function NewtonGPS2_1( )

% % Simultaneous positions
A1 = 15600;
B1 = 7540;
C1 = 20140;

A2 = 18760;
B2 = 2750;
C2 = 18610;

A3 = 17610;
B3 = 14630;
C3 = 13480;

A4 = 19170;
B4 = 610;
C4 = 18390;

% Time intervals in seconds
t1 = 0.07074;
t2 = 0.07220;
t3 = 0.07690;
t4 = 0.07242;
% % Speed of light measured in km/sec
 c = 299792.458;

    r11 = -2*(A1-A2);
    r12 = -2*(B1-B2);
    r13 = -2*(C1-C2);
    r14 = 2*c^2*(t1-t2);
    r15 = -(A1^2-A2^2)-(B1^2-B2^2)-(C1^2-C2^2)+c^2*(t1^2-t2^2);
    
    r21 = -2*(A1-A3);
    r22 = -2*(B1-B3);
    r23 = -2*(C1-C3);
    r24 = 2*c^2*(t1-t3);
    r25 = -(A1^2-A3^2)-(B1^2-B3^2)-(C1^2-C3^2)+c^2*(t1^2 - t3^2);
    
    r31 = -2*(A1-A4);
    r32 = -2*(B1-B4);
    r33 = -2*(C1-C4);
    r34 = 2*c^2*(t1-t4);
    r35 = -(A1^2-A4^2)-(B1^2-B4^2)-(C1^2-C4^2)+c^2*(t1^2-t4^2);
    b=[r15; r25; r35];
    A=[r11 r12 r13 r14;
        r21 r22 r23 r24;
        r31 r32 r33 r34];
    r = [A b];
    
    u = rref(r);
    
    a=u (1,4)^2+u (2,4)^2+u (3,4)^2-c^2;
    b=2*u(1,4)*(A1-u(1,5))+2*u(2,4)*(B1-u(2,5))+2*u(3,4)*(C1-u(3,5))+2*t1*c^2;
    e=(A1-u(1,5))^2+(B1-u(2,5))^2+(C1-u(3,5))^2-(c^2*t1^2);
    d=(-b+sqrt(b^2-4*a*e))/(2*a)
       
    x = -u(1,4)*d + u(1,5)
    y = -u(2,4)*d + u(2,5)
    z = -u(3,4)*d + u(3,5)
end