Tip: Planes with "No callsign" in FlightRadar24

[Mick West]

Some ADS-B planes have "No callsign", but if you look 40 miles behind them (in the US) you can often find the plane from FAA data.



Notice the different photos. The ADS-B data has the callsign, so they use a photo of the actual plane. The FAA data does not have the callsign, so is using a generic image of a 737

 

[Jason]

I also noticed this a few weeks ago when you provided the link to that site to help someone identify what was the cause of contrails over his home near LA. Does the call sign come from the Transponder or is it assigned to the plane via the airport or company they are flying for. Does each plane have their own call sign, or do the call signs change from flight to flight. I ask this because when I was fooling around with the site you provided I began to notice some planes without call signs. I thought these planes might be government contract planes or planes that fly around politicians and event the president. I noticed this was more evident in the DC area, so thats why I assumed that. So who assigns the call sign

 

[Mick West]

I also noticed this a few weeks ago when you provided the link to that site to help someone identify what was the cause of contrails over his home near LA. Does the call sign come from the Transponder or is it assigned to the plane via the airport or company they are flying for. Does each plane have their own call sign, or do the call signs change from flight to flight. I ask this because when I was fooling around with the site you provided I began to notice some planes without call signs. I thought these planes might be government contract planes or planes that fly around politicians and event the president. I noticed this was more evident in the DC area, so thats why I assumed that. So who assigns the call sign

The call sign is generally the flight number for scheduled flights (e.g. SWA2447 for SouthWest flight 2447), or the plane's "N-number" for private planes (like N612JM, which is equivalent to a car license plate). For a flight number lke SWA2447, different planes can have that same number for that flight on different days. But N-numbers are essentially fixed with the plane.




A regular transponder just sends out a "squawk" code, a four digit code assigned by ATC, this can change several times during flight.

ADS-B transmission from the plane seem to vary. I think there are a variety of allowable transmissions.

 

[WeedWhacker]

And the transponder squawk code is for ATC use...it's how the computers will identify a particular flight. Also, the ATC computers already have the filed Flight Plan, specific to each IFR flight. As Mick said, the Xpondr codes can change for any given flight, even when en-route...this usually happens when two ATC facilities hundreds of miles apart coincidentally assign the same code (it is a 4-digit combination, but only the numerals zero through seven. Allows for about 4096 combinations, and some of those are reserved for specific usages).

'0000' is never used, for example. '1200' is for certain VFR traffic, and '7500', '7600' and '7700' are specific emergency codes.

Actually, Wiki has a nice article which explains well:
http://en.wikipedia.org/wiki/Transponder_(aviation)

 

[Jason]

A regular transponder just sends out a "squawk" code, a four digit code assigned by ATC, this can change several times during flight.

How often does the transponder communicate with the ATC. Is it instantaneous or delayed?

 

[Mick West]

How often does the transponder communicate with the ATC. Is it instantaneous or delayed?

As often as the ATC interrogates them. For practical purposes it's instant.
See: http://en.wikipedia.org/wiki/Secondary_surveillance_radar

 

[Jason]

As often as the ATC interrogates them. For practical purposes it's instant.
See: http://en.wikipedia.org/wiki/Secondary_surveillance_radar

Where are the transponders located in the aircraft? They were primarily installed so they could identify if the plane was friend or foe. Can transponders be reprogrammed or switched out of a plane, and whats the sense of having a "switch" so that a pilot could turn it off. Why is the transponder switch available to disengage in the cockpit?

 

[Mick West]

Where are the transponders located in the aircraft? They were primarily installed so they could identify if the plane was friend or foe. Can transponders be reprogrammed or switched out of a plane, and whats the sense of having a "switch" so that a pilot could turn it off. Why is the transponder switch available to disengage in the cockpit?

Transponders are switched off when on the ground to avoid taxiing aircraft being mistaken for nearby aircraft.

The transponder unit and controls are in the cockpit. There's an external antenna.

A regular transponder is pretty generic, so there's nothing to switch out. An ADS-B transmitter can be easily reprogrammed with a different call-sign and n-number.

 

[WeedWhacker]

Where are the transponders located in the aircraft? They were primarily installed so they could identify if the plane was friend or foe. Can transponders be reprogrammed or switched out of a plane, and whats the sense of having a "switch" so that a pilot could turn it off. Why is the transponder switch available to disengage in the cockpit?

The Xpndr is controlled via a panel, just as other avionics. The physical device (in the case of large airliners) is located in the lower part of the fuselage, forward and under the Flight Deck in the "E&E Bay" (Electronics and Equipment).


"Reprogramming" is not relevant...changing the squawk code is about all, and the TCAS functions are now incorporated into the control heads.

The Xpndr needs to be "OFF" (really it's usually called "StandBy") to prevent unnecessary radar screen clutter. Up until a few years ago it was frowned upon to have the Xpndr on at any time when on the ground (except during takeoff and landing when on the runway).

Nowadays some airports are incorporating ground movement control via the Xpndr, so the "rule" varies.

 

[Jason]

The Xpndr needs to be "OFF" (really it's usually called "StandBy") to prevent unnecessary radar screen clutter. Up until a few years ago it was frowned upon to have the Xpndr on at any time when on the ground (except during takeoff and landing when on the runway).

Thanks guys, are they coming up with automated systems so that the pilot has no access to these xpndrs. If they can get planes to land while on autopilot, it seems it would make sense to install a sensor that would allow the xpndr to be turned on and off automatically so that the pilot has no access to it. Given the recent events, and those that happened on 9/11, it just seems like a logical step in the right direction. But if you say they are incorporating "ground movement" controls into radar so there's less confusion then its probably best to update all airports in the US anyway, so that pilots can't shut em off.

Up until a few years ago it was frowned upon to have the Xpndr on at any time when on the ground (except during takeoff and landing when on the runway).

Just out of curiosity, based on this statement do pilots actually forget to turn em on, or is it something ATC reminds them to do when they are taxiing for take off.

And shutting off a transponder doesn't make a plane disappear, does it? I mean they can still be seen on radar right transponder or no transponder? If thats the case then why does the media make it a big deal, as if terrorist did this to hide the airplane?

 

[WeedWhacker]

...are they coming up with automated systems so that the pilot has no access to these xpndrs.

No need to.

Just out of curiosity, based on this statement do pilots actually forget to turn em on, or is it something ATC reminds them to do when they are taxiing for take off.

LOL...every day, even though it's on the 'Before Takeoff Checklist', someone somewhere forgets to turn it on...but, all that happens is the controller asks (usually nicely, like "Check transponder please", while probably thinking "Stupid pilots")

Same with the 'After Landing Checklist', if a flight is still squawking when it should not be.

And shutting off a transponder doesn't make a plane disappear, does it?

Only on Secondary Radar (SSR) A primary target ("skin paint") will show up as long is it's within the radar antenna range and sweep. Controllers can switch their displays to eliminate Primary hits, if it is causing too much clutter.

Here is a pretty good video about how ATC works (in Canada...eh?) Only gripe is the narrator used the word "tarmac"...it's like nails on a chalkboard to me when used as a generic for an Airport Operations Area.


This one is older, but might also help be informative:

 

[Jason]

Only on Secondary Radar (SSR) A primary target ("skin paint") will show up as long is it's within the radar antenna range and sweep. Controllers can switch their displays to eliminate Primary hits, if it is causing too much clutter.

So just to be clear, and I will use 9-11 as an example. If your flying in a heavily constricted zone, like the northeast where there are multiple air ports and NORAD, even if you shut your transponder off, you flight would still be visible on Primary Paint Radar. Correct. What is the minimum height a plane can fly before it drops off of radar, and does that vary depending on the area, like a mountainous area

 

[WeedWhacker]

What is the minimum height a plane can fly before it drops off of radar, and does that vary depending on the area, like a mountainous area

The minimum for radar coverage would be a height above the ground, compared to the distance and line-of-sight of any radar facilities in a particular region. There is no simple answer, because the available radar coverage varies so greatly. And yes, mountainous terrain presents a problem, but so do man made obstructions.

 

[Balance]

If it helps, this is my understanding of radar and transponders:

Radar has both a transmitter and receiver. Transponders have both transmitters, receivers and responders.
Radar emits a sweeping "signal" from it's transmitter and the receiver collects any "bounced" signals, ie a plane in the air, and shows them on a display. You often see a circular display depicting a 360degree field of view the signal is sweeping across. A dot appears for each bounced signal received.
Transponders transmits "are you there" type requests and will respod to such request with not only affirmation but other useful information such as ID, position, speed, etc which the receivers accepts and displays on a display.