Section 6. Direction Finders
3-6-1. DF ANTENNA
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
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.
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
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
the appropriate Technical Operations Aviation System Standards, National
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
for the use of normal and emergency air/ground frequencies in DF radio
communications (VHF, 118-156 MHz).
Terminal Operations Area office must determine requirements for
establishment of DF approach procedures at airports with suitably located
DF antenna sites.
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 FSS/ATCT/ARTCC. Flight Standards will submit data to be
published in the AIM. Approach details will be published only on FAA Form
e. Prepare a
facility directive specifying the service to be provided. Include the
(a) The DF
indication and the bearing of a known transmitter site be compared once
reception of all frequencies be checked once each day if there is a
suitable signal available.
DF approach procedures.
safe DF altitudes.
with DF must:
a. Check the
equipment at least once each 8 hours, and advise any affected radar
control facilities and FSSs of a DF outage.
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.)
periods when DF information is not required, the frequency selected must
be left to the discretion of the facility as determined by local
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
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.
cross bearings to establish a fix where the communications range of two or
more installations overlap.
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
procedures may be canceled if deemed appropriate by the Terminal
Operations Area office and Technical Operations Aviation System Standards,
National Flight Procedures.