npts
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y1(i)=y1(i)*a1
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y2(i)=y2(i)*a2
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y3(i)=y3(i)*a3
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y4(i)=y4(i)*a4
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enddo
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call sync(y1,y2,y3,y4,npts,jpk,baud,bauderr)
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C Decimate y arrays by NSPD
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ndits=npts/NSPD - 1
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do i=1,ndits
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y1(i)=y1(jpk+(i-1)*NSPD)
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y2(i)=y2(jpk+(i-1)*NSPD)
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y3(i)=y3(jpk+(i-1)*NSPD)
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y4(i)=y4(jpk+(i-1)*NSPD)
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enddo
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C Now find the mod3 phase that has no tone 3's
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n4(0)=0
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n4(1)=0
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n4(2)=0
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do i=1,ndits
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ymax=max(y1(i),y2(i),y3(i),y4(i))
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if(y1(i).eq.ymax) dit(i)=0
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if(y2(i).eq.ymax) dit(i)=1
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if(y3(i).eq.ymax) dit(i)=2
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if(y4(i).eq.ymax) then
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dit(i)=3
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k=mod(i,3)
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n4(k)=n4(k)+1
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endif
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enddo
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n4min=min(n4(0),n4(1),n4(2))
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if(n4min.eq.n4(0)) jsync=3
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if(n4min.eq.n4(1)) jsync=1
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if(n4min.eq.n4(2)) jsync=2
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C Might want to notify if n4min>0 or if one of the others is equal
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C to n4min. In both cases, could then decode 2 or 3 times, using
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C other starting phases.
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C Finally, decode the message.
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msg=' '
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msglen=ndits/3
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msglen=min(msglen,40)
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do i=1,msglen
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j=(i-1)*3+jsync
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nc=16*dit(j) + 4*dit(j+1) +dit(j+2)
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msg(i:i)=' '
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if(nc.le.47) msg(i:i)=c(nc+1:nc+1)
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enddo
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return
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end
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