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Localizer and Localizer - Back Course approaches are different from
FMS initiated ILS approaches in that the localizer signal from the ILS
receiver is processed and retransmitted from the FMS, along with
vertical guidance (pseudo glideslope) using barometric altimetry
from the aircraft air data computer. This feature allows fully coupled
3D ILS-type approaches when there is no ground-based glideslope
signal available.
In the case of LOC approaches the localizer signal is retransmitted
to the EFIS/FGS in its raw form with the same scaling as received
from the ILS receiver.
In the case of LOC-BC approaches, the localizer signal is inverted
and mathematically modified to provide scaling similar to a front
course localizer signal (i.e. ±350 ft full scale at the runway
threshold). It should be noted that the HSI desired track pointer will
indicate the inbound course to the landing runway (unlike normal
LOC-BC operation in which the desired track pointer is aligned with
the front course inbound setting). The reason for this is to allow
display of pseudo glideslope on the HSI.
Section V
Operations
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Change 1
VOR Approaches
VOR data are provided to the FMS either from a digital VOR
receiver, or from a UNS Radio Reference Sensor. A collocated
DME is not required at the VOR station; however, there must be at
least one other DME in the terminal area in order to provide
approach guidance.
For VOR approaches the DME is tuned to each of the closest DMEs
once every four seconds.
RNAV Approaches (Including VOR-DME)
RNAV and VOR-DME approaches require a collocated DME with the
VOR ground station. These types of charted approaches should be
defined as RNAV approaches since the DME will be scanned much
more often than for VOR type approaches. VOR data are provided
to the FMS from a digital VOR receiver, with DME provided through
the blind channel of a suitable DME. A UNS Radio Reference
Sensor, if installed, is capable of providing both the VOR and DME
data.
TACAN Approaches
TACAN approaches require the capability of obtaining both azimuth
and distance from a TACAN interrogator / receiver or from a UNS
Radio Reference Sensor. TACAN typically provides a considerably
more accurate and stable (i.e., non-scalloping) source of azimuth
data than conventional VOR. Distance accuracy is the same as a
DME.
Since TACAN is primarily a military navigation aid, TACAN
approaches are most frequently found at military airfields. TACAN
ground stations are collocated with many VORs throughout the
airspace system.
Section V
Operations
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GPS Approaches
GPS “Overlays” are only applied to VOR, VOR-DME, NDB and
RNAV approaches. ILS, LOC, LDA and SDF approaches are not
eligible for GPS overlays. Currently GPS supports only nonprecision
approaches. (Unaugmented GPS does not have sufficient accuracy
-- especially in the vertical plane -- to support precision approaches.
It is anticipated that Differential GPS will be required to attain the
accuracy required for precision approaches.)
GPS Use During Approaches
It is most important to note that there are some very specific
requirements, which must be met in order to execute a GPS overlay
approach:
1. The GPS sensor must conform to FAA Technical Standard
Order (TSO) C-129 class A1, B1 or C1;
2. The approach waypoint coordinates must conform to World
Geodetic System - 1984 (WGS-84) coordinate system (or
equivalent).
3. The approach must be approved as a GPS or GPS overlay
approach. On Jeppesen charts, the legend “GPS” contained in
parenthesis indicates a Phase I or Phase II Overlay approach.
A Phase III approach is indicated in the title of the approach;
e.g., “GPS or VOR Rwy 26”. Due to the very large number of
approaches being converted to GPS, charts for all approved
approaches may not have been updated. Jeppesen provides a
listing of additional approved Phase III overlay in the “Terminal”
section of the Airway Manual.
Section V
Operations
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Therefore, the FMS meets the above requirements when using its
internal GPS (if applicable) or is interfaced with a UNS 764-2
GPOSS, GPS-1000 or GPS-1200, and is operating in a region of the
world which uses WGS-84 coordinate system. The navigation data
base specifically codes approaches which are GPS approved.
Under these conditions, when an RNV, VOR, TACAN, ILS, NDB,
LOC, BC or VFR approach is armed and activated then the GPS
sensor will remain selected as a navigation sensor and provide
　
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