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How Does The Tcas Work?  
User currently offlineSfilipowicz From Netherlands, joined Jul 2002, 327 posts, RR: 1
Posted (10 years 5 months 3 weeks 5 days 4 hours ago) and read 11927 times:

Hi All,

I was wondering if somebody could tell me how the TCAS works?!

Does it work in combination with the transponder and with the ATC?
Would the TCAS be able to 'see' different aircraft when flying above the oceans?

Thanks!

16 replies: All unread, jump to last
 
User currently offlineIkarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 1, posted (10 years 5 months 3 weeks 5 days 3 hours ago) and read 11771 times:

Please add on to that questions:

How does TCAS work at low altitudes (i.e. when a downwards diversion would be problematic) and during final approach?

Technically, I believe it uses transponder information to trace other aircraft and spot any risk of collision. Correct me if I'm wrong....

Cheers

Ikarus


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 2, posted (10 years 5 months 3 weeks 5 days 3 hours ago) and read 11720 times:

It uses timing for direction, mode C information for altitude and a directional antenna for a (quite unreliable) indication of direction.

It then uses the rate of closure to decide on a tau value, or time to collision. The tau value used to trigger traffic alerts and resolution advisories is dependant upon altitude. At lot altitudes, the inability to go down will be considered. Eventually, all RAs will be inhibited.

The aircraft will coordinate their RAs over the mode S datalink.

That is a very brief description. It is all rather complicated, and I do not even claim to know all the intricasies, especially when it comes to performance and the effect on RAs issued. I'm an engineer, much more concerned with the bit sequencing than the algorithms used.  Big grin

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineBen From Switzerland, joined Aug 1999, 1391 posts, RR: 50
Reply 3, posted (10 years 5 months 3 weeks 5 days 3 hours ago) and read 11707 times:

Light aircraft now have a similar system but it doesn't coordinate RA's because there is no data link to allow them to talk to eachother. That's why it's now being advised that all aircraft have mode-c squawking, even when in the pattern/circuit.

Light aircraft transponders which do not use mode-c can trigger alerts on airliners flying far above, because without the altitude encoding, TCAS can't determine your vertical separation.


User currently offlineSfilipowicz From Netherlands, joined Jul 2002, 327 posts, RR: 1
Reply 4, posted (10 years 5 months 3 weeks 5 days 2 hours ago) and read 11664 times:

Hi Fred,

Thanks for your answer!
What are RA's ???

Is there a new system in the making for TCAS?

Thanks again!

Steven.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 5, posted (10 years 5 months 3 weeks 5 days 1 hour ago) and read 11662 times:

RAs are Resolution Advisories, vertical path guidance commands issued by the TCAS system to avoid a potential collision.

TCAS II provides this. TCAS III was supposed to include lateral guidance (turn commands) as well, but has now been scrapped to the best of my knowledge.

The new system in the making is Automatic Dependent Surveillance Broadcast, ADS-B, where the aircraft and ground stations are all communicating over TCP/IP inspired datalinks, telling each other where they are and what they are up to. Thus, no radars are required. Each pilot can have the full picture presented right there in the cockpit, just as if they had a radar scope today (if not better). This will open entirely new possibilities as far as airspace management goes! How about free flight? Or even self-separating traffic in trail of one another along the airways?

TCAS II requires a mode S transponder, something you won't see in many light aircraft.

There is also a "faux TCAS" available for GA, which is not a TCAS at all since it doesn't interrogate the transponders of other aircraft. It merely listens in on the responses transmitted to other interrogations, and tries do determine the distance based on other parameters such as signal strength. Anyone who has tried it and would like to chime in with how well it works?

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineBuckfifty From Canada, joined Oct 2001, 1316 posts, RR: 20
Reply 6, posted (10 years 5 months 3 weeks 5 days 1 hour ago) and read 11663 times:

RA's are resolution advisaries (considered a warning). Which are basically instructions from the TCAS telling you to either climb or descend with traffic approaching. TA's are traffic advisories (considered only a caution), which is when the TCAS will visually and aurally warn you of approaching traffic, but will not instruct you to do anything.

I've forgotten when the range of TA's and RA's are activated, but I believe TA's are when the target is in a probable collision course, at 40 seconds, and RA's are activated when that same target is within 20 seconds. Someone please correct me if I'm wrong.

And an aircraft equipped with Mode C only will still generate a TA/RA on an aircraft equipped with TCAS and Mode S. It's just that it won't be reciprocated.

The system in use for light aircraft is called TCAD. It's based on Mode C transponders, and will give traffic advisories in terms of height above or below, and the rate of vertical approach or separation.


User currently offlineBuckfifty From Canada, joined Oct 2001, 1316 posts, RR: 20
Reply 7, posted (10 years 5 months 3 weeks 5 days 1 hour ago) and read 11645 times:

There is also a "faux TCAS" available for GA, which is not a TCAS at all since it doesn't interrogate the transponders of other aircraft. It merely listens in on the responses transmitted to other interrogations, and tries do determine the distance based on other parameters such as signal strength. Anyone who has tried it and would like to chime in with how well it works?

Hehe. It's crap, it's annonying, and I think it's dangerous, especially to low hour pilots. It dings an awful lot, especially in crowded training areas, and people I think tend to look less carefully at other traffic with this device on board. And it can give a lot of heart attacks too, because it generates a lot of nuisance and false warnings that'll really give you a scare.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (10 years 5 months 3 weeks 5 days 1 hour ago) and read 11636 times:

Buckfifty,
as I said above, the tau value limits are altitude dependant. I'm fast, am I not? Corrected you before you posted! Big grin

Interesting to hear that those not-really-TCAS-systems live up to my expectations - crap.  Big grin

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineBuckfifty From Canada, joined Oct 2001, 1316 posts, RR: 20
Reply 9, posted (10 years 5 months 3 weeks 5 days 1 hour ago) and read 11636 times:

Buckfifty,
as I said above, the tau value limits are altitude dependant. I'm fast, am I not? Corrected you before you posted!


Hehe. Well, I'm always being corrected, even in my past life, it seems. Big grin


User currently offlineBMAbound From Sweden, joined Nov 2003, 660 posts, RR: 4
Reply 10, posted (10 years 5 months 3 weeks 5 days 1 hour ago) and read 11629 times:

Fred, I know, I should know the answer (  Big grin ) , but does the TCAS on the heavy iron pick up light A/C that has squawked VFR?

cheers

johan



Altitude is Insurance - Get Insured
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 11, posted (10 years 5 months 3 weeks 5 days ago) and read 11621 times:

I should know too, but I could just picture that they have put in a filter to avoid nuisance alerts. Would be tricky though, as the VFR code isn't the same all over...

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineBen From Switzerland, joined Aug 1999, 1391 posts, RR: 50
Reply 12, posted (10 years 5 months 3 weeks 4 days 23 hours ago) and read 11655 times:

The "faux TCAS" in my experience isn't reliable even when working.

The systems for displaying/alerting are inconsistent between models.

I have had one which was just 2 red L.E.D. numbers that indicate the distance in nautical miles to the nearest 'threat'. Not all that useful.... if you notice the numbers counting down, it can be distracting as you frantically search around in every direction for the other aircraft. Most of the time it is just ignored because you're too busy actually flying and looking out.

A lot of light aircraft either dont have or dont use their Mode C so it wont spot them anyway.

I usually fly with it turned off, if one is installed, but always make sure I squawk VFR Mode C which is 7000 in the UK.

[Edited 2004-02-02 18:00:00]

User currently offlineJumbojet From United States of America, joined Dec 2003, 1159 posts, RR: 0
Reply 13, posted (9 years 4 months 2 weeks 3 days 8 hours ago) and read 11043 times:

picking up where this left off, will TCAS work over the open oceans?

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 14, posted (9 years 4 months 2 weeks 3 days 6 hours ago) and read 11031 times:

For the record, this question was answered in another thread just a short while ago.  Smile


I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 15, posted (9 years 4 months 2 weeks 2 days 1 hour ago) and read 10828 times:

This is classic A.net stupidity.....

Because somebody (I have my suspicions) cryed, the only post on this entire thread that contained correct information got deleted...

I guess you guys prefer to wallow in ignorance.

[Edited 2005-03-13 16:36:42]

User currently offlineKAUSpilot From United States of America, joined Jan 2002, 1958 posts, RR: 33
Reply 16, posted (9 years 4 months 2 weeks 1 day 22 hours ago) and read 10838 times:

Fred, I know, I should know the answer ( ) , but does the TCAS on the heavy iron pick up light A/C that has squawked VFR?


TCAS definitely picks up VFR traffic in the aircraft I fly. Just the other day we got an RA from an aircraft flying 500 ft above class B airspace, obviously VFR with his xponder turned on.

Last week we got a TA from a VFR aircraft doing aerobatics off our right side on approach to MHT. If there is a feature in TCAS which blocks VFR targets, I'm certainly not aware of it.


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