TCAS and Nil Mode C Targets
Thread Starter
Join Date: Jan 2006
Location: Straya
Posts: 537
Likes: 0
Received 0 Likes
on
0 Posts
TCAS and Nil Mode C Targets
Question from an ATC point of view.
We often get a lot of aircraft beneath our approaches who for one reason or another (poor training / inexperience / unfamiliarity with panel?) only select ON instead of ALT on their transponder.
TCAS equipped aircraft flying the approach get a bit nervous about them, and rightly so - However, aircraft overflying the same no Mode C aircraft at say 10000FT don't even see it.
My question is, how does your TCAS determine when a non-Mode C target is going to display on your TCAS unit as traffic, whether proximate or otherwise? Obviously it cannot be based on a pre-set altitude (cannot be determined) so how does it do it?
By the way I have asked a lot of the local pilots operating in to our area, and none have been able to give me a defintive answer - I think they are in the dark as much as me to be honest.
We often get a lot of aircraft beneath our approaches who for one reason or another (poor training / inexperience / unfamiliarity with panel?) only select ON instead of ALT on their transponder.
TCAS equipped aircraft flying the approach get a bit nervous about them, and rightly so - However, aircraft overflying the same no Mode C aircraft at say 10000FT don't even see it.
My question is, how does your TCAS determine when a non-Mode C target is going to display on your TCAS unit as traffic, whether proximate or otherwise? Obviously it cannot be based on a pre-set altitude (cannot be determined) so how does it do it?
By the way I have asked a lot of the local pilots operating in to our area, and none have been able to give me a defintive answer - I think they are in the dark as much as me to be honest.
Last edited by Shitsu_Tonka; 7th Jul 2006 at 01:40.
Join Date: Jul 2000
Location: ask crewing
Posts: 168
Likes: 0
Received 0 Likes
on
0 Posts
TCAS Targets
I can only answer based on the type I fly (DHC8 Q400), though I assume other types are the same in principle if not the specifics.
Our TCAS has three settings for the altitude bands it displays: NORMAL, ABOVE, and BELOW.
On NORMAL, it diplays targets 2,700 ft above and below our level,
on ABOVE, it shows 9,000 ft above and 2,700 ft below,
on BELOW, it shows 2,700 ft above and 9,000 ft below.
The display is also limited to 40nm in front, and only a few miles behind, and roughly 45 degress either side of the aircraft heading.
It doesn't matter if another aircraft has mode C turned on or off (or mode S for that matter), if our radar picks up a mode A squawk, and it's within the area mentioned above, it displays it on screen.
So if we are below 9,000ft with the mode set to BELOW every aircraft that passes below us with a mode A squawk, produces a TA and we get "TRAFFIC TRAFFIC" shouted at us by the ACAS. Very annoying. If the aircraft had mode C turned on, it wouldn't be considered a threat and the display would just show the aircraft as normal, with an indication of its level relative to us.
For that reason, we tend to use ABOVE from T/O to TOC, then switch to NORMAL for the cruise and only change to BELOW around initial approach. Or some people just leave it in NORMAL all the time.
Hope that's of some use.
Our TCAS has three settings for the altitude bands it displays: NORMAL, ABOVE, and BELOW.
On NORMAL, it diplays targets 2,700 ft above and below our level,
on ABOVE, it shows 9,000 ft above and 2,700 ft below,
on BELOW, it shows 2,700 ft above and 9,000 ft below.
The display is also limited to 40nm in front, and only a few miles behind, and roughly 45 degress either side of the aircraft heading.
It doesn't matter if another aircraft has mode C turned on or off (or mode S for that matter), if our radar picks up a mode A squawk, and it's within the area mentioned above, it displays it on screen.
So if we are below 9,000ft with the mode set to BELOW every aircraft that passes below us with a mode A squawk, produces a TA and we get "TRAFFIC TRAFFIC" shouted at us by the ACAS. Very annoying. If the aircraft had mode C turned on, it wouldn't be considered a threat and the display would just show the aircraft as normal, with an indication of its level relative to us.
For that reason, we tend to use ABOVE from T/O to TOC, then switch to NORMAL for the cruise and only change to BELOW around initial approach. Or some people just leave it in NORMAL all the time.
Hope that's of some use.
Join Date: Oct 2003
Location: If this is Tuesday, it must be?
Posts: 651
Likes: 0
Received 0 Likes
on
0 Posts
Cloud Chaser,
That is how the TCAS works when the other transponder has a mode C; all mode A only targets are displayed because it doesn't know their level in order to apply the filter. It will then assume the target is at the same level as you and issue a TA if appropriate, but it will not issue any RA's since it is unable to compute a solution.
Edit - many apologies, I've just read your post properly and what you said is absolutely correct - I'll just crawl back into my hole
****su Tonka,
I'm afraid I don't know the situation in your part of the globe, but here in the UK the majority of light piston GA machines haven't got mode C. Primarily because the electronics cost as much as the rest of the aircraft. That may be the reason why they don't switch it on! I hope my clarification for CC above is also useful to you.
All the best,
BJJ
That is how the TCAS works when the other transponder has a mode C; all mode A only targets are displayed because it doesn't know their level in order to apply the filter. It will then assume the target is at the same level as you and issue a TA if appropriate, but it will not issue any RA's since it is unable to compute a solution.
Edit - many apologies, I've just read your post properly and what you said is absolutely correct - I'll just crawl back into my hole
****su Tonka,
I'm afraid I don't know the situation in your part of the globe, but here in the UK the majority of light piston GA machines haven't got mode C. Primarily because the electronics cost as much as the rest of the aircraft. That may be the reason why they don't switch it on! I hope my clarification for CC above is also useful to you.
All the best,
BJJ
Join Date: Jul 1999
Location: Europe
Posts: 352
Likes: 0
Received 0 Likes
on
0 Posts
If the intruder is Non-Altitude Reporting (NAR), TCAS will display only the range and bearing. It can issue a Traffic Advisory based on distance and direction of flight but will not generate a Resolution Advisory. TCAS assumes Non-Altitude Reporting (NAR) traffic is at the same altitude as your own aircraft.
TCAS does not display NAR traffic above 14,500 feet.
The NORMAL, ABOVE, and BELOW setting are for display limits only. The selection has no effect on TA or RA issuance.
TCAS does not display NAR traffic above 14,500 feet.
The NORMAL, ABOVE, and BELOW setting are for display limits only. The selection has no effect on TA or RA issuance.
Thread Starter
Join Date: Jan 2006
Location: Straya
Posts: 537
Likes: 0
Received 0 Likes
on
0 Posts
Thanks, but I am still confused.
Here in Oz most Transponders are Mode C capable - it's just that some of the pilots arent! (I put it down to training and telling them what ALT means on the KT76 or whatever.)
The situation we usually get is aircraft on descent to an IAF for the ILS, say thru 5 or 6000 FT, and getting traffic on a Mode A target. Traffic passed, not much else to say.
What I am wondering is why that same Mode A target does not become traffic for an overflying or departing aircraft at say 10000FT? i,e,, as discussed in the above posts, if the TCAS unit does not know the ALT of the intruder, how does it filter it out? And yes, I have asked the TCAS aircraft if anything observed on TCAS - "Negative".
Thanks for the replies on the over/under/normal. Not sure if this applies to the aircraft in question (mainly 737/A320/A330/767/747), although we also have a few DHC8-400's.
Here in Oz most Transponders are Mode C capable - it's just that some of the pilots arent! (I put it down to training and telling them what ALT means on the KT76 or whatever.)
The situation we usually get is aircraft on descent to an IAF for the ILS, say thru 5 or 6000 FT, and getting traffic on a Mode A target. Traffic passed, not much else to say.
What I am wondering is why that same Mode A target does not become traffic for an overflying or departing aircraft at say 10000FT? i,e,, as discussed in the above posts, if the TCAS unit does not know the ALT of the intruder, how does it filter it out? And yes, I have asked the TCAS aircraft if anything observed on TCAS - "Negative".
Thanks for the replies on the over/under/normal. Not sure if this applies to the aircraft in question (mainly 737/A320/A330/767/747), although we also have a few DHC8-400's.
Join Date: May 2001
Location: Location Location
Posts: 376
Likes: 0
Received 0 Likes
on
0 Posts
Cloud Chaser
Point of order
If you change the 9000 ft you mention to 8700 ft
and the max range forward is aprox 20nm to see the intruder (With a max range selected of 40nm)
Hope I'm not being to picky
If you change the 9000 ft you mention to 8700 ft
and the max range forward is aprox 20nm to see the intruder (With a max range selected of 40nm)
Hope I'm not being to picky
Remember different aircraft have different height thresholds.
TCAS works on closure rate so even if there is no MODE C, if the target aircraft is a few thousand feet below it will have a different closure rate to an aircraft that is 500 ft below, provided all aircraft are in level flight. So some non MODE C will be TAs and some will be proximate or other.
See this eurocontrol ACAS Bulletin 4
TCAS works on closure rate so even if there is no MODE C, if the target aircraft is a few thousand feet below it will have a different closure rate to an aircraft that is 500 ft below, provided all aircraft are in level flight. So some non MODE C will be TAs and some will be proximate or other.
See this eurocontrol ACAS Bulletin 4
Moderator
Join Date: Jul 1997
Location: Europe
Posts: 3,228
Likes: 0
Received 0 Likes
on
0 Posts
TCAS works on closure rate so even if there is no MODE C, if the target aircraft is a few thousand feet below it will have a different closure rate to an aircraft that is 500 ft below, provided all aircraft are in level flight. So some non MODE C will be TAs and some will be proximate or other.
See this eurocontrol ACAS Bulletin 4
I don't think this is true. Without Mode C (Altitude Reporting) all that can ever be displayed is a TA. The TCAS equipment can't work out a vertical closure rate since it will not know the intruders height, and the horizontal closure rate will be the same whether the intruder aircraft is physically above or below the TCAS equipped aircraft. Won't it ??
Like the old railway analogy of stuff you can see out the window appears to pass you at different speeds depending on range. Thus something a few hundred feet away but missing will have a different rate to something a 1000 ft away and missing to something that will hit. TCAS can crudely decide if a non C is a threat BUT crucially can not give an RA because because it can not give an accurate miss manouvre without height readout.
Tracking:
Using surveillance reports each second, the CAS logic tracks the slant range and closing speed of each target aircraft, to determine the time in seconds to the CPA. If the target is equipped with an altitude-coding transponder, the CAS logic also tracks the altitude reports to project the altitude of the target at the CPA. The CAS logic determines the vertical rate of the target by measuring the time it takes to traverse successive 100-foot increments of altitude.
The CAS logic uses the data from its own aircraft pressure altimeter either directly from the altitude encoder or as processed by an air data computer, to determine own aircraft altitude, vertical rate, and the relative altitude of each target. The Outputs from the tracking algorithm (target range and range-rate, relative altitude and vertical rate) from the inputs for the traffic advisory and threat detection algorithms.
Below 1700 feet AGL, the CAS logic uses the difference between its own aircraft pressure altitude and radar altitude to determine the approximate elevation of the ground above sea level. It then subtracts the latter value from the pressure altitude value received from the target, to determine the approximate altitude of the target above the ground. If this altitude is less than 180 feet. TCAS considers the target to be on the ground, so the CAS logic does not generate a TA.
Traffic Advisory:
Range and altitude tests are performed on each altitude-reporting target. A proximity target is displayed if the target does not quality for a TA but is currently within 6 NM and 1200 feet of the TCAS aircraft.
A non-altitude reporting target is declared an intruder it the range test alone shows that the calculated tau is within the RA tau threshold associated with the current SL being used. This varies from 20 seconds for SL 4 to 35 seconds for SL 7. When TCAS is in SL 2 the tau threshold used is 15 seconds.
Using surveillance reports each second, the CAS logic tracks the slant range and closing speed of each target aircraft, to determine the time in seconds to the CPA. If the target is equipped with an altitude-coding transponder, the CAS logic also tracks the altitude reports to project the altitude of the target at the CPA. The CAS logic determines the vertical rate of the target by measuring the time it takes to traverse successive 100-foot increments of altitude.
The CAS logic uses the data from its own aircraft pressure altimeter either directly from the altitude encoder or as processed by an air data computer, to determine own aircraft altitude, vertical rate, and the relative altitude of each target. The Outputs from the tracking algorithm (target range and range-rate, relative altitude and vertical rate) from the inputs for the traffic advisory and threat detection algorithms.
Below 1700 feet AGL, the CAS logic uses the difference between its own aircraft pressure altitude and radar altitude to determine the approximate elevation of the ground above sea level. It then subtracts the latter value from the pressure altitude value received from the target, to determine the approximate altitude of the target above the ground. If this altitude is less than 180 feet. TCAS considers the target to be on the ground, so the CAS logic does not generate a TA.
Traffic Advisory:
Range and altitude tests are performed on each altitude-reporting target. A proximity target is displayed if the target does not quality for a TA but is currently within 6 NM and 1200 feet of the TCAS aircraft.
A non-altitude reporting target is declared an intruder it the range test alone shows that the calculated tau is within the RA tau threshold associated with the current SL being used. This varies from 20 seconds for SL 4 to 35 seconds for SL 7. When TCAS is in SL 2 the tau threshold used is 15 seconds.
A non-altitude reporting target will trigger the
generation of a TA if the range test is satisfied, on
the basis of the same tau values associated with the
RA.
generation of a TA if the range test is satisfied, on
the basis of the same tau values associated with the
RA.
Join Date: Aug 1999
Location: England
Posts: 1,050
Likes: 0
Received 0 Likes
on
0 Posts
OK - what the hell is Mode A + C without Altitude reporting?
So a transponder without Altitude Reporting can still reply to a mode C interrogation by sending the pulse frame. The pulse frame itself will just not contain any data. But since there is a response, a radar can do the time delay related to speed of light calculation to work out the range.
Since TCAS want to know the altitude of the intruder it needs to interrogate Mode C. It has no interest in the 4096 ident, so to avoid congestion on the SSR frequencies it does not interrogate Mode A , at all.
The basic problem here is that when folks say they are mode A, what they usually mean is "Mode A + C without Altitude reporting". When they say they are mode C what they mean is "Mode A + C with Altitude reporting".
Note the 2nd and 3rd quotes in mr ripleys post. There is no mention of A or C, only altitude reporting or non-altitude reporting.
OK, some might say I'm being picky. Certainly the distinction between mode C and altitude reporting is one that many people do not follow, but if you are going to go delving into the guts of TCAS technology understanding the difference will make it easier to understand.
Strictly speaking, references that say TCAS can give a TA against mode A are incorrect.
CPB
Join Date: Jul 2000
Location: ask crewing
Posts: 168
Likes: 0
Received 0 Likes
on
0 Posts
dv8
If you change the 9000 ft you mention to 8700 ft
and the max range forward is aprox 20nm to see the intruder (With a max range selected of 40nm)
Hope I'm not being to picky
and the max range forward is aprox 20nm to see the intruder (With a max range selected of 40nm)
Hope I'm not being to picky
Join Date: Jul 2000
Location: ask crewing
Posts: 168
Likes: 0
Received 0 Likes
on
0 Posts
I stand corrected dv8, have just dug said manual out from under the cobwebs and see the Canadians do know after all. Sorry.
Although, as I see we are on the same side, if you refer to B4-2.43-4 you will see the source of my confusion. SNAFU
Although, as I see we are on the same side, if you refer to B4-2.43-4 you will see the source of my confusion. SNAFU
