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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. 

## Localizer Approaches 

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 

approach system: 

• Localizer approach 

• Localizer/DME approach 

• Localizer back course approach 

• Localizer-type directional aid (LDA) 

## Localizer and Localizer DME 

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 

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