function y = F(X)
x = X(1); y = X(2); z = X(3); d = X(4);

A_1 = 15600; A_2 = 18760; A_3 = 17610; A_4 = 19170;
B_1 = 7540;  B_2 = 2750;  B_3 = 14630; B_4 = 610;
C_1 = 20140; C_2 = 18610; C_3 = 13480; C_4 = 18390;

t_1 = 0.07074; t_2 = 0.07220; t_3 = 0.07690; t_4 = 0.07242;

c = 299792.458;

x_1 = (x - A_1)^2 + (y - B_1)^2 + (z - C_1)^2 - c^2 * (t_1 - d)^2;
x_2 = (x - A_2)^2 + (y - B_2)^2 + (z - C_2)^2 - c^2 * (t_2 - d)^2;
x_3 = (x - A_3)^2 + (y - B_3)^2 + (z - C_3)^2 - c^2 * (t_3 - d)^2;
x_4 = (x - A_4)^2 + (y - B_4)^2 + (z - C_4)^2 - c^2 * (t_4 - d)^2;

y = [x_1, x_2, x_3, x_4]';



