function y = obs_nlinear_lat(ts, x, u, param)

% Function to compute the observation variables (i.e., RHS of observation equations): 
% Lateral motion
% Nonlinear model in terms of dimensional derivatives as function of
% variables in the stability axes (V, alfa)   
%                  states  - beta, p, r, phi, psi
%                  outputs - beta, p, r, phi, psi, ay
%                  inputs  - da, dr

d2r = pi/180;

% Geometry data
G0     =    9.80665D+0;
global stables;
U0     = stables(1);  % トリム状態における対気速度[m/s]
W0     = stables(2);  % トリム状態における下方向速度[m/s]
THETA0 = stables(3);  % トリム状態におけるピッチ角(安定軸)

% State Variables
Bet = x(1);
Prate = x(2);
Rrate = x(3);
Phi = x(4);
Psi = x(5);

% Input variables
da    = u(1);
dr    = u(2);

% Parameters
Yb   = param( 1);
Yp   = param( 2);
Yr   = param( 3);
Ydr  = param( 4);
Lb   = param( 5);
Lp   = param( 6);
Lr   = param( 7);
Lda  = param( 8);
Ldr  = param( 9);
Nb   = param(10);
Np   = param(11);
Nr   = param(12);
Nda  = param(13);
Ndr  = param(14);

Betdot = (Yb * Bet + (W0 + Yp) * Prate + (Yr - U0) * Rrate ...
    + G0 * cos(THETA0) * Phi + Ydr * dr) / U0;

% Observations
y(1)  =  Bet;
y(2)  =  Prate;
y(3)  =  Rrate;
y(4)  =  Phi;
y(5)  =  Psi;
y(6)  =  Betdot + Rrate * U0 - G0 * cos(THETA0) * sin(Phi);

% y must be a column vector
y = y';

return
% end of function