On Apr 4, 4:47*pm, Papa3 wrote:
Hi,

Quick google search on "TCAS filter slow" and similar didn't get me
what I wanted, so here goes... *Do TCAS systems filter out targets
which have slow/no ground speed?

Reason for asking... *I got into wave yesterday, and while climbing
through 11,000 MSL NW of Allentown PA had an American Airlines CRJ
pass nearby. * I would guesstimate that he was about 500 above and a
half mile horizontal. * It wasn't really close - I had him in sight
for over a minute and was edging away laterally to gain separation.
But, it was a little closer than the published TA/RA numbers I've seen
in manuals. * * I was essentially stationary for 20 minutes.

By comparison, later in the flight I watch an A300 make a sweeping
turn around me at 9,000 as I crossed a well-published STAR. * I also
checked in with Allentown approach, and they painted me just fine on
their ground radar.

Curious...

P3

No TCAS I or II do not velocity filter on ground speed. This would be
bad as its foreseeable light aircraft might well have near zero ground
speed. And one reason TCAS got mandated is because light aircraft have
taken down airliners.

This is an old wives tale that I've heard before. The relative speed
(not ground speed) between the threat aircraft and TCAS equipped
aircraft is used to calculate time to collision/proximity parameters
that drive the systems overall TA and RA behavior.

At least one reason this old-wives tale exists seems to be a belief
that aircraft on the ground do not trigger a TA/RA from nearby
airborne aircraft and this must be because there us ground speed
filtering. That's just wrong.

TCAS II (the CRJ will have TCAS II) will have very good altitude data
on you and slightly fuzzy direction data. But its still using that to
projected paths to calculate collision risks if he is not actually
pointed your way with a chance of a collision it should have very low
chance of generating an RA. If your transponder was on and working and
squawking an accurate altitude they may have seen you on their panel
(and hopefully got you visual) or even have had a TA and not needed to
take any action.

Darryl