diff --git a/lib/superfox/sym_prob.f90 b/lib/superfox/sym_prob.f90
new file mode 100644
index 000000000..acbe25845
--- /dev/null
+++ b/lib/superfox/sym_prob.f90
@@ -0,0 +1,58 @@
+subroutine sym_prob(s3,rxdat,rxprob,rxdat2,rxprob2)
+
+! Demodulate the 64-bin spectra for each of 63 symbols in a frame.
+
+! Parameters
+!    rxdat    most reliable symbol value
+!    rxdat2   second most likely symbol value
+!    rxprob   probability that rxdat was the transmitted value
+!    rxprob2  probability that rxdat2 was the transmitted value
+
+  use sfox_mod
+  implicit real*8 (a-h,o-z)
+  real*4 s3(0:NQ-1,0:NN-1)
+  integer rxdat(0:NN-1),rxprob(0:NN-1),rxdat2(0:NN-1),rxprob2(0:NN-1)
+
+  afac=1.1
+  scale=255.999
+
+! Compute average spectral value
+  ave=sum(s3)/(NQ*ND)
+  i1=1                                      !Silence warning
+  i2=1
+
+! Compute probabilities for most reliable symbol values
+  do j=0,NN-1                               !Loop over all symbols
+     s1=-1.e30
+     psum=0. 
+     do i=0,NQ-1                            !Loop over frequency bins
+        x=min(afac*s3(i,j)/ave,50.d0)
+        psum=psum+s3(i,j)
+        if(s3(i,j).gt.s1) then
+           s1=s3(i,j)                       !Find max signal+noise power
+           i1=i                             !Find most reliable symbol value
+        endif
+     enddo
+     if(psum.eq.0.0) psum=1.e-6             !Guard against zero signal+noise
+
+     s2=-1.e30
+     do i=0,NQ-1
+        if(i.ne.i1 .and. s3(i,j).gt.s2) then
+           s2=s3(i,j)                       !Second largest signal+noise power
+           i2=i                             !Bin number for second largest power
+        endif
+     enddo
+     p1=s1/psum                             !p1, p2 are symbol metrics for ftrsd
+     p2=s2/psum
+     rxdat(j)=i1
+     rxdat2(j)=i2
+     rxprob(j)=scale*p1                     !Scaled probabilities, 0 - 255
+     rxprob2(j)=scale*p2
+  enddo
+
+!  Once, ntest.lt.50 or nlow.gt.20 were used to flag bad data:
+  ntest=sum(rxprob)
+  nlow=count(rxprob.le.5)
+
+  return
+end subroutine sym_prob