ATC questions - tracking and transponders

[Edx]

Hi there,

I have been a debating a conspiracy theorist on a forum where he claims he is a pilot.

He is as far as I know making erroneous claims about how the ATC system works, I don't want to debate 9/11 here, I just want to get the opinion of professionals that can set him straight and correct those misconceptions on how the ATC system does and doesn't work. I hope that's okay!

The entire thread is here (http://forums.eidosgames.com/showthread.php?t=116145&page=6), but here are the relevant parts:

You still don't understand that without transponder information they had very little to go on and had to rely on the primary radar

Transponder data is fed into the system along with primary radar data. These are not separate systems. They work together. Every dot from primary radar shows up on the main screen. Transponder data shows up alongside if available. There is no problem finding the blip on main screen without transponder data. That's how the system works.

Again, if the blips without transponder data DID NOT show up on main screen, ATC would not be aware of the aircraft with no transponder that accidentally wondered into Class B air space. They would be unaware of it right up until the collision, which is exactly why it DOES show up on main screen, and why it's immediately possible to see which airplane has no transponder.

Yeah, you wouldn't be able to tell which flight it is, and outside of the Mode C zone, you wouldn't be able to tell it from any number of private planes with no transponder. But once the A 77 was in Mode C zone, it would light up on main screen. And that's 50nm from impact site

Do you know how a transponder works? It works as an extension of radar detection, not as a separate system. When a radar sends a ping, it's not just a short burst of radio frequency. It's a short code. When transponder picks up that code, it sends a burst of its own, replying to the sent code and also encoding the squak code, and depending on the system some additional information that is difficult for radar to acquire. Mode C specifically, which is what required within 30nm of Class B sends back pressure altitude.

This is very important for the ATC operations in large airports. A squak code is used to identify aircraft, so that the ATC can give specific instruction to a specific blip on the screen. The altitude is important because radar cannot pick it up, and it's another piece of information that the ATC will require.

Now why am I claiming that it's a load of crap that they couldn't find a plane? What happens if I'm flying my crop duster, and I flew into a busy Class B airport area? Am I simply not going to show up on the radar screen which ATC are using to steer the planes? Are you seriously that naive? Do you know what will happen if that crop duster ends up on the path of a 747 going for final? You'll have 300 dead bodies. That's if the 747 doesn't end up dropping on a residential area.

Everything that's big enough to show up on radar shows up on the screen to which all the transponder data is fed. If something does not have transponder, it will be shown without a squak and without an altitude. You see a dot on your radar without a squak in Class B, you report it immediately, because it's a hazard to all aircraft in the area. That's how the system works. That's how people get busted for flying within 30nm of a Class B without a working Mode C Transponder.

[niknak]

It appears that you are feeding a Troll.

Nothing you say will change the minds of people like this, they are not open to rational debate and are almost always people with an awful lot to say about very little.

[sambatc]

what is it exactly Edx that you disagree with?

[Edx]

@sambatc,

I believe he is wrong about his idea of how the ATC system works or worked 10 years ago and in terms of procedure how they would deal with the situations he brought up.

That is why I wanted to ask experts how exactly he gets it wrong! :)

For example he believes it was easy on 911 to track planes with transponders turned off and the "excuse" that this is hard is nonsense. If anyone with experience can explain why they had problems and why his understand is faulty I would appreciate it, I am also curious.

[BOAC]

what is it exactly Edx that you disagree with? - indeed - I'd go with that. You would have to assume:
a) The controllers are awake
b) they do not have 'primary' returns suppressed on the displays

[Blockla]

I would suggest it would have been difficult (but not impossible) for a number of reasons and make no specific comments about this area of the USA dicussed.

1) Not everywhere is within primary radar coverage, many areas have SSR only radar coverage; no transponder means no paint. Some areas are 'suppressed' that is the radar head can see the paint but the target is not displayed on ATC screens. Most places on the globe have no radar coverage at all.

2) Primary paints can be followed provided they stay within range of the primary radar head and on the range of the ATC tracking it; assuming someone is tracking it.

3) If the aircraft went from one centre to another, the coverage may vary and the coordination required between centres may be difficult. (access to radar feeds vary, they don't all get all the feeds only the ones pertinent to their area for their type of role, ie the terminal radars for a certain aerodrome may not even be provided to the overlying high level airspace)

4) Modern ATC systems can 'filter out' non-transponder aircraft; if this was initially selected then the aircraft may have simply disappeared... (for a while)

There is a significant difference between secondary (transponders -interrogated signal) and primary radar coverage (blips - bounce back); whilst integrated they are certainly different. We can lose SSR tracks or Primary tracks but still have the other system working normally. Generally SSR and Primary equipment are co-located ie the secondary array is on the primary antennae, but not always. Then there is the scan rate variation from different radar heads... But in lay terms there isn't a whole lot wrong in the quotes above from my very quick read.

In modern digitised systems the paints are not displayed as 'primary and secondary' they are a single blip (you may get an indication that there is both primary and secondary or just one or the other on that single blip.

There are various reasons why tracking of primary paints isn't practical or why a primary paint may be ignored; aircraft can operate under CTA steps without transponders, military aircraft conduct runs without transponders etc.

you see a dot on your radar without a squak in Class Bwithout a transponder you cannot be certain it's in Class B, it could be in E underneath in G... etc... even with a transponder you cannot be assured as the level information may not be verified (accurate).

[BOAC]

For example he believes it was easy on 911 to track planes with transponders turned off and the "excuse" that this is hard is nonsense. If anyone with experience can explain why they had problems and why his understand is faulty I would appreciate it, I am also curious- I have not read the thread to which you link, but you need to remember the a/c were under ATC control until hi-jacked.

If a squawk disappears on a controlled a/c (and the primary return is not seen either), one would hope that anyone with half a brain would take appropriate action?