Garmin GTS 855 - Manual (Page 11)

7 190-00587-02 Rev. A Garmin GTS 8XX Series Traffic Advisory System
Bottom Mounted Monopole Antenna (optional)
The addition of an optional bottom-mounted monopole antenna will improve
visibility of intruder aircraft by reducing shading and providing altitude and
distance of threat aircraft. Bearing cannot be determined from replies received
on a bottom-mounted monopole antenna.
Figure 3 Bottom-Mounted Monopole Antenna
gpA 65 pA / LNA Module (gTs 820 & 850 only)
The GPA 65 is a combined Power Amplifier and Low Noise Amplifier module
used in the higher power GTS 820 and 850.
Figure 4 GPA 65 PA/LNA Module