function [ echo_sFFT_mtx, angle_index ] = spatial_FFT(echo_mtx, d, lambda)
% FFT for echo space domain
%
% Usage: 
% [echo_sFFT_mtx, angle_index] = spatial_FFT(echo_mtx, d, lambda)
%
% Inputs:
%   echo_mtx:     Matrix containing the echo data
%   d:            Antenna spacing
%   lambda:       Wave length
%
% Outputs:
%   echo_sFFT_mtx:  Processed echo matrix
%   angle_index:    True value of the angle


    % parameters
    [lenA, lenR, lenP] = size(echo_mtx);
    
    % calculate angle
    angle_index = asin(-1 * (-1/2: 1/lenA: 1/2 - 1/lenA) * lambda / d) / pi * 180;
    
    % spatial FFT
    echo_mtx = reshape(echo_mtx, [lenA, lenR * lenP]);
    echo_sFFT_mtx = zeros(size(echo_mtx));
    for i = 1: lenR * lenP
        echo_sFFT_mtx(:, i) = fftshift(fft(echo_mtx(:, i))) ./ sqrt(lenA);
    end
    echo_sFFT_mtx = reshape(echo_sFFT_mtx, [lenA, lenR, lenP]);
    
end