If you use the two-output syntax for the max() function, the second output is the index of the maximum:
clear all
clc
set(0,'defaultlinelinewidth',1)
True_range=1000;%Range to target
el=1.619*10^(-19);%electron charge
pulse_width=10e-9;
PRF=10;%pulse per second
P_avg=0.4; %Average laser power
E_t=P_avg/PRF;%energy per pulse in units of joule
sigma_w=2e-9; %standard deviation second
theta_t=.01; % divergence beam in radiance
theta_r=pi;% Reflection angle prom target
tau_atm=1;%atmospheric transmission loss
tau_opt=1;%reciever optics transmission
rho_t=.1;%target reflection
reciever_focal=.1;% reciever focal length
delta=1e-4;%physical size of the detectorin the ladar recieverin meters
dA=(True_range*delta/reciever_focal)^2;%area of target illuminated by light
ap_diameter=.01;
Rmin=990;
minT=Rmin*2/3e8;
Rmax=1010;
maxT=Rmax*2/3e8;
deltat=sigma_w/10; %sample time to ensure good pulse shape sampling
t=minT:deltat:maxT; %range of times over which the return signal is measured
P_t=(E_t/(sqrt(2*pi)*sigma_w))*exp(-((t-True_range*2/3e8).^2)/(2*sigma_w^2));
I_target=4*tau_atm*P_t/(pi*(True_range^2)*(theta_t^2));
P_ref=I_target*dA*rho_t; %Reflected power
I_reciever=tau_atm*P_ref/(theta_r*True_range^2); %intensity at the aperture
P_rec= tau_opt*(ap_diameter^2)*pi*I_reciever/4; %recieved signal power after reciever optics
quantum_eff=.075;
h=6.626e-34;
v=3e8/1.55e-6;
N=P_rec*deltat*quantum_eff/(h*v);%generated photoelectron
figure;
plot(N,'b.-')
% Changed this line to output index:
[N_max,index_of_N_max]=max(N);
sprintf('Maximum of N occurred at index position %d.',index_of_N_max)
