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visual) approach to the active runway. It may also be used in precision area navigation
(RNAV) when coupled with the elevation and distance measuring equipment that are the
remaining transmitted signals.
The Azimuth and Back Azimuth signal beam shapes are both fan-shaped in a vertical
plane formed along any of the antenna’s radials. As viewed by the pilot of an aircraft on
final approach to the runway, the azimuth beam is swept from the right-most coverage
angle to the left-most in the “To” scan, and is then returned from the left to right coverage
angle in the “From” scan after a specified delay at the left-most limit. The time difference
between the reception of the two scans is determined. A third signal transmitted is the
elevation signal. This fan-shaped beam is transmitted from the Elevation antenna located
abeam the MLS datum point. The beam originates at an angle near horizontal (minimum
elevation angle), scans to the upper elevation angle limit in an upward direction (the “To”
scan), and then returns (the “From” scan). The time interval between the “To” and “From”
scans is measured in the receiver and based on the data transmitted from the ground
(concerning site geometry and configuration) the elevation angle of the aircraft is
determined.
The MLS system has 200 channels, with a 1MHz channel spacing (ILS has only 40
channels). Technically, this allows for a nearly unlimited number of MLS installations
without interference. However, low operational need means that not all of the available
channels are currently required.
3.4.4 Operational Requirements
3.4.4.1 General navigation requirements
Introduction to RNAV and RNP
RNAV (or Area Navigation) is a method which permits aircraft operation on any desired
flight path within the coverage of referenced navigation aids, or within the limits of the
capabilities of self-contained aids (i.e. IRS/INS), or both. The Required Navigation
Performance (RNP) determines the accuracy of the RNAV system to determine the
aircraft’s absolute geographical position (instead of its position relative to a navigation aid,
as is the case with VOR/DME etc).
The RNP for aircraft in the ECAC area is mandated to RNP 5 (i.e. the containment value
is 5nm) – also known as B-RNAV (Basic area navigation). States must ensure that the
navigational infrastructure provided adequately supports the prescribed RNP type in a
specific area (or on a specific route).
RNAV represents a fundamental change in navigation philosophy. Instead of flying
to/from specific navaids, aircraft can now determine their absolute position from a variety
of inputs (e.g. VOR, DME, GNSS and INS). As such, the coverage of these navaids
should be tailored to ensure that at all times the aircraft can determine its position to
within 5nm (or whatever the RNP value is set to). Note that not all navaids are required –
whatever is provided must be sufficient to provide the required performance (and possibly
include a redundant system due to the criticality of the application). As a result of this
concept, the practice of ‘point-to-point’ navigation is becoming rare in civil aviation.
In order to facilitate this move to RNAV, many aircraft are equipped with Multi-Mode
Receivers, or MMR, which integrate L-band, VHF, UHF and C-band signals in the flight
management system.
EN-ROUTE NAVIGATION AID REQUIREMENTS
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