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Most of the TRACONs in the US are colocated with a control tower. The location IDs of these match their airport IDs, which makes sense.

There are also several TRACONs that are not located at a control tower. Most of these are designated with a letter and two numbers, such as A11 for Anchorage. The first letter usually matches the TRACON name, but not always. Where did these IDs come from? Does the number signify anything?

Also, there are a few of these non-tower TRACONs that use three letters and no number - what makes them special?

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An "up/down" facility is where the tower and TRACON are in the same building and most of the traffic is to/from that airport (typically one class C or TRSA), so they share the identifier. They do provide services for satellite (class D or untowered) airports, but their traffic levels are usually negligible.

A "standalone" facility is where the TRACON is separate building, often on the grounds of a large airport but not the same building, and serves multiple airports (typically one or more class Bs plus any underlying class Cs, plus any satellites) so it gets a distinct identifier with numbers, which can't be confused with an up/down. I've never found a source for where the specific numbers come from; they don't seem random, but there isn't any obvious pattern either.

Recently, the FAA has created "consolidated" TRACONs that merge several neighboring standalone facilities. I'm not sure why the FAA has assigned them all-letter identifiers (e.g. PCT for Potomac Consolidated TRACON), but perhaps it's because there are so few of them (so far) and they cover such large areas that they're relatively well known and there isn't much risk of confusion.

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    $\begingroup$ But how are the IDs of standalone facilities determined? $\endgroup$ – expeditedescent Oct 22 '19 at 19:43
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This is anecdotal evidence gleaned from various forum and reddit comments, but I think it makes enough sense that I can justify making it an answer.

FAA Air Route Traffic Control Center (ARTCC) sectors are identified by a name and a number, or only a number. Boston Center, for example, publishes a sector map in a Letter to Airmen; a Syracuse Approach controller trying to contact the Boston Center controller working the "HNK23" sector might say either "Hancock, Syracuse, apreq" or "Two-Three, Syracuse, apreq." The TRACON systems and the ARTCC systems both keep track of ARTCC sectors by assigning each sector a one-letter two-number code, like C23 for "Center, sector 23."

Back when standalone TRACONs first came online, the ARTCC automation system (which may have been HOST or may have been an even earlier system) did not have any possible designation for a sector besides XNN. This is important, because the system needs to know who exactly should be on the receiving end of an automated handoff (where the identification of a radar target is transferred from one controller to another via computer connections, instead of the controllers verbally discussing the location of the target). So the new facilities were added to the center systems as if they were just another sector at an external ARTCC, using the same designator format. By the time facilities like PCT, NCT, and SCT came into existence, the systems had been upgraded and no longer required the workaround.

...Of course this doesn't explain how the system would deal with a normal non-separated TRACON like SYR, so perhaps this answer isn't correct after all. But I have seen it a couple of different places.

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