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I know that the TCAS II system has a lower antenna which can be either omnidirectional or directional. How does the functionality of the system change? If the antenna is omni then I think / assume that it cannot know the bearing of an intruding aircraft and therefore can not provide data to position it on a display. Is it only used for transmitting interrogation and/or receiving other data?

Thanks,

Ori

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The directionality of a TCAS antenna is ONLY used for positioning the bogey on the TCAS display, it is NOT used for determining time-to-contact. So you're still going to get TAs and RAs from the Omni antenna even if the upper antenna is blanked, but you won't get the display of that traffic.

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  • $\begingroup$ Thanks for your answer. My question is what would happen if the lower (omni) antenna is partially obstructed. The upper (directional) antenna is not obstructed. $\endgroup$ – user1412867 Jun 27 '19 at 9:15

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