fm-cw radar

this is the code of rx of fm-cw radar to get beat frequency is this correct?

   %receiver of fm-cw radar 
  clear all;clc;close all;
  %sample frequency
  fs=1000;         
  %time period
  t=0:1/fs:1;
  %estimat range
  r=randint(1,1,100);
  rf=randint(1,1,[50 120])
  %phase shift
  phi=4*pi*r*rf/(3e8);
  %reflected signal
  echo=cos(2*pi*rf*t-phi);
  subplot 211; plot(t,echo);xlabel('t/sec');ylabel('reflected signal/v');title('reflected signal');
  %refrance signal
  c=cos(2*pi*t*50);
  %mixer output
  x=c.*echo;
  subplot 212; plot(t,x);xlabel('t/sec');ylabel('mixer signal/v');title('mixer output signal');
  %low pass filter
  [b a]=butter(8,70/500);
  %output of LPF
  a=filter(b,a,x);
  figure;
  subplot 211; plot(t,a);xlabel('t/sec');ylabel('filter signal/v');title('LPF output signal');
  %calculation of power of extracted signal
  op=fft(a,1001)/1000;
  op=fftshift(op);
  omga0=1; omga=-500:omga0:500;
  subplot 212; plot(omga,abs(op));xlabel('FREQ./HZ');ylabel(' FFT');title('amplitude spectrum');
  power=sumsqr(abs(op));
  %claculation of beat frquency
  f=find(op>max(op)-.00001);
  fb=501-min(f)