NOTE − 1 − 1 − 10. Simplified Directional Facility (SDF) EXAMPLE − (1) No critical area protective action is provided under these conditions. (2) A flight crew, under these conditions, should advise the tower that it will conduct an AUTOLAND or COUPLED approach. 3. Aircraft holding below 5,000 feet between the outer marker and the airport may cause localizer signal variations for aircraft conducting the ILS approach. Accordingly, such holding is not autho- rized when weather or visibility conditions are less than ceiling 800 feet and/or visibility 2 miles. 4. Pilots are cautioned that vehicular traffic not subject to ATC may cause momentary deviation to ILS course or glide slope signals. Also, critical areas are not protected at uncontrolled airports or at airports with an operating control tower when weather or visibility conditions are above those requiring protective measures. Aircraft conducting coupled or autoland operations should be especially alert in monitoring automatic flight control systems.  (See FIG 1 − 1 − 7.) g. Identification consists of a three − letter identifi- er transmitted in Morse Code on the SDF frequency. The appropriate instrument approach chart will indicate the identifier used at a particular airport. f. The SDF signal is fixed at either 6 degrees or 12 degrees as necessary to provide maximum flyability and optimum course quality. e. The SDF antenna may be offset from the runway centerline. Because of this, the angle of convergence between the final approach course and the runway bearing should be determined by reference to the instrument approach procedure chart. This angle is generally not more than 3 degrees. However, it should be noted that inasmuch as the approach course originates at the antenna site, an approach which is continued beyond the runway threshold will lead the aircraft to the SDF offset position rather than along the runway centerline. d. Usable off − course indications are limited to 35 degrees either side of the course centerline. Instrument indications received beyond 35 degrees should be disregarded. c. The approach techniques and procedures used in an SDF instrument approach are essentially the same as those employed in executing a standard localizer approach except the SDF course may not be aligned with the runway and the course may be wider, resulting in less precision. b. The SDF transmits signals within the range of 108.10 to 111.95 MHz. a. The SDF provides a final approach course similar to that of the ILS localizer. It does not provide glide slope information. A clear understanding of the ILS localizer and the additional factors listed below completely describe the operational characteristics and use of the SDF. (2) Glide Slope Critical Area. Do not authorize vehicles or aircraft operations in or over the area when an arriving aircraft is inside the ILS outer marker (OM), or the fix used in lieu of the OM, unless the arriving aircraft has reported the runway in sight and is circling or side − stepping to land on another runway. (b) Weather Conditions. At or above ceil- ing 800 feet and/or visibility 2 miles. missed approach, vehicles and aircraft are not authorized in or over the critical area when an arriving aircraft is inside the outer marker (OM) or the fix used in lieu of the OM. Additionally, whenever the official weather observation is a ceiling of less than 200 feet or RVR less than 2,000 feet, do not authorize vehicles or aircraft operations in or over the area when an arriving aircraft is inside the MM, or in the absence of a MM,  ½  mile final. Unless otherwise coordinated through Flight Standards, ILS signals to Category I runways are not flight inspected below the point that is 100 feet less than the decision altitude (DA). Guidance signal anomalies may be encountered below this altitude. AIM 8/15/19  12/31/20 Denver Tower, United 1153, Request Autoland/Coupled Approach (runway) ATC replies with: United 1153, Denver Tower, Roger, Critical Areas not protected. 1 − 1 − 15 Navigation Aids