There are two forms of separation used in air traffic. Radar and Non-radar. While we are almost always using radar data to separate aircraft, we still use forms of separation that do not require radar to get the job done. For instance, any time we have vertical separation (two planes at two different altitudes), that is considered Non-radar separation. The radar comes in when we have planes not yet at different altitudes, but we can assure that by the time they are within 5 miles of each other the vertical separation is there. We can keep planes at the same altitude if we turn one or both of them away from each other and thus use lateral separation.
Whenever I am driving to work, I tend to see things around me as they would relate to how we control airplanes. For example, each lane in of the highway is an altitude. Cars in the right lane are always separated from cars in the middle lane, which are always separated from the cars in the left lane. The key is that each lane is going its own speed. If one car in the right lane slows down, the rest of the cars in the right lane must slow down as well, or, move into the middle lane so they can speed up. In addition, when cars are exiting or entering the highway via the right lane, the cars in the right lane must use their judgment and adjust their speed accordingly to allow the cars to enter or exit without hitting them. In air traffic, this is Radar separation. Determining the sequence and using vectors and speed control to make it work as it happens is the job of the Radar controller, the controller talking to the airplanes.
The other option is to simply switch lanes to the middle lane where you can then go your own pace and there is much less work involved. Cars can get on easily, and you don't have to slow down, speed up, and get stressed out over how to let everyone in. Remember the analogy, changing lanes is like changing altitudes. The key is the look ahead, see the cars approaching on the on-ramp, and switch lanes to make life easier for all. This is the job of the D-side (Radar associate/data controller). D-sides watch the edges of the airspace for aircraft approaching the sector which are in conflict with other planes either already inside the sector or approaching it from other sectors (aircraft which are being handed off from the same sector must be separated when they enter another sector, but very often, aircraft converge into your sector from two or three different sectors). The D-side must then call the other sector on the land line and issue control instructions to that aircraft to change altitudes so they are separated by altitude when they enter the sector.
It is not always that easy, though often times it is. The D-side works with the Radar controller to make a crucial decision as to which of the two aircraft to move. For example, lets assume two aircraft are converging over Ithaca, NY. One is coming from New York Center, the other from Cleveland Center. One aircraft is landing Boston at FL350, the other is flying to Frankfort at FL350. Chances are the overseas flight can't go any higher due to fuel weight, so lets descend the Boston guy. Ideally, we try to keep planes at the correct altitude for direction, so FL330. But if there is a HPN arrival below at Fl330, FL340 may have to be used. This is the basic job function of a D-side.
In the last few years, D-sides have been called upon to make more radar decisions. Perhaps giving the overseas flight a shortcut to the right would cause the flight paths not to cross. Maybe a similar shortcut would make the flight paths cross 40 miles east of where they originally would have crossed in conflict, allowing the radar controller to descend the aircraft arriving HPN first, then incrementally descend the BOS arrival on top of him. Sometimes the best bet is to turn one of the planes 30 degrees off course to fall behind the other aircraft. It is the D-side's job to FIND the conflict and advise the Radar controller. Sometimes the radar controller has a definite plan, and simply informs the D-side what they would like done, other times its a team effort to come up with the best plan.
In addition, the D-side keeps track of all the flight plans on the URET screen. The URET will often alert the D-side of potential conflicts between aircraft with Red boxes. This does not guarantee that the aircraft will be in conflict, nor does a lack of conflict guarantee there won't be a conflict. The key is to understand how each sector works and how that will affect each aircraft. The Frankfort and Boston example is straightforward, but it gets much more complicated when you have to decide between JFK and PIT, PHL and HPN, ALB and SWF, BTV and PSM, MDT and MGW. The key is knowing exactly what each aircraft must do in order to leave your sector correctly, and what the next sector can and won't accept. Aircraft performance and weather also play key rolls in the decision making.
Finally, D-sides handle most, if not all, land line calls to other sectors. These behind-the-scenes calls are to coordinate approval requests, pointouts, and anything else that is out of the ordinary and must be talked over with the other sector.
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