This represents a portion of the airspace I worked at Cleveland Center. Sort box 581 had Clearfield as preferred, with Pittsburgh as supplemental. That scenario I just went through on the previous slide was a true life example. On the one hand we (FAA) require aircraft to have a transponder, have it turned on, have it reporting altitude, have it certified every two years, yet on the other hand we take that perfectly good radar data from the transponder and don't necessarily process it for display! (And that assumes the transponder system itself is working reliably. For more on that angle, read my 1999 letter about a transponder stubbornly invisible to just one radar site.)
As I recall, after my paper was published in the Journal of ATC, sort box 581 got reprogrammed with Pittsburgh as preferred, and Clearfield as supplemental. However, does that solve our problems? In that particular scenario, sure. But what about all the other locations that lie midway between long-range radar sites? Which radar site has the absolute best coverage? One radar may see better in one portion of the box, whereas another may see better elsewhere in that box. However, for almost every midpoint situation, one-and-only-one radar is assigned as preferred. This can result in a low-altitude aircraft, that is adequately detected by radar, not being on the controller's display.
I recall when our backup radar display computer, which is named DARC (stands for Direct Access Radar Channel), was upgraded. It used to be that when the computer that drove the main computers would fail, one "switch" was thrown (probably a computer entry) so that all control positions would see the backup display. As I say, DARC was upgraded, and one of the improvements was that we now had individual control at each position for when we wished to switch to DARC. We could switch any time we wished. However, DARC was a lot less capable. For instance, it didn't have radar sort boxes at that time. One-and-only-one radar site could be selected at any one time. Changing the radar site was accomplished by an entry at the position. I distinctly recall my surprise one time when I was working the Clarion sector and I switched to DARC, the "less capable" backup computer. Much to my surprise, just west of the Clarion VOR, I saw two VFR targets flying around at low altitudes. Their transponders were working beautifully, and they were showing altitude. I switched back to our main display computer, the one we used all the time, and these two aircraft were invisible. Nada. Nothing. Zero. Zip. Not a trace of these aircraft existed. I switched back to DARC, and there they were. It was one of those enlightening experiences that drove me this far.
[Back] [Forward] [To Slide Show Introduction]
© 2000 by Lusch's Midair Collision Investigations. All rights reserved.