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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.)
NOTEUnless
otherwise coordinated through Flight Standards,
ILS signals to Category I runways are not flight inspected
below 100 feet AGL. Guidance signal anomalies may be
encountered below this altitude.
1-1-10. Simplified Directional Facility
(SDF)
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.
b. The SDF transmits signals within the range of
108.10 to 111.95 MHz.
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.
d. Usable off-course indications are limited to
35degrees either side of the course centerline.
Instrument indications received beyond 35 degrees
should be disregarded.
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.
AIM 2/14/08
1-1-12 Navigation Aids
FIG 1-1-7
FAA Instrument Landing Systems
2/14/08 AIM
Navigation Aids 1-1-13
f. The SDF signal is fixed at either 6 degrees or
12degrees as necessary to provide maximum
flyability and optimum course quality.
g. Identification consists of a three-letter identifier
transmitted in Morse Code on the SDF frequency.
The appropriate instrument approach chart will
indicate the identifier used at a particular airport.
1-1-11. Microwave Landing System (MLS)
a. General
1. The MLS provides precision navigation
guidance for exact alignment and descent of aircraft
on approach to a runway. It provides azimuth,
elevation, and distance.
2. Both lateral and vertical guidance may be
displayed on conventional course deviation indicators
or incorporated into multipurpose cockpit
displays. Range information can be displayed by
conventional DME indicators and also incorporated
into multipurpose displays.
3. The MLS supplements the ILS as the standard
landing system in the U.S. for civil, military, and
international civil aviation. At international airports,
ILS service is protected to 2010.
4. The system may be divided into five
functions:
(a) Approach azimuth;
(b) Back azimuth;
(c) Approach elevation;
(d) Range; and
(e) Data communications.
5. The standard configuration of MLS ground
equipment includes:
(a) An azimuth station to perform functions
(a) and (e) above. In addition to providing azimuth
navigation guidance, the station transmits basic data
which consists of information associated directly
with the operation of the landing system, as well as
advisory data on the performance of the ground
equipment.
(b) An elevation station to perform
function(c).
(c) Distance Measuring Equipment (DME) to
perform range guidance, both standard DME
(DME/N) and precision DME (DME/P).
6. MLS Expansion Capabilities. The standard
configuration can be expanded by adding one or
more of the following functions or characteristics.
(a) Back azimuth. Provides lateral guidance
for missed approach and departure navigation.
(b) Auxiliary data transmissions. Provides
additional data, including refined airborne positioning,
meteorological information, runway status, and
other supplementary information.
(c) Expanded Service Volume (ESV) proportional
guidance to 60 degrees.
7. MLS identification is a four-letter designation
starting with the letter M. It is transmitted in
International Morse Code at least six times per
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