7 ms·

I wonder if it's because can fly higher than 65,535ft.

Retric 12y ago
"the maximum altitude transmitted by a U-2 transponder is FL600: even if the aircraft is flying well above it, the Dragon Lady’s mode C will show no higher than 60,000 feet." Which might be the issue if there was an off by one error in the code.
- abruzzi 12y ago
  I want to know why a spy plane has a transponder. Can it be turned off when flying over Cuba?
  - mikeash 12y ago
    Because it's a legal requirement to fly in a lot of airspace, to ensure that you show up on radar, and it's far easier to install one than to wrangle an exception. Of course it can be turned off, just like any other piece of equipment.
    - dsl 12y ago
      > far easier to install one than to wrangle an exception You are talking about an agency that builds and operates planes that nobody else knows about. They can have any "exception" they want by just shrugging and saying the aircraft in question does not even exist. All military aircraft have transponders because the air bases have ATC as well and (at least in the US) are near commercial airports they have to work with to avoid collisions during takeoff and landing.
      - mikeash 12y ago
        Flying a military airplane through civilian airspace without participating in civilian ATC sounds like an excellent way to end one's career in the military. Sure, they can just say the aircraft does not exist, if they stick to their own space. A U-2 is not going to do that.
    - twistedpair 12y ago
      Even the B-2 bomber had a retractable radar reflector to defeat it's stealth. When flying in civilian airspace the reflector would extend so ATC could track the plane. Standard stuff to prevent collisions.
      - mikeash 12y ago
        Why would they have a retractable reflector rather than simply using a standard radar transponder? Sounds like BS.
        twistedpair 12y ago
        To start with, a reflector is passive, so it can't fail like a transponder. Failing is a problem when flying an intentionally very hard to see/detect airframe in dense civilian airways. Transponders only provide a secondary radar hit, while many (older) systems are looking for a primary hit to locate aircraft, which a reflector would do. Before calling BS you could just Google it too. ;) For example, here is a nice explanation of the F117's reflector. http://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk http://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk
        mikeash 12y ago
        Any system that relies on a primary hit is going to have trouble. Something as simple as a composite glider is not going to show up well[1]. I'm not aware of any ATC system that relies on primary returns. As for Googling it, that's an interesting picture. It's not retractable, but I suppose the point is made. Perhaps the B-2's is retractable. I did Google it this time and found nothing but some random airliners.net forum posts, not exactly reliable. [1] http://www.dg-flugzeugbau.de/radar-reflector-e.html http://www.dg-flugzeugbau.de/radar-reflector-e.html
  - mjlee 12y ago
    A spy plane has a transponder so you don't shoot a friendly/neutral aircraft down (or waste time classifying it as friendly/neutral.) The codes it squawks can be modified in various ways but there will be scenarios when you want your transponder, such as flying in an area where you have air superiority.
  - Crito 12y ago
    I think the idea of the U-2, which was developed before stealth was really a thing, is that although you can see it, it flies so high it is out of range of your interceptors. After Gary Powers, they probably double-check that the countries they fly it over don't have modern AA capabilities.
  - ceejayoz 12y ago
    Even commercial airliners' transponders can be trivially switched off in the cockpit.
- gcb0 12y ago
  Nice. So the system depends entirely on transponder? What will happen when one plane is at the right altitude but report one higher by human or machine error on a day where everyone is flying blind? ...not that we have any blind day in LA but still...
  - bdonlan 12y ago
    It is standard procedure for ATC to request altitude (or, more commonly, to get unsolicited altitude reports) on initial contact, and to not trust the Mode C data until it can be cross-checked for exactly this reason.
deleted 12y ago
[deleted]
userbinator 12y ago
That crossed my mind; I really hope this isn't the case, because that would mean a lot of people who should be expected to know these sorts of things with such familiarity that it's like their lives depended on it, actually don't. Now thanks to 2048, if it did turn out to be 16-bit overflow, maybe quite a lot more of the general public would understand this too...
ams6110 12y ago
Seems unlikely; altitudes are normally managed in multiples of 100, e.g. FL350 = 35,000 ft. But of course it depends on the system's internal data structures.
Theodores 12y ago
Whoosh!!!

"the maximum altitude transmitted by a U-2 transponder is FL600: even if the aircraft is flying well above it, the Dragon Lady’s mode C will show no higher than 60,000 feet." Which might be the issue if there was an off by one error in the code.

abruzzi 12y ago
I want to know why a spy plane has a transponder. Can it be turned off when flying over Cuba?
- mikeash 12y ago
  Because it's a legal requirement to fly in a lot of airspace, to ensure that you show up on radar, and it's far easier to install one than to wrangle an exception. Of course it can be turned off, just like any other piece of equipment.
  - dsl 12y ago
    > far easier to install one than to wrangle an exception You are talking about an agency that builds and operates planes that nobody else knows about. They can have any "exception" they want by just shrugging and saying the aircraft in question does not even exist. All military aircraft have transponders because the air bases have ATC as well and (at least in the US) are near commercial airports they have to work with to avoid collisions during takeoff and landing.
    - mikeash 12y ago
      Flying a military airplane through civilian airspace without participating in civilian ATC sounds like an excellent way to end one's career in the military. Sure, they can just say the aircraft does not exist, if they stick to their own space. A U-2 is not going to do that.
  - twistedpair 12y ago
    Even the B-2 bomber had a retractable radar reflector to defeat it's stealth. When flying in civilian airspace the reflector would extend so ATC could track the plane. Standard stuff to prevent collisions.
    - mikeash 12y ago
      Why would they have a retractable reflector rather than simply using a standard radar transponder? Sounds like BS.
      - twistedpair 12y ago
        To start with, a reflector is passive, so it can't fail like a transponder. Failing is a problem when flying an intentionally very hard to see/detect airframe in dense civilian airways. Transponders only provide a secondary radar hit, while many (older) systems are looking for a primary hit to locate aircraft, which a reflector would do. Before calling BS you could just Google it too. ;) For example, here is a nice explanation of the F117's reflector. http://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk http://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk
        mikeash 12y ago
        Any system that relies on a primary hit is going to have trouble. Something as simple as a composite glider is not going to show up well[1]. I'm not aware of any ATC system that relies on primary returns. As for Googling it, that's an interesting picture. It's not retractable, but I suppose the point is made. Perhaps the B-2's is retractable. I did Google it this time and found nothing but some random airliners.net forum posts, not exactly reliable. [1] http://www.dg-flugzeugbau.de/radar-reflector-e.html http://www.dg-flugzeugbau.de/radar-reflector-e.html
- mjlee 12y ago
  A spy plane has a transponder so you don't shoot a friendly/neutral aircraft down (or waste time classifying it as friendly/neutral.) The codes it squawks can be modified in various ways but there will be scenarios when you want your transponder, such as flying in an area where you have air superiority.
- Crito 12y ago
  I think the idea of the U-2, which was developed before stealth was really a thing, is that although you can see it, it flies so high it is out of range of your interceptors. After Gary Powers, they probably double-check that the countries they fly it over don't have modern AA capabilities.
- ceejayoz 12y ago
  Even commercial airliners' transponders can be trivially switched off in the cockpit.
gcb0 12y ago
Nice. So the system depends entirely on transponder? What will happen when one plane is at the right altitude but report one higher by human or machine error on a day where everyone is flying blind? ...not that we have any blind day in LA but still...
- bdonlan 12y ago
  It is standard procedure for ATC to request altitude (or, more commonly, to get unsolicited altitude reports) on initial contact, and to not trust the Mode C data until it can be cross-checked for exactly this reason.

deleted 12y ago

[deleted]

userbinator 12y ago

That crossed my mind; I really hope this isn't the case, because that would mean a lot of people who should be expected to know these sorts of things with such familiarity that it's like their lives depended on it, actually don't. Now thanks to 2048, if it did turn out to be 16-bit overflow, maybe quite a lot more of the general public would understand this too...

ams6110 12y ago

Seems unlikely; altitudes are normally managed in multiples of 100, e.g. FL350 = 35,000 ft. But of course it depends on the system's internal data structures.

Theodores 12y ago

Whoosh!!!