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entry of the 5-digit channel number. An interface based on approach selection through a flight management function similar
to current practice with ILS is also possible. The GBAS channel number may be stored in an on-board navigation database as
part of a named approach. The approach may be selected by name and the channel number can automatically be provided to
the equipment that must select the appropriate GBAS approach data from the broadcast data. Similarly, the use of the GBAS
positioning service may be based on the selection of a 5-digit channel number. This facilitates conducting operations other
than the approaches defined by the FAS data. To facilitate frequency tuning, the GBAS channel numbers for neighbouring
GBAS ground subsystems supporting positioning service may be provided in the Type 2 message additional data block 2.
7.7.2 A channel number in the range from 20 001 to 39 999 is assigned when the FAS data are broadcast in the Type 4
message. A channel number in the range from 40 000 to 99 999 is assigned when the FAS data associated with an APV are
obtained from the on-board database.
7.8 Reference path data selector and reference station data selector
A mapping scheme provides a unique assignment of a channel number to each GBAS approach. The channel number consists
of five numeric characters in the range 20 001 to 39 999. The channel number enables the GBAS airborne subsystem to tune
to the correct frequency and select the final approach segment (FAS) data block that defines the desired approach. The
correct FAS data block is selected by the reference path data selector (RPDS), which is included as part of the FAS definition
data in a Type 4 message. Table D-6 shows examples of the relationship between the channel number, frequency and RPDS.
The same mapping scheme applies to selection of the positioning service through the reference station data selector (RSDS).
The RSDS is broadcast in the Type 2 message and allows the selection of a unique GBAS ground subsystem that provides
the positioning service. For GBAS ground subsystems that do not provide the positioning service and broadcast the additional
ephemeris data, the RSDS is coded with a value of 255. All RPDS and RSDS broadcast by a ground subsystem must be
unique on the broadcast frequency within radio range of the signal. The RSDS value must not be the same as any of the
broadcast RPDS values.
ATT D-25 23/11/06
Annex 10 — Aeronautical Communications Volume I
Table D-6. Channel assignment examples
Channel number (N) Frequency in MHz (F)
Reference path data
selector (RPDS)
or
Reference station data
selector (RSDS)
20 001 108.025 0
20 002 108.05 0
20 003 108.075 0
…. …. ….
20 397 117.925 0
20 398 117.95 0
20 412 (Note) 108.025 1
20 413 108.05 1
…. …. ….
Note.— Channels between 20 398 and 20 412 are not assignable because the channel algorithm maps
them to frequencies outside the range of 108.025 MHz and 117.950 MHz. A similar “gap” in the channel
assignments occurs at each RPDS transition.
7.9 Assignment of RPDS and RSDS by service provider
RPDS and RSDS assignments are to be controlled to avoid duplicate use of channel numbers within the protection region for
the data broadcast frequency. Therefore, the GBAS service provider has to ensure that an RPDS and RSDS are assigned only
once on a given frequency within radio range of a particular GBAS ground subsystem. Assignments of RPDS and RSDS are
to be managed along with assignments of frequency and time slots for the VHF data broadcast.
7.10 GBAS identification
The GBAS identification (ID) is used to uniquely identify a GBAS ground subsystem broadcasting on a given frequency
within the coverage region of the GBAS. The aircraft will navigate using data broadcast from one or more GBAS broadcast
stations of a single GBAS ground subsystem (as identified by a common GBAS identification).
7.11 Final approach segment (FAS) path
7.11.1 FAS path is a line in space defined by the landing threshold point/fictitious threshold point (LTP/FTP), flight
path alignment point (FPAP), threshold crossing height (TCH) and glide path angle (GPA). These parameters are determined
from data provided in a FAS data block within a Type 4 message or in the on-board database. The relationship between these
parameters and the FAS path is illustrated in Figure D-6.
7.11.1.1 FAS data blocks for SBAS and some GBAS approaches are held within a common onboard database
supporting both SBAS and GBAS. States are responsible for providing the FAS data to support APV procedures when the
Type 4 message is not broadcast. These data comprise the parameters contained within the FAS block, the RSDS, and
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