specify link as joint to world in urdf

When ever i launch my urdf velodyne sensor in gazebo, it starts to rotate and than it vanishes in the world by jumping from one place to another. I tried to apply similar method as in gazebo but seems like in xacro file it donot worlks. i.e i cant keep the velodyne base link only fixed with the ground. I also used true in my xacro file, but the hole velodyne is not rotating after this, the top link has to rotate. Than i tried below code which works for sdf file, but it is not working for xacro file: I inserted element so that gazebo will know the element world in parent link. My full xacro file of velodyne sensor is: transmission_interface/SimpleTransmissionEffortJointInterfaceEffortJointInterface1 So how can i remove this problem. What i want is when i launch this file, i dont want my velodyne sensor to jump cas of rotation and stay in one place.