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Microwave Landing System (MLS)
The MLS provides precision navigation guidance for exact
alignment and descent of aircraft on approach to a runway.
It provides azimuth, elevation, and distance. Both lateral
and vertical guidance may be displayed on conventional
course deviation indicators or incorporated into multipurpose
flight deck displays. Range information can be displayed
by conventional DME indicators and also incorporated into
multipurpose displays. [Figure 7-41]
The system may be divided into five functions, which are
approach azimuth, back azimuth, approach elevation, range;
and data communications. The standard configuration of
MLS ground equipment includes an azimuth station to
perform functions as indicated 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.
Approach Azimuth Guidance
The azimuth station transmits MLS angle and data on one
of 200 channels within the frequency range of 5031 to 5091
7-46
Figure 7-41. MLS Coverage Volumes, 3-D Representation.
MHz. The equipment is normally located about 1,000 feet
beyond the stop end of the runway, but there is considerable
flexibility in selecting sites. For example, for heliport
operations the azimuth transmitter can be collocated with
the elevation transmitter. The azimuth coverage extends
laterally at least 40° on either side of the runway centerline
in a standard configuration, in elevation up to an angle of 15°
and to at least 20,000 feet, and in range to at least 20 NM.
MLS requires separate airborne equipment to receive and
process the signals from what is normally installed in general
aviation aircraft today. It has data communications capability,
and can provide audible information about the condition
of the transmitting system and other pertinent data such as
weather, runway status, etc. The MLS transmits an audible
identifier consisting of four letters beginning with the letter
M, in Morse code at a rate of at least six per minute. The
MLS system monitors itself and transmits ground-to-air data
messages about the system’s operational condition. During
periods of routine or emergency maintenance, the coded
identification is missing from the transmissions. At this time
there are only a few systems installed.
Required Navigation Performance
RNP is a navigation system that provides a specified level
of accuracy defined by a lateral area of confined airspace
in which an RNP-certified aircraft operates. The continuing
growth of aviation places increasing demands on airspace
capacity and emphasizes the need for the best use of the
available airspace. These factors, along with the accuracy of
modern aviation navigation systems and the requirement for
increased operational efficiency in terms of direct routings
and track-keeping accuracy, have resulted in the concept
of required navigation performance—a statement of the
navigation performance accuracy necessary for operation
within a defined airspace. RNP can include both performance
and functional requirements, and is indicated by the RNP type.
These standards are intended for designers, manufacturers,
and installers of avionics equipment, as well as service
providers and users of these systems for global operations.
The minimum aviation system performance specification
(MASPS) provides guidance for the development of airspace
and operational procedures needed to obtain the benefits of
improved navigation capability. [Figure 7-42]
The RNP type defines the total system error (TSE) that
is allowed in lateral and longitudinal dimensions within
a particular airspace. The TSE, which takes account of
navigation system errors (NSE), computation errors, display
errors and flight technical errors (FTE), must not exceed the
specified RNP value for 95 percent of the flight time on any
part of any single flight. RNP combines the accuracy standards
laid out in the ICAO Manual (Doc 9613) with specific
accuracy requirements, as well as functional and performance
standards, for the RNAV system to realize a system that
can meet future air traffic management requirements. The
functional criteria for RNP address the need for the flight paths
of participating aircraft to be both predictable and repeatable
to the declared levels of accuracy. More information on RNP
is contained in subsequent chapters.
The term RNP is also applied as a descriptor for airspace,
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