be modified by the terms “rapidly” and “slowly” (e.g., “well  above glidepath, coming down rapidly”).  Range from touchdown is given at least once each mile. If  an aircraft is observed by the controller to proceed outside  of specified safety zone limits in azimuth and/or elevation  and continue to operate outside these prescribed limits, the  pilot will be directed to execute a missed approach or to fly a  specified course unless the pilot has the runway environment  (runway, approach lights, etc.) in sight. Navigational  guidance in azimuth and elevation is provided to the pilot  until the aircraft reaches the published decision altitude (DA)/ DH. Advisory course and glidepath information is furnished  by the controller until the aircraft passes over the landing  threshold, at which point the pilot is advised of any deviation  from the runway centerline. Radar service is automatically  terminated upon completion of the approach. Radar equipment is specifically designed to detect all  principal features on the surface of an airport, including  aircraft and vehicular traffic, and to present the entire image  on a radar indicator console in the control tower. It is used  to augment visual observation by tower personnel of aircraft  and/or vehicular movements on runways and taxiways. Airport Surface Detection Equipment Radar Limitations 1.  It is very important for the aviation community to  recognize the fact that there are limitations to radar  service and that ATC may not always be able to issue  traffic advisories concerning aircraft which are not  under ATC control and cannot be seen on radar. 2.  The characteristics of radio waves are such that they  normally travel in a continuous straight line unless  they are “bent” by abnormal atmospheric phenomena  such as temperature inversions; reflected or attenuated  by dense objects such as heavy clouds, precipitation,  ground obstacles, mountains, etc.; or screened by high  terrain features. 3.  Primary radar energy that strikes dense objects is  reflected and displayed on the operator’s scope,  thereby blocking out aircraft at the same range and  greatly weakening or completely eliminating the  display of targets at a greater range. 4.  Relatively low altitude aircraft are not seen if they are  screened by mountains or are below the radar beam  due to curvature of the Earth. 5.  The amount of reflective surface of an aircraft  determines the size of the radar return. Therefore,  a small light airplane or a sleek jet fighter is more  difficult to see on primary radar than a large  commercial jet or military bomber. 6.  All ARTCC radar in the conterminous United States  and many ASR have the capability to interrogate Mode  C and display altitude information to the controller  from appropriately-equipped aircraft. However,  a number of ASR do not have Mode C display  capability; therefore, altitude information must be  obtained from the pilot. 9-48