INSTALLATION MANUAL
CMA-5024 GLSSU
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without prior written permission by CMC Electronics Inc.
Page 4
Revision 1 Jan 19, 2009
in the absence of the SBAS signal through the use of Fault Detection and Exclusion (FDE). The GLSSU
also includes a navigation function to provide path deviations relative to a selected final approach path
and distance to the runway threshold.
In a suitable aircraft installation, the GLSSU supports the functions of FAA TSO-C145b and TSO-C146b
as follows:
- Used as Class Beta-3 equipment (TSO-C145b compliant), the GLSSU outputs position and integrity
data for use by the external equipment driving the aircraft on the desired flight path.
- Used as a Class Delta-4 equipment (TSO-C146b compliant), the GLSSU outputs the path
deviations to drive directly the aircraft on the desired flight path.
The discrete input/output signals and the data busses of the GLSSU are described in details in
Appendices A and C.
The GLSSU provides digital DME/ILS Look-Alike outputs that are intended to replace the output of
conventional digital DME/ILS instruments when performing an approach of type LP or LPV. The desired
approach path is extracted from an SBAS Approach Database, either built-in the GLSSU or an external
database, typically from an FMS.
Note: The GLSSU expects to receive the SBAS approach data as a Final Approach Segment
(FAS) data block formatted in accordance with Appendix D of DO-229D and ARINC-755.
The FAS data formatting and the computation of its CRC words require careful attention to
the details of the format. For purposes of checking, please refer to the sample FAS data
block in Appendix G.
Depending upon the local conditions, the pilot will select the approach distance/deviation data source
(ILS source using the conventional DME/ILS instruments, or GLS source using the GLSSU) by means
of a manual switch that controls a High Integrity Switch (HIS) built-in the GLSSU. Accordingly, the HIS
will either forward transparently the DME/ILS distance/deviation data from the conventional on-board
flight instruments, or it will substitute the look-alike GLS distance/deviation data that is computed
internally using the GPS/SBAS navigation solution.
Note: The use of the HIS in an aircraft installation is optional. In particular, it is not required for
retrofitting legacy ARINC-743A installations.
The pilot manages the SBAS approach by means of a Controller Unit that typically will be an FMS.
SBAS approach selection is similar to ILS approach tuning as follows:
- For ILS approach tuning, pilots identify the tuning frequency of the appropriate ILS station from an
aviation approach plate, enter that frequency into the ILS system, and then listen for the Morse code
identification signal.
For SBAS approach selection, the pilot enters the appropriate SBAS channel ID taken from an aviation
approach plate. Since lacking the ILS audio feedback, pilots must also select/confirm the desired
approach from a list of approaches, extracted from the SBAS Approach Database, that share this
channel ID.
C. Major characteristics (GLSSU)
The GLSSU operates from the RF signals received via a separate antenna, supplied by the installer,
from the NAVSTAR GPS satellite constellation and from the SBAS satellites. The primary signal
processing functions of the GLSSU are:
The document reference is online, please check the correspondence between the online documentation and the printed version.
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