User's Manual Part 1

T
2
CAS / Part No. 9000000
SYSTEM DESCRIPTION AND INSTALLATION MANUAL
34-43-20
15 Feb 2003
Use or disclosure of information on this page is subject to the restrictions in the proprietary notice of this document.
1--39
D. GPS Antenna and Coax
For T
2
CAS installations that include the optional internal GPS module, a compatible
ARINC 743A active GPS antenna and coax interface shall be included in the T
2
CAS
installation design.
The antenna used for the GNSSA Receiver Module is required to meet the requirements
of RTCA/DO --228, Change 1, and TSO--C144. A potentially suitable antenna is listed
below, although antennas are available from different vendors:
• Manufacture: AeroAntenna
• Part Number: AT575--143WAC-- TNCF-- 000--26--NM
• Description: 26 dB gain, 12 volt, Active Antenna
The range of antenna gains which are acceptable is dependent on the coax length and
resulting dBs of loss. Generally, an antenna gain of 20 -- 30 dB is desired. Higher gain
antennas introduce the amplification of noise as well as the desired signal. This can
saturate the GPS input circuitry, reducing its effectiveness. If the antenna gain is low and
the resulting limit of allowable cable loss is exceeded, excessive signal degradation will
occur and the GNSSA Receiver Module may not be able to meet its performance
specifications.
Do -- 228 Change 1 compliant antennas are required. Change 1 introduced rejection at 5
to 10 dB higher that the original DO -- 228 requirements at the SATCOM frequency.
Therefore system performance under interference can not be assured with the
non--Change 1 compliant antennas. When subjected to DO229 normal interference
levels, system performance can be degraded.
Figure 1--11 shows a typical T
2
CAS GPS antenna coax sub kit with the sub kit parts listed
in Table 1-- 7. Figure 2--4 shows AeroAntenna’s AT575-- 143 active GPS antenna series
outline drawing.
The GPS coax installation should include one disconnect near the T
2
CAS computer tray
to facilitate the removal of the equipment shelf from the aircraft. The T
2
CAS computer
provides +12 Vdc to the active GPS antenna through the coax’s center conductor.