# Studying VOR Signals with the RTL-SDR

VHF Omni Directional Radio Range (VOR) signals are used in aviation as a short range radio navigational system. Amateur radio hobbyist F4GKR decided to study these VOR signals by recording them using his RTL-SDR, and then analyzing them in MATLAB. On his post he shows his method of analysis and discusses his results.

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I was trying to replicate the analysis result using 2.4 Ms per second wav file captured by HDSDR provided by the author. I am unable to get 30 Hz signal as achieved by Author.

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% Close all previous figures
close all;
clear;

No_Of_Sample=400000;
%Original Sampling is at 2.4 Mhz

Fs_Original=Fs;
%Down Sample to 30Khz
Fs_Downsample=30e3;
%Downsample Factor
Downsample_Factor=(Fs_Original/Fs_Downsample);
%Downsample_Factor=80;
size(y(:,1))
y_d=downsample(y(:,1),Downsample_Factor);
%y_d=decimate(y(:,1),Downsample_Factor);
%y_d= resample(y(:,1),1,Downsample_Factor);
clear y;

% clear y_d;
% y_d=out;

Fech = Fs_Downsample ;
L = length(y_d);

freqs = -Fech/2:Fech/L:Fech/2-Fech/L;
plot( freqs,10*log10(abs(fftshift(fft(y_d)))));

xlabel(‘Frequency (Hz)’,’fontsize’,9);
ylabel(‘Level’,’fontsize’,9);

nyq_freq=Fs_Downsample/2;
fc=40;
fn=Fs_Downsample;
cutoff_norm =fc/nyq_freq;
order=12;
fir_coeff = fir1(order, cutoff_norm);
filtered_signal = filter(fir_coeff, 1, y_d);
figure;

plot((filtered_signal));

figure;
plot(abs(fft(filtered_signal)));

grid;
zoom;