Section 6. Direction Finders

3-6-1. DF ANTENNA SITE

NOTE-
The DF antenna may be located at a site other than the airport proper. If the antenna is remoted, it is important that specialists recognize that the bearing information obtained is in relation to the antenna site rather than to the airport.

3-6-2. STROBE LINE INDICATION

Each time an aircraft transmits on the designated frequency, bearing information is provided from the antenna site. This data can be presented directly on the surveillance scope by strobe line indication. It also appears on the DF mechanical azimuth indicator.

3-6-3. EQUIPMENT LIMITATIONS

Equipment specifications require a bearing accuracy of plus or minus 6 degrees on the azimuth indicator. This deviation may be increased depending on the site, terrain, or other factors. A small additional error is introduced when the strobe line indication is placed on the surveillance scope. Additionally, as in the case of any electronic equipment, the DF unit has certain limitations.

3-6-4. INACCURATE BEARING INDICATION

DF equipment is designed to furnish bearing information from any transmitting station within communications range on the frequency selected. Any signal within range affects it. Therefore, when two or more aircraft are transmitting simultaneously, an inaccurate bearing indication results.

3-6-5. COMMISSIONING DF EQUIPMENT

When DF equipment is commissioned:

a. Notify the appropriate Technical Operations Aviation System Standards, National Flight Procedures.

b. Coordinate with the Technical Operations Aviation System Standards, National Flight Procedures to establish minimum safe DF altitudes for a 100 mile radius. See FAAO 8260.3, United States Standards for Terminal Instrument Procedures (TERPS).

c. Provide for the use of normal and emergency air/ground frequencies in DF radio communications (VHF, 118-156 MHz).

d. The Terminal Operations Area office shall determine requirements for establishment of DF approach procedures at airports with suitably located DF antenna sites.

NOTE-
The Technical Operations Aviation System Standards, National Flight Procedures will document and approve the procedure (restrictively identified for emergency use only) on FAA Form 8260 and supply copies to the associated AFSS/FSS/ATCT/ARTCC. Flight Standards will submit data to be published in the AIM. Approach details will be published only on FAA Form 8260.

e. Prepare a facility directive specifying the service to be provided. Include the following:

1 Instructions that:

(a) The DF indication and the bearing of a known transmitter site be compared once each watch.

(b) The reception of all frequencies be checked once each day if there is a suitable signal available.

(c) Approved DF approach procedures.

(d) Minimum safe DF altitudes.

3-6-6. OPERATING PROCEDURES

Facilities equipped with DF shall:

a. Check the equipment at least once each 8 hours, and advise any affected radar control facilities and AFSSs of a DF outage.

b. Maintain the equipment in a state of readiness that will allow for operation in the shortest time possible. (This may be "off," "standby," or "continuous" depending on the operational characteristics of the equipment.)

c. During periods when DF information is not required, the frequency selected shall be left to the discretion of the facility as determined by local requirements.

3-6-7. ASR-ASSOCIATED DF

VHF/DF equipment is installed at many airports equipped with ASR. The information obtained furnishes the heading needed to bring the aircraft within the ASR pattern. Specialists may use this equipment to assist in obtaining an accurate bearing to:

a. Locate a lost aircraft or one in distress which is within communications range but outside radar coverage.

b. Vector an aircraft directly to an airport.

c. Coordinate cross bearings to establish a fix where the communications range of two or more installations overlap.

3-6-8. ASSIGNING HEADING USING DF/ASR

If the DF antenna and the ASR antenna are not collocated, the surveillance compass rose centers on the ASR antenna site rather than on the DF antenna site. When assigning an aircraft a heading for homing purposes using the DF equipment under these circumstances, make the required allowance for the location of the two sites.

3-6-9. CANCELING DF APPROACH PROCEDURES

DF approach procedures may be canceled if deemed appropriate by the Terminal Operations Area office and Technical Operations Aviation System Standards, National Flight Procedures.