approach system:  facility that has an antenna within 20 NM of the airport  and meets the equipment requirements outlined in FAA  Order 8200.1, U.S. Standard Flight Inspection Manual. ASR  approaches are not authorized for use when Center Radar  ARTS processing (CENRAP) procedures are in use due to  diminished radar capability.  The final approach course for an ASR approach is aligned  with the runway centerline for straight-in approaches  and aligned with the center of the airport for circling  approaches. Within the final approach area, the pilot is also  guaranteed a minimum of 250 feet obstacle clearance. ASR  descent gradients are designed to be relatively flat, with an  optimal gradient of 150 feet per mile and never exceeding  300 feet per mile.  The localizer approach system can provide both precision  and non-precision approach capabilities to a pilot. As a  part of the ILS system, the localizer provides horizontal  guidance for a precision approach. Typically, when the  localizer is discussed, it is thought of as a non-precision  approach due to the fact that either it is the only approach  system installed, or the glideslope is out of service on the  ILS. In either case, the localizer provides a non-precision  approach using a localizer transmitter installed at a specific  airport. [Figure 4-58]  TERPS provides the same alignment criteria for a localizer  approach as it does for the ILS, since it is essentially the  As an approach system, the localizer is an extremely flexible  approach aid that, due to its inherent design, provides  many applications for a variety of needs in instrument  flying. An ILS glideslope installation may be impossible  due to surrounding terrain. The localizer is able to provide  four separate types of non-precision approaches from one  Localizer Approaches  Localizer and Localizer DME  4-75 Figure 4-46. Simultaneous Independent Approach Example Using ILS Approaches.  12R 12L  3,200'  2,200'  2,200'  3,200'  NO TRANSGRESSION ZONE (NTZ)  Radar monitoring provided to ensure  separation during simultaneous  approaches. A breakout will be  directed if an aircraft enters the NTZ.  Intercept glideslope at 2,200 feet  Intercept glideslope at 3,200 feet  Radar monitoring provided to ensure  separation during simultaneous  approaches. A breakout will be  directed if an aircraft enters the NTZ.  Independent approaches to runway  centerlines spaced 4,300 feet or more  —radar monitoring required.  Normal Operating Zone (NOZ)  • Localizer approach • Localizer/DME approach • Localizer back course approach • Localizer-type directional aid (LDA)