FredT From United Kingdom, joined Feb 2002, 2184 posts, RR: 26 Reply 2, posted (9 years 3 months 3 weeks 4 days 11 hours ago) and read 8688 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.
Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
Ben From Switzerland, joined Aug 1999, 1391 posts, RR: 52 Reply 3, posted (9 years 3 months 3 weeks 4 days 11 hours ago) and read 8675 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.
FredT From United Kingdom, joined Feb 2002, 2184 posts, RR: 26 Reply 5, posted (9 years 3 months 3 weeks 4 days 9 hours ago) and read 8630 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.
Buckfifty From Canada, joined Oct 2001, 1314 posts, RR: 21 Reply 6, posted (9 years 3 months 3 weeks 4 days 9 hours ago) and read 8631 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.
Buckfifty From Canada, joined Oct 2001, 1314 posts, RR: 21 Reply 7, posted (9 years 3 months 3 weeks 4 days 9 hours ago) and read 8613 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.
FredT From United Kingdom, joined Feb 2002, 2184 posts, RR: 26 Reply 11, posted (9 years 3 months 3 weeks 4 days 8 hours ago) and read 8589 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.
Ben From Switzerland, joined Aug 1999, 1391 posts, RR: 52 Reply 12, posted (9 years 3 months 3 weeks 4 days 7 hours ago) and read 8623 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.
Jumbojet From United States of America, joined Dec 2003, 1159 posts, RR: 0 Reply 13, posted (8 years 2 months 2 weeks 2 days 16 hours ago) and read 8011 times:
picking up where this left off, will TCAS work over the open oceans?
KAUSpilot From United States of America, joined Jan 2002, 1955 posts, RR: 37 Reply 16, posted (8 years 2 months 2 weeks 1 day 5 hours ago) and read 7806 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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