% Range of IR Missile Seeker
% ---------------------------

clear;clf;clc;

% Input Seeker Parameters

J=1000;			% Target Radiant Intensity - w/sterandian
Do=7.5;			% Optics Diameter - cm
NA=1.7;			% Numerical Aperture
ao=.7;			% Optics Transmittance
Ds=1e11;			% Detector D*
phi=1.5;			% Optics FOV - degrees
df=100;			% Noise Bandwidth - Hz
snr=30;			% Voltage Signal-to-Noise Ratio - dB
Ls=8;				% System and Processing Loss - dB

snr=10^(snr/20);Ls=10^(Ls/10);
fov= (phi*pi/180)^2;                  % FOV in steradians

% Compute Range With No Atmospheric Attenuation

tarfac=J^.5;
optfac=(pi*Do*NA*ao/2)^.5;
detfac=Ds^.5;
sysfac=(((fov*df)^.5)*snr*Ls)^1/2;
Ro=tarfac*optfac*detfac/(sysfac*1e5); % Range in km

% Input Atmospheric Attenuation vs Altitude

alt=[200 2000 5000 10000]; % Atmospheric Attenuation -dB/km
att=[.88 .37 .16 .06];

% Compute Range with Atmospheric Attenuation
% Using Newton's Method

for j=1:4;
R(1)=Ro;
atn=att(j);				
a=atn/20;

for i=1:10;
   x=.4343/R(i)+a;
   y=log10(R(i))-log10(Ro)+a*R(i);
   dR=y/x;
   R(i+1)=R(i)-dR;
   if abs(dR)<1e-6;break;end;
end
Rx(j)=R(i);
end;

% Plot Range vs Altitude
% Smooth Data using Polynomial Fit

z=0:10000;
f=polyval(polyfit(alt,Rx,3),z);
plot(z,f);grid;
xlabel('Altitude - meters');
ylabel('Range - km');
title('Range of IR Seeker With Altitude');
axis([0,10000,0,14]);