Transponder Sequence of Events

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Randall45

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Randall
Assumption 1: When your transponder is set to altitude, the air traffic controller sees a target and your altitude.

Assumption 2: When you are assigned a squawk, your target is paired with a call sign and other information.

If assumption 1 and 2 are correct, how does the air traffic controller know your altitude if you weren't assigned a squawk? Does the transponder transmit position too? I can understand how a radar wave will detect a piece of metal in the sky, and its position, but how does it know that the transmitter transmitting x altitude is in fact the same target as the airplane?
 
Controller just sees many VFR (eg 1200 code) targets on their screen. The transponder is a secondary radar return and many controllers only look at this data. They can see a VFR aircraft in a certain location at a certain altitude but don't know anything else about it other than track direction and ground speed. This will change with ADS-B since the tail number is hardcoded into the transponder response.
 
Ahah! I see. So, as I suspected, only track and ground speed.

Followup: So during flight following, if the traffic aircraft doesn't have a squawk, the controller could only tell me direction and not altitude?
 
Randall45 said:
Followup: So during flight following, if the traffic aircraft doesn't have a squawk, the controller could only tell me direction and not altitude?
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They could tell you altitude if the traffic aircraft has Mode C
 
An unverified altitude as well. "Altitude indicates..."
 
Randall45 said:
Assumption 1: When your transponder is set to altitude, the air traffic controller sees a target and your altitude.

Assumption 2: When you are assigned a squawk, your target is paired with a call sign and other information.

If assumption 1 and 2 are correct, how does the air traffic controller know your altitude if you weren't assigned a squawk? Does the transponder transmit position too? I can understand how a radar wave will detect a piece of metal in the sky, and its position, but how does it know that the transmitter transmitting x altitude is in fact the same target as the airplane?
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Good question. What we call radar, is not actually a radar in the traditional sense. Primary radar is based on the ground radar broadcasting a signal and looking for returns that bounce off the aircraft and are received by the radar.

A transponder is part of a system that is called secondary radar and isn't radar at all. A directional antenna broadcasts a pulse sequence on one frequency (1030 MHz) and depending on the timing of the pulses the transponder recognizes this as an inquiry for the transponder code or an altitude. The transponder delays a short period of time and then broadcasts the reply on a separate frequency (1090 MHz). If the inquiry was for the transponder code, the transponder code is broadcast. If the inquiry was for the altitude, then the altitude is broadcast. The ground antenna receives the reply and is able to relate it to the inquiry it sent (for a transponder code or an altitude). The time of arrival of the response can be used to determine distance from the radar site and the azimuth that the antenna was pointing at can be used to determine position. After all is said and done, the secondary ground system has the distance, the azimuth, the code, and the altitude.
 
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John Collins said:
A transponder is part of a system that is called secondary radar and isn't radar at all.
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It is IN fact. It uses radio to detect direction and range quite similarly to the primary radar other than the fact that it uses the transponder return rather than the reflected radio waive.

The time of arrival of the response can be used to determine distance from the radar site and the azimuth that the antenna was pointing at can be used to determine position. After all is said and done, the secondary ground system has the distance, the azimuth, the code, and the altitude.
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Entirely, like the way primary radar works except for the nature of the thing that turns around the return signal.
 
flyingron said:
It is IN fact. It uses radio to detect direction and range quite similarly to the primary radar other than the fact that it uses the transponder return rather than the reflected radio waive.

Entirely, like the way primary radar works except for the nature of the thing that turns around the return signal.
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I was simply pointing out that a totally different principal is used than (primary) radar which bounces a signal off of the target and measures the round trip time to determine the distance. Secondary radar uses totally different technology, uses two frequencies, uses encoded pulse trains, depends on a transponder, and the time between the interrogation and the return signal is not a simple measurement of the round trip time based on the speed of light in the atmosphere.
 
John's explanation is sound. Nicely put.

I see VFR targets (on ARTS it's a square symbol, on ERAM it's a V or I symbol) with Mode C altitude on my scope. Since I have no way of verifying the validity of the readout, I state in a traffic call "altitude indicates."

As an aside, this is also why controllers require pilots to state their current altitude on initial check on.
 
While we're on the topic, I've always been assigned a code simply by "squawk XXXX". Yesterday I was told "reset transponder and squawk XXXX". Reset transponder was a new one to me.

I had flight following and was cleared through a couple restricted areas, got a squawk and then was handed off. Then after a few minutes told to reset and squawk a different code.

Does that literally mean "turn your transponder all the way off and adjust squawk code then turn it back on", or was there an error in transponder reporting? After that I was handed off a few more times and was incident free.
 
gitmo234 said:
While we're on the topic, I've always been assigned a code simply by "squawk XXXX". Yesterday I was told "reset transponder and squawk XXXX". Reset transponder was a new one to me.

I had flight following and was cleared through a couple restricted areas, got a squawk and then was handed off. Then after a few minutes told to reset and squawk a different code.

Does that literally mean "turn your transponder all the way off and adjust squawk code then turn it back on", or was there an error in transponder reporting? After that I was handed off a few more times and was incident free.
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Yes. Turn the transponder off, then turn it back on and set the code. I had the reasoning explained years ago in training, but have long since forgotten.

Personally I only do it if a mode c or discrepancy persists. Otherwise it's, "squawk XXXX."
 
So its relatively standard procedure, not meaning my transponder is jacked up. I'm guessing it may be to prevent accidental "7X00" squawks?
 
gitmo234 said:
So its relatively standard procedure, not meaning my transponder is jacked up. I'm guessing it may be to prevent accidental "7X00" squawks?
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That's firing off a few neurons in my noodle, could have something to do with it. 99% of the time, it's technique.

Don't sweat it, if something is amiss, the controller will let you know.
 
Speaking of transponder codes, is there a list of which facilities assign which codes? GRR always gives me 42xx for VFR and (I think) 0xxx for IFR. Just a curiosity.
 
EdFred said:
Speaking of transponder codes, is there a list of which facilities assign which codes? GRR always gives me 42xx for VFR and (I think) 0xxx for IFR. Just a curiosity.
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Yes. Each facility gets assigned a subset of codes for internal usage.
 
MarkZ said:
Yes. Each facility gets assigned a subset of codes for internal usage.
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And those internal codes don't get picked up by radar tracking sites like flight aware. If you're flying a longer VFR cross country that passes though sectors controlled by different facilities you'll usually get given a code that's in the national airspace system and thus gets picked up by the tracking sites, but sometimes a facility will still just dump you and have you request following from the next sector.

When flying in somewhere I'll often tell the friends/family I'm meeting there that they can't track my flight on these sites but need to explain the nuances. I tell them if my radar return just disappears, don't worry I didn't pull a Malasyan Airlines on you.
 
Fascinating, John! That's exactly what I was looking for. I knew there had to be a way to pair the aircraft with the altitude even if multiple are equidistant from the radar return.
 
Randall45 said:
Ahah! I see. So, as I suspected, only track and ground speed.

Followup: So during flight following, if the traffic aircraft doesn't have a squawk, the controller could only tell me direction and not altitude?
Click to expand...

Incorrect. The transponder transmits from the same location as the primary target and overlays on ATC's radar screen. When you are VFR with a Mode C transponder on Alt, they will see your target with 1200 and your altitude display.
 
rocketflyer84 said:
And those internal codes don't get picked up by radar tracking sites like flight aware. If you're flying a longer VFR cross country that passes though sectors controlled by different facilities you'll usually get given a code that's in the national airspace system and thus gets picked up by the tracking sites, but sometimes a facility will still just dump you and have you request following from the next sector.

When flying in somewhere I'll often tell the friends/family I'm meeting there that they can't track my flight on these sites but need to explain the nuances. I tell them if my radar return just disappears, don't worry I didn't pull a Malasyan Airlines on you.
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That's incorrect. I've gotten flight following out of GRR and headed to Philly and I show up on Flight aware without changing codes.
 
MarkZ said:
Yes. Turn the transponder off, then turn it back on and set the code. I had the reasoning explained years ago in training, but have long since forgotten.
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What if the only way to turn off the transponder is to switch off the avionics master? My primary transponder is in the tailcone and is commanded by my 480. There is no power switch for that transponder that I can access in flight.

I've gotten the "reset transponder" instruction before and had no idea you were supposed to do that. I always just change the code and have never had ATC come back with "reset transponder" a second time. :dunno:
 
This is an interesting readout.

ATC-Radar-Display.png


Here's another:

aircraft_tag.jpg


Maybe a controller can chime in and tells us how these different configurations might be used.
 
Top configuration is terminal, bottom is enroute. I can only speak to the terminal one. The T for TCAS is something I've never seen, we locally at least use that location to identify aircraft on RNAV SIDS/STARS.An indent will show up as ID flashing to the right of the T. We also have which scope owns which tag exactly how ito is in the enroute example. Our handoff status is indicated between altitude and speed with a single character.

On fusion radar the return looks different. You get a blue ball.
 
azure said:
What if the only way to turn off the transponder is to switch off the avionics master? My primary transponder is in the tailcone and is commanded by my 480. There is no power switch for that transponder that I can access in flight.

I've gotten the "reset transponder" instruction before and had no idea you were supposed to do that. I always just change the code and have never had ATC come back with "reset transponder" a second time. :dunno:
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"Reset transponder" or "recycle transponder" is usually ATC speak for "You idiot, you took off with an incorrect transponder code... don't you use your checklist? Please update the code." But "reset transponder" sounds nicer over the radio. (So the guys up in the tower tell me).
 
rocketflyer84 said:
"Reset transponder" or "recycle transponder" is usually ATC speak for "You idiot, you took off with an incorrect transponder code... don't you use your checklist? Please update the code." But "reset transponder" sounds nicer over the radio. (So the guys up in the tower tell me).
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Yep, but it wasn't just "reset transponder", it was "Nxxxxx, reset transponder squawk wxyz", when I had been squawking abcd. For all I know I could have had two digits reversed, but this was a totally different code - often happens when I'm started out with a local code, then switched to something appropriate for use across sectors.

If they also give that instruction when there's some undefined problem with the signal, as someone said earlier, it would be good to know how I can "reset" my transponder when there's no "reset" button to press.
 
EdFred said:
That's incorrect. I've gotten flight following out of GRR and headed to Philly and I show up on Flight aware without changing codes.
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Just because you show up on FA for one flight does not mean you'll show up on the next. It's been discussed on here before. It has to do with how the controller enters you into the system. I've filed VFR Flight plans and had my track show up on FA, and the very next day done the same flight, filed in the same way and not shown up. The only guarantee that you'll show up is to fly IFR AFAIK.
 
I find if I get my squawk on the ground, I pretty much always show up on Flight Aware.
 
azure said:
What if the only way to turn off the transponder is to switch off the avionics master? My primary transponder is in the tailcone and is commanded by my 480. There is no power switch for that transponder that I can access in flight.

I've gotten the "reset transponder" instruction before and had no idea you were supposed to do that. I always just change the code and have never had ATC come back with "reset transponder" a second time. :dunno:
Click to expand...

"Reset (or recycle) transponder squawk XXXX" just means "here comes a new setting: XXXX", or "make sure you're squawking code XXXX". It does not mean you need to do anything special except make sure you are squawking XXXX, and if it's the same as before, make sure you are getting a good reply light and it's set to mode C.
 
StinkBug said:
Just because you show up on FA for one flight does not mean you'll show up on the next. It's been discussed on here before. It has to do with how the controller enters you into the system. I've filed VFR Flight plans and had my track show up on FA, and the very next day done the same flight, filed in the same way and not shown up. The only guarantee that you'll show up is to fly IFR AFAIK.
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He said they don't - as in never. They do.
 
MarkZ said:
Yes. Each facility gets assigned a subset of codes for internal usage.
Click to expand...

That's true, but may confuse folks...

Here in the SF Bay Area, Norcal Approach uses 0xxx squawks for flights that won't leave their airspace. They use 4xxx squawks for aircraft they're going to hand off to Center, or Travis approach, or Bakersfield approach.

So, two meanings of internal. 0xxx squawks fly completely internal to Norcal airspace. 4xxx codes are consumed internally, for aircraft not staying internal to their airspace.

All codes can be reused in every center's airspace. But there is some coordination to allow a flight to transit multiple center airspace without code changes... Usually. 0xxx codes can be used multiple times in each center's airspace by the contained approach controls.

Paul
 
>> The only guarantee that you'll show up is to fly IFR AFAIK.

There are no guarantees, I'm afraid. But I've had nearly 100% success showing up in FA VFR by *filing* IFR, then just requesting flight following. Since the IFR flight plan is in the computer, whatever needs to happen seems to happen.

Paul
 
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