Do combat planes have transponders that can be turned off to evade...

[maveric]

radar? The planes involved in 9/11 had their transponders turned off as to not be detected by radar. Now radar is used to track combat planes. Right? Can the fighters or bombers simply turn off their transponders to evade radar? That would make radar useless in those situations. Is their different a different type of radar thats used for commercial flights? or is radar, radar?
I'm just having a tough time believing that all four of those jets were uncrackable simply because the transponders were turned off, where if combat aircraft were heading toward a target they would show up on the radar.
Are there any pilots or air traffic controllers here to help me with my ignorance of radar systems?

[Zero Gravitas]

1. transponders

you can't "evade" radar by turning a transponder off. The transponder signals identifying information and things like altitude and speed. Turning off the transponder doesn't stop a plane from being tracked by radar, it is still visible it is just not identifying itself. The 9/11 hijacked planes were tracked by radar, no-one has claimed otherwise as far as I am aware. The only time those or any other plane might drop off the radar screen is id they flew close to the ground.

Fighter jets do have transponders to help identify friend from foe.

[Silverhair]

5. On the Air Traffic Controllers radar screen the controller...

he sees the dot that is the actual radar reflection from the aircraft, and below that dot he sees some information that the transponder transmits. They aren't supposed to, but sometimes, to reduce screen clutter, a particular ATC may turn down the sensitivity so that he doesn't see the dot, just the info. If the transponder is then turned off, the ATC sees nothing. There are special codes that the pilot can send via the transponder for different emergencies.

A military aircraft, flying in a friendly radar cover will usually have the transponder on, set to the authorized code. When in enemy radar he will turn it off as he doesn't want to help them know where he is.

[MercutioATC]

13. It doesn't work quite that way....

An enroute system is a strange animal. The "dot", as you call it, is a computer-generated symbol placed where the computer THINKS it belongs based on data from multiple radar sites in an enroute environment. It's usually accurate, but sometimes it's not.

We display a flight as a combination of two data streams. One is the computer's interpretation of the radar data and the other is the computer's prediction of the flight's progress based on its filed flight plan. Neither is very accurate sometimes.

[maveric]

7. So as soon as the 9/11 planes went off course radar should have

picked them up? Then in that case fighters should have been scrambled at that time?

[MercutioATC]

14. Nope. Planes go off course and/or stop talking to us all of the time.

Remember, we're talking about millions of flights a year. If we scrambled fighters for every one of those, the military would be VERY busy, indeed.

We DID pick up the deviations, but the culture at the time was to assume mechanical failure, not a hijack.

[htuttle]

2. I'm no pilot...

But I believe radar bounces off of metal. The planes should show up as 'blips', but woudn't have the usual transponder-provided information on what they were.

On edit: To avoid technical picking, I should correct that and say that radar bounces off of most solid objects, some more than others.

[MercutioATC]

16. That's an accurate accessment...No transponder does NOT equal no target.

[ThomWV]

3. No

Military aircraft are not required to have transponders, but about a third of them do as I recall. There was a congressional inquiry into the use of them by the military a couple of years ago. I think a military transport aircraft had gone down and it took them a while to find it and it found its way to a minor hearing. I recall some Congressmen being outraged that the things weren't on all aircraft primarily enguaged in the transport of personell. Memorys a little sketchy on it but I think that's how it went.

[MercutioATC]

15. I believe the number is much higher (like 100%).

Imagine you're flying in an F-18 Hornet and your onboard radar shows a target approaching you. How much you wanna bet that the military DOES actually want you to know if it's an F-16 or not???

[Fovea]

4. Radar has nothing to do with an IFF transponder

Aircraft transponders have to do with Air Traffic Control and IFF, they put out an identifier signal that can be triangulated via radio recievers. That is what you are referring to, their abscence from the ATC system.

Radar reads the reflected ping from a radar transmitter off a target, like sonar. You can't turn it off from inside your plane, you are either visible to radar, or you are stealth.

I am not a pilot, but that is how I understand the difference.

[mike1963]

8. To you and ThomWV both...yes all military a/c have transponders. In fact

over 90% of ALL aircraft have them now. You can't fly IFR without one or go into many areas. And they have everything to do with radar...when the xponder receives a radar scan it transponds (or responds) with a digital code that adds the additional information mentioned in earlier posts. And yes they can be turned off. Anything in an airplane can be turned off except for the ELT (emergency locator transmitter.)

[Flightful]

20. ELTs can be shut off

The ELT has a three-position switch- OFF, ON or AUTO.

[Don_G]

6. As I Understand It

The Air Traffic Control system has a minimal Radar capability.

Transponders were dictated by Federal Law to enable commercial flights to expand the Radar signature with a tracking code to accomodate the air traffic over the US Air-Space.

Once the Transponder loses power, the enhanced Radar signal disappears off the screens and a 757 several hundred miles out disappears in the Radar clutter that a nearby flock of birds will provide.

I could be wrong, but I think the person to ask would be a trained Naval Officer currently serving on an Aircraft Carrier who's up to speed with general Intel and Civilan Flight Regs with internet access.

I suggest we ask Hawker Hurricane; I was only a Navy Brat whose father rebuilt "Navy Stuff" at a Naval Ordenance Station ten years ago.

[mike1963]

10. It's hardly "minimal"...there are probably less than a couple hundred

square miles in the CONUS out of ATC radar coverage these days (unless, of course the target is very low to the ground (in the 'grass'), I mean at normal flight altitudes.

[MercutioATC]

11. Except in certain locations, that's not true.

There are really two civilain systems, terminal and enroute. Terminal syetems work off of a single radar site and provide real-time data as the radar sweeps (metal reflects radio waves...you're visible unless you're stealth). Enroute systems work on a "radar mosaic" principle wherein the data from multiple radar sites is collected by a computer and the most likely position is depicted on the screen. In the center of one of these areas, the computer is pretty good at this. On the fringes (where two radar sites have nearly equal priority) we get "target hops". The depiction of the plane on our scopes will literally "hop" a couple of miles in random directions until it's firmly located insire one radar site's "territory".

There are plenty of other quirks that we have to get used to, but a 757 doesn't "disappear in the Radar clutter that a nearby flock of birds will provide". This is NOT a precise science, but we rarely lose raw radar data (what we call "primary targets", the actual depiction of the radar capture of the aircraft) except in low-altituse situations. Remember, RADAR is a line-of-sight tool...if there are mountains in the way, we lose coverage.

[Postman]

9. Ahhh...my specialty!...I was in the US Navy as a Radarman....

...Radarman is the older term used for what is now called an Operations Specialist, which is what I was.

I'll try to explain it to you as well as I can keeping in mind that you are unfamiliar with such things.

I will not be giving you classified data, so not to worry that I'm compromising national security or fingering CIA agents or anything of that magnitude.

Aircraft DO have what is known as a "transponder." But first, how it relates to radar.

When a radar "paints" an aircraft the radars'electronic beam hits the aircraft, that pulse is then reflected and sent back to the radar's receiver which interprets the bearing and range of the target....(here is where the transponder comes into play)..

..At the same time that the radar beam is painting the aircraft, the transponder is receiving an interrogation from the radar's IFF antennae. In essence, the radar is asking the transponder on the aircraft who the aircraft is-(commercial aircraft?)and what altitude he's flying at.

The radar operator will get on his display - the aircraft (the notorious "radar blip" on the screen, this is also called raw video)) and the IFF response (which looks like a longer radar blip that shows up behind the actual raw video).

Now, judging from what I've read about various reports of planes "falling off the radar"...what that tells me is that these FAA enroute centers or whoever it was that was tracking these planes, where not using raw video when tracking the planes.

So when the hijackers switched off the transponders, the planes "disappeared" from the radar screens because the operators were looking at IFF responses and shutting off the transponders effectively took them off the radar screen.(some radars have the ability to eliminate the raw video and just track aircraft using the IFF responses. This kind of clears up the screen somewhat and makes it easier to track known planes. It also may be something that is "built-in" to modern aircraft tracking systems and may not be helped)

I have no knowledge of how civilian aircraft tracking systems work. This was just a guess based on my experience.

[MercutioATC]

12. As far as the enroute centers thing goes, you're right and wrong...

"Now, judging from what I've read about various reports of planes "falling off the radar"...what that tells me is that these FAA enroute centers or whoever it was that was tracking these planes, where not using raw video when tracking the planes.

So when the hijackers switched off the transponders, the planes "disappeared" from the radar screens because the operators were looking at IFF responses and shutting off the transponders effectively took them off the radar screen.(some radars have the ability to eliminate the raw video and just track aircraft using the IFF responses. This kind of clears up the screen somewhat and makes it easier to track known planes. It also may be something that is "built-in" to modern aircraft tracking systems and may not be helped)"

We DO use a "radar mosaic' system. The computer is essentially showing us what it THINKS is going on. However, we DO see primary targets (which directly reflect raw radar data) SOME of the time. Frankly, our margins of separation are built the way they are to compensate for the fact that, while we have a damn good idea where all of those planes are, we can only really be precise to within a couple of miles. It's NOT a black and white issue, which is something that ATCs understand. I realize the flying public might have a different view of the system, but that doesn't make it so,,,

[RH]

18. "the flying public might have a different view"

Yes, indeed, if it means that at any given time ATC may see no more than a part of the full monty.

For the sake of safety should we not expect that for as far as possible anything at all in an air lane should be visible?

[MercutioATC]

22. There are a couple of issues with that...

1) Radar is line-of-sight. Without dotting the landscape with radar sires (which are expensive) we're going to have dead spots in low areas due to terrain.

2) There is an incredible amount of data being fed to the HOST computer and we're asking that computer to provide a picture of what's going on. When the computer recieves conflicting data about a target from 2 (or more) different radar sites, it interprets that data to complete the picture. Sometimes it gets it wrong.

Thankfully, the system does work well most of the time (and the problems are mostly ones we're familiar with, so we know how to work around them). It's also a big sky...most of the time they just miss by themselves.

[RH]

17. Was this supposed to be taken seriously?

As if all that an enemy would then have to do to locate a fighter would be to tune into the transponder?

[BeFree]

19. Good discussion about radar!! Thanks...

I knew a little about RADAR before, but know even more now.

Question. In Colonel Scott's report to the 9/11 Commission, he is quoted as saying this:

At 9:05, FAA reports a possible hijack of United 175. Again, that's three minutes after the impact in the tower. That's how long it is taking now the information to flow through the system to the command and control agencies and through the command and control agencies to the pilots in the cockpit. At 9:09, Langley F-16s are directed to battle stations, just based on the general situation and the breaking news, and the general developing feeling about what's going on. And at about that same time, kind of way out in the West, is when America 77, which in the meantime has turned off its transponder and turned left back toward Washington, appears back in radar coverage. And my understanding is the FAA controllers now are beginning to pick up primary skin paints on an airplane, and they don't know exactly whether that is 77, and they are asking a lot of people whether it is, including an a C-130 that is westbound toward Ohio.

The question I have is this.....when he says that America 77, "appears back in radar coverage", what is he trying to say? That it had first disappeared?
How can a plane the size of American 77 disappear from radar?

[Flightful]

21. Altitude

Radar cannot see over the horizon and ATC radar units cover a 60-mile radius, so depending on the distance to the nearest station you can fall off fairly quickly. At 20 miles it's about 600 feet. I once dropped out of radar at 2500 feet.