Note: The following information applies specifically to Boston Center ARTCC. The 'special purpose' squawk information applies nationwide. For more information, see here.
Often times when monitoring aircraft, especially when listening to them take off, you'll hear the controller say something like "N46F, squawk 6252". What does this mean, and what information, if any, can we gain from this?
First, a little background. Aircraft transponders exist to help identify the aircraft on the air traffic controller's radar screen, and on collision avoidance systems. To aid in this, aircraft equipped with a transponder will be assigned a four digit code to "squawk" from their transponder. This is old, technology, dating back to the 1950's, so instead of being a decimal system, it's an octal one. This means there are no 8's or 9's, or, more simply, each transponder has four dials that can each be positioned from 1 to 7. This leaves 4096 possible codes.
Transponder codes are assigned by air traffic control, but the possibility exists for more than 4096 flights nationwide at the same time, so the codes must be carefully managed. In addition, some codes have designated purposes, and aren't available for routine flight operations.
Let's start with the codes that have set meanings:
0000: Not used
0100-0400: Allocated to Service Area Operations for assignment for use by Terminal/CERAP/Industry/Unique Purpose/Experimental Activities
1000: Used exclusively by ADS-B aircraft to inhibit Mode 3A transmit
1200: For use by VFR aircraft not in radio contact with ATC
1201: VFR aircraft not in radio contact with ATC
1202-1272: Used by VFR aircraft that will be entering a US Air Defense Identification Zone
1255: Firefighting aircraft
1273-1275: Used by calibration monitoring equipment
1276: Air Defense Identification Zone (ADIZ) penetration when unable to establish communication with ATC or aeronautical facility
1277: Search and Rescue aircraft
4400: SR-71, F-12, U-2, B-57, pressure suit flights and aircraft operations above FL 600
4401-4433: Federal Law Enforcement
4434-4437: Weather reconnaissance
4440-4441: Operations above FL600 for Lockheed/NASA from Moffett Field
4442-4446: Operations above FL600 for Lockheed from Air Force Plant 42
4447-4452: Operations above FL600 for SR-71/U-2 operations from Edwards AFB
4453: High balloon operations – National Scientific Balloon Facility, Palestine TX
4454-4465: Air Force operations above FL600
5000-5060, 5400-5477, 6100-6177, 6400-6477, and 7501-7577: Reserved for military operations. Can only be assigned by NORAD. (Note: Most military flights are not assigned these codes)
7500: Hijacked aircraft
7600: Communications failure
7601-7607: FAA Special Use
7700: General emergency
7701-7707: FAA Special Use
7777: DOD interceptor aircraft on active air defense missions and operating without ATC clearance
So, more or less the rest of these codes are available for assignment. When assigning discrete codes, obviously they can't be shared by aircraft in the same area. Sometimes re-assignments are made in the air, but this is also accomplished via careful management of which codes are assigned by which area, and for what purpose. Certain code blocks are designated to be used for aircraft that will complete their flight plan without leaving the ARTCC area, and other blocks are for flights departing the area. In other words, when you are hearing the squawk get assigned, you can tell if the flight is local or not. For flights taking off inside Boston Center, the assignments are as follows:
For flights staying inside the ZBW ARTCC:
Primary: 4600-4677
Secondary: 5300-5377, 5500-5577, 4700-4777, and 0001-0077
For flights departing the ZBW ARTCC:
Primary: 3400-3477 and 3500-3577
Secondary: 7300-7377, 2000-2077, 1400-1477, and 1300-1377
Tertiary: 7000-7077 and 2400-2477
Remember that this only works for flights originating within the Boston Center ARTCC, which covers New England and part of New York state.
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