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Shitsu_Tonka
7th Jul 2006, 02:11
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.

Cloud Chaser
7th Jul 2006, 15:54
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.

BizJetJock
7th Jul 2006, 16:06
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.:eek:
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:uhoh:
****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

Clarence Oveur
7th Jul 2006, 16:08
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.

Shitsu_Tonka
8th Jul 2006, 01:50
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.

Capt Pit Bull
8th Jul 2006, 07:56
If ypu genuinely do have Mode A only traffic (as opposed to Mode A and C without altitude reporting) TCAS will NOT see it.

TCAS doesn't interrogate Mode A.

CPB

Shitsu_Tonka
8th Jul 2006, 11:15
OK - what the hell is Mode A + C without Altitude reporting?

dv8
8th Jul 2006, 12:41
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

mr ripley
8th Jul 2006, 16:12
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 (http://www.eurocontrol.int/msa/gallery/content/public/documents/Safety/ACAS_Bulletins4-BUL2-D-2.0.pdf)

PPRuNe Radar
8th Jul 2006, 17:04
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.

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 ??


See this eurocontrol ACAS Bulletin 4 (http://www.eurocontrol.int/msa/gallery/content/public/documents/Safety/ACAS_Bulletins4-BUL2-D-2.0.pdf)


All the examples in the bulletin are based on both aircraft having Mode C.

mr ripley
8th Jul 2006, 17:42
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 ??


Correct all that can ever be displayed is a TA without C. I did not say this was the case. However TCAS works on closure rates as well as altitude. It just measures the change in time taken for successive interrogations to be replied to. Its all to do with slant range.

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.

and
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.

see http://www.eurocontrol.int/ra-downlink/Library/ACAS_training_ver20.pdf

Shitsu_Tonka
9th Jul 2006, 00:48
Thanks Mr ripley - thats the stuff I am after.

Capt Pit Bull
9th Jul 2006, 08:41
OK - what the hell is Mode A + C without Altitude reporting?

OK, both mode A and mpde C replies contain a some pulses, the timing of which marks whether the reply is mode A or Mode C. These also serve to provide a frame inside which data can be sent.

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

PPRuNe Radar
9th Jul 2006, 13:14
Thanks for the clarifications Mr Ripley and Captain Pit Bull :ok:

Shitsu_Tonka
9th Jul 2006, 13:26
I doff my cap in awe.

Cloud Chaser
12th Jul 2006, 14:27
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
You should tell the Canadians, it says 9000' in the book!:hmm:

dv8
14th Jul 2006, 10:31
My book is the Flight Safty Canada Q400 FCOM Vol 2

Crew Alerting System Capter
Page 6-25

ABOVE +8700 to -2700
NORM +2700 to -2700
BELOW -8700 to +2700

Cloud Chaser
14th Jul 2006, 11:57
I stand corrected dv8, have just dug said manual out from under the cobwebs and see the Canadians do know after all. Sorry.:ouch:
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 :ugh: