Distence calculation between two GPS points

gistfile1.java

    private double gps2m(float lat_a, float lng_a, float lat_b, float lng_b) { 
         float pk = (float) (180/3.14169); 
       
         float a1 = lat_a / pk; 
         float a2 = lng_a / pk; 
         float b1 = lat_b / pk; 
         float b2 = lng_b / pk; 
       
         float t1 = FloatMath.cos(a1)*FloatMath.cos(a2)*FloatMath.cos(b1)*FloatMath.cos(b2); 
         float t2 = FloatMath.cos(a1)*FloatMath.sin(a2)*FloatMath.cos(b1)*FloatMath.sin(b2); 
         float t3 = FloatMath.sin(a1)*FloatMath.sin(b1); 
         double tt = Math.acos(t1 + t2 + t3); 
         
         return 6366000*tt; 
    }