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  1. #1
    Edx
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    Transponders and primary and secondary radar

    Hi there!

    I am posting here to get the opinion of experts in ATC systems on a debate I'm having on a forum with someone that claims to be a pilot.

    The debate is regarding 9/11, he seems to be making some extreme claims regarding the ATC systems and I was wondering if any of you guys could set the record straight and explain exactly what he is missing?

    He already said he is not an expert in ATC systems, but then talks as if he is and acts like its all so obvious. I however can find nothing supporting how he understands it and no one else seems to talk about the system like he does.

    I hope this subject is not offtopic as it is in some forums, but I really don't want to debate 9/11 here, merely deal with the facts of how the system works.

    The entire thread is here, but the relevant parts are below.

    Quote Originally Posted by K^2
    Quote Originally Posted by Edx
    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.
    And earlier in the conversation he says...

    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.

  2. #2
    flyin_sooner's Avatar
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    Re: Transponders and primary and secondary radar

    I don't claim to be an expert either but here is a quick overview of what I can recall from memory. If I am incorrect on anything I'm sure someone here will.
    Primary radar- sends out a signal which bounces back and shows that something (not specific) is out there. This something can be aircraft, weather, large flocks of birds, etc. Primary radar is divided into two subsections. Linear and circular radar. Linear radar was first and therefore the least sophisticated. It showed anything and everything the radar could pick up birds, planes, storm clouds, rain anything big enough to bounce back a signal. Later they came out with circular. Circular radar still shot out signals which bounced back but sent them out in a curved path which essentially filtered out a lot of noise from weather related radar feedback. In other words you would be more likely to see the plane near the storm clouds instead of just one big blob. Later they came out with secondary radar which is your transponder system. This system does not use a radar signal which bounces back but instead sends a out and when a transponder receives it the transponder responds with information about the aircraft such as altitude. Information such as flight number and aircraft type are not sent out by the transponder. When aircraft are assigned a squak code this information (A/C type, etc.) is assigned to the code in the ATC system which gives said info to the controllers. I am 99% sure about above info. Below info I am less sure on but will tell you my understanding on. I believe the two systems (primary and secondary radar) are separate systems. Secondary is really all that is in use anymore in the US. If primary exists at a facility I believe it is mainly as a back up system. According to one of my instructors (again going from memory) most secondary radar systems are backed up by other secondary systems so most controllers now days never see primary radar. For this reason most planes have transponders. If Farmer Dan makes a homebuilt aircraft and does not put a transponder in it and then goes and flies around I do not believe he would show up at all with the exception being a facility using a primary radar system which I believe are all phased out but someone else here will be able to confirm or deny this. Hope This lengthy, late night, sleep deprived, from memory, college student response helps!

  3. #3
    mtlatc's Avatar
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    Re: Transponders and primary and secondary radar

    I quit reading when he said they are not separate systems....
    This space for rent.

  4. #4
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    Re: Transponders and primary and secondary radar

    I couldn't read and reread as much as I'd like due to a fun case of pinkeye, but since I can't sleep I might as well do my best on this one. This is from a terminal guy, in case that makes a difference.

    It seems like he knows MOST of what he's talking about. I can track a primary all day long no problem. There are some flaws with his logic though. Due to being far away from the radar site or at a lower altitude and depending on terrain you can very easily never see an aircraft. Radar is line of sight after all. Another problem is airports/aircraft below the fight path of said aircraft (and below controlled airspace) and birds. I can daily see the primaries in the pattern at dirt farmer field, but never get a mode C hit. I'll also spend months watching the migratory patters of birds flood the scope. What he said about us finding people who bust the mode C veil. Meh. I imagine after the 1,000th time that day of me trying to get Farmer Joe at pattern altitude (read as too low for the mode C blade to get a good hit) in his Cub or Benny the Canadian Goose (we're assuming Benny, like most Geese are /T) for a violation they'd haul me to the loony bin.

    Its also a very real possibility to have a solid hit on a low flier at your boundary. Lose their mode C then 5 miles later, whoops, where'd he go? Not so much as a primary. From my experiences with Center, I do not believe their radar can see primaries.

    I'm probably missing the question or point, but eh. Need something to do. I absolutely missed where he said they are not separate systems. Wow. Really?
    Last edited by Genot; 01-25-2011 at 01:55 AM. Reason: Missed something.

  5. #5
    mtlatc's Avatar
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    Re: Transponders and primary and secondary radar

    Quote Originally Posted by Genot View Post
    I absolutely missed where he said they are not separate systems. Wow. Really?
    Second sentence, hope your eye gets better...lol
    This space for rent.

  6. #6
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    Re: Transponders and primary and secondary radar

    Quote Originally Posted by Genot View Post
    From my experiences with Center, I do not believe their radar can see primaries.
    We have the ability to track primaries. In some cases we actually share radar sites with approach controls. Seems like I remember seeing/hearing that at some point. Since they are only using the one radar site they are able to use the 3nm for separation though. The only place we use secondary only is high altitude to eliminate clutter on the scope. No one is flying around at 37,000 feet without a transponder, at least not normally. We can turn on primaries there as well; but I've never seen it done out of necessity. All it takes is the click of a button, so it's no big deal to switch back and forth.
    CPC @ ZAU

  7. #7
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    Re: Transponders and primary and secondary radar

    +1 for Genot's post. In all honesty, that guy isn't terribly far off from the truth of the matter.

    I personally think when he says they are "not a separate system," it's in the sense that both primary and secondary targets are shown on a radar scope at all times for low sectors and TRACONs. He could be a little more articulate certainly, but I think his overall argument is close to accurate.

    Toward mbalunda's point, center can certainly track primary targets. The system produces what I think of as a hybrid digitized target. Even if HOST is getting both primary and beacon hits on a guy, as long as it's getting a beacon target, it will display a beacon target only, in preference to the primary. If there's only primary though, that's what itt'l show (assuming the filter key is set to display primaries). The mosaic can pull in ARSR and ASR data depending on how the system is adapted. TRACONs though, itt'l normally show beacon and primary targets together.

    ~Nate

  8. #8
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    Re: Transponders and primary and secondary radar

    I think the main thing he omits is human factors and equipment capabilities at the time. Once that transponder was turned off, the chances of the primary being lost, missidentified, etc... went up greatly, especially considering the multiple sectors involved in trying to find it from different perspectives. Let's not forget the chaos of the moment either. It's very different to talk about it after the fact than to live it real time. I think the guy is coming from the mindset of "perfect world", which most know is fantasy speak.

  9. #9
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    Re: Transponders and primary and secondary radar

    Absolutely. If a track becomes disassociated from its target and the guy is primary only... identification could be damned near impossible, especially if it's a nice day (like it was) and subsequent VFR traffic, clutter, birds, and everything else associated with a nice fall day makes the radar look "busy" to the eye.

  10. #10
    StuSEL's Avatar
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    Re: Transponders and primary and secondary radar

    The guy seems like he's looked into radar systems a little bit, but not enough to really understand what he's talking about. I'm citing this: "It works as an extension of radar detection, not as a separate system." Everyone who studies radar in-depth knows that the primary and secondary signals are two completely independent systems, besides for the fact that their antennae might rotate on the same block of concrete at the radar site.

    When a radar sends a ping, it's not just a short burst of radio frequency.
    This is correct if you're talking about a secondary radar antennae, but he didn't specify. Primary radar sends out only the radio signal and picks up the deflected signal that it sent out.

    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.
    "Difficult for the radar to acquire." Again, any completely knowledgeable person would know that it's not "difficult" -- it's impossible. Radar itself cannot pick up altitudes or flight plans. Period.
    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.
    "Steer the planes?" Really? And more likely than not you won't have a collision if ATC vectors (not steers) aircraft around the primary "blip."
    And hey, I just checked the charts, and it's almost 50nm from Mode C boundary of Washington Dulles to Pentagon going from direction that AA 77 reportedly came in from.
    How would this be relevant if you have a terrorist who turns off the Mode C? It's not. They don't care about the Mode C veil.

    This guy does not know what he's talking about.

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