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If I understand holding patterns correctly, several planes stack up vertically, each circulating at its assigned altitude until given permission to exit the pattern and enter an approach for landing.

My question is, don't the vertically stacked airplanes appear to be superimposed on the ATC radar? I am wondering how the ATC can keep them sufficiently separated on the screen so they can track them individually.

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    $\begingroup$ It would be really rare for them to all be superimposed, that implies that they are at exactly the same position in the racetrack and traveling at exactly the same speed and turning at exactly the same rate. Radar is also designed to declutter and move labels to free area's of the screen. $\endgroup$ – Ron Beyer Jan 21 at 4:38
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    $\begingroup$ Really rare, yes, but it does happen every now and then in congested airspace for two acft, three and more is exponentially rare. This condition rarely will last long, as there is minor variation in speeds, and flying a holding pattern is not dead accurate even by autopilots. $\endgroup$ – Jpe61 Jan 21 at 9:41
  • $\begingroup$ Why would it matter if they can be distinguished from each other? $\endgroup$ – Mike Brockington Jan 22 at 20:47
  • $\begingroup$ @MikeBrockington Potential human error on the ATC's part that might cause mid-air collisions if the top plane starts losing altitude, or the bottom plane starts gaining altitude and the ATC doesn't notice. $\endgroup$ – nick012000 Jan 23 at 11:22
  • $\begingroup$ @nick012000 Well, either ATC notices the change of altitude, or they don't. Surely still doesn't matter which blip is which? $\endgroup$ – Mike Brockington Jan 25 at 11:43
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Sometimes we can't.

While having two radar targets presented directly on top of each other is not really an issue (most radar presentation systems lets you move the data label around freely, so you can prevent overlap), the radar system itself can sometimes confuse two targets that are very close. A radar will search for aircraft by sending out a signal and waiting for the aircraft transponder to respond. When two aircraft are very close, they will receive the request from the radar almost at the same time, so their responses will be at the same time as well. The radar will receive the two replies on top of each other, which can result in garbling (see this related question).

In some situations, the radar software can also confuse two close targets, swapping the label between them, so aircraft A is presented to the controller as aircraft B and vice versa. We also sometimes see this close to an airport, where a departing aircraft can "steal" the label from an aircraft on short final, making it look like the departing aircraft is actually the one just about to land.

As a result of these issues, we are not allowed to use normal radar separation to individual aircraft in a holding stack. Instead, radar separation will be provided to a defined holding area, which is a geographical area established around a holding stack, large enough to ensure that anyone holding will remain within. Inside a holding stack, as you correctly noted, only vertical separation is used.

Mind you, most of the time, aircraft in a holding pattern are not flying directly on top of each other. That would require them to enter the holding at exactly the same time, fly at the exact same speed and turn at the exact same rate, which just doesn't happen. So the issues described above are not a problem most of the time, but they are common enough that we have specific procedures to mitigate the risks imposed.

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    $\begingroup$ Do you have only Mode A/C radars where you work? Mode-S should prevent all the radar problems you describe. $\endgroup$ – DeltaLima Jan 21 at 8:27
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    $\begingroup$ @DeltaLima We have mode S, but correlation is based on mode A $\endgroup$ – expeditedescent Jan 21 at 9:50
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    $\begingroup$ In that case the radar will interrogate the aircraft one-by-one and the replies will not overlap. $\endgroup$ – DeltaLima Jan 21 at 10:16
  • $\begingroup$ I'd be interested to know if in strong cross-winds (component perpendicular to the inbound holding direction) you find airplanes equipped with only traditional radio-nav equipment flying a different shape of hold compared to the FMC equipped. $\endgroup$ – skipper44 Jan 21 at 20:08
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    $\begingroup$ Do you work in the US? I think your answer is very specific to a country. In Europe we dont experience such issues and we do indeed seperate to Single aircraft with normal radar separation in a holding stack! $\endgroup$ – pcfreakxx Jan 22 at 20:32
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ATC's radar is of the type 'secondary radar' where at the beam does not simply bounce off the object being detected or tracked as it was earlier on the primary radars of old.

On first contact or earlier a 4 digit transponder code is assigned by ATC to each airplane by using the phrase "Squawk xxxx". This code is only valid for that particular flight, or segment of flight.

Secondary radar uses a coded signal from an Interrogator unit which is identified, decoded and responded to by an onboard transponder so that each airplane returns certain data such as callsign, altitude etc. so that each airplane is uniquely identified.

There is also a facility for identifying a blip representing an airplane by pressing an Ident button on the transponder panel in the cockpit which then highlights the blip/symbol of that airplane. This is called "Identing" and is normally initiated by the controller only by saying "Squawk Ident"

Beyond this, the info can be displayed and have many facilities limited only by software processing, the info can be processed and displayed in a customized manner, to make it more useful to the radar controller.

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