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From Are squawk codes used with ADS-B?:

Yes, squawk codes are still used and required with ADS-B.

The post is 3 years old though. And from Wikipedia List of transponder codes:

1000: Used exclusively by ADS-B aircraft to inhibit Mode 3A transmit (USA)

The way I understand that, if the flight will be in complete ADS-B coverage, say a domestic flight, it is assigned 1000.

Is 1000 indeed assigned to ADS-B flights, or are they given discrete* codes still?


* Oops. I didn't mean discrete as in hex discrete, but the 4-digit one that changes from flight to flight.

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Currently, FAA's "fusion" software uses discrete squawk codes to merge targets generated by different surveillance systems, i.e. SSR (often several of them) and ADS-B. They plan to upgrade the software so it can merge targets based on Mode S hex code as well, but (as of early 2019) that hasn't been rolled out yet. Once that is in place, ATC will switch to telling Mode S targets to squawk 1000. Mode A/C targets will still need discrete codes, though, including Mode S aircraft in areas that only have Mode C SSR.

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  • $\begingroup$ I found this based on the concept of fusion: "The current terminal and en route surveillance/automation systems in the United States have no way of associating an aircraft with a flight plan except by use of a 4096 code." (FAA, 2007) It's over a decade old though. $\endgroup$ – ymb1 Mar 26 '19 at 11:23
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1000 is a blockout code that prevents the ADS-B transmitter from also sending its discrete code, if you enter 1000 on your transponder no Mode 3/A code is sent in the ADS-B OUT message. Its part of AC-20-165B that outlines ADS-B

Mode 3/A Code.

Currently ATC automation relies on the Mode 3/A code to identify aircraft under radar surveillance and correlate the target to a flight plan. The mode 3/A code is a four digit number ranging from 0000 to 7777. Secondary Surveillance Radars (SSR) and ADS-B will concurrently provide surveillance, so the Mode 3/A code is included in the ADS-B OUT message and is required to be transmitted by § 91.227.

Note: ADS-B systems will not transmit the Mode 3/A code if the Mode 3/A code is set to 1000.

As of last weekend when I was flying (in an ADS-B equipped aircraft through controlled airspace) discrete codes are still being assigned. So then why would you need a squak code that overrides the very function of a transponder by preventing the sending of your "unique" squak? every aircraft is assigned a discrete 24-bit address so ADS-B is capable of unique identification without traditional 4 digit squak codes.

ICAO 24-bit Address.

The ICAO 24-bit address is a unique address assigned to an aircraft during the registration process. ICAO 24-bit addresses are defined blocks of addresses assigned for participating countries or states worldwide. In the United States, civil aircraft are assigned an address from an encoding scheme based on the aircraft registration number (“N” number). Additional information regarding the 24-bit address can be found in ICAO Annex 10, Part I, Volume III, appendix to Chapter 9, A World-Wide Scheme for the Allocation, Assignment and Application of Aircraft Addresses.

Since they don't offer an explanation as to why the 1000 code exists my educated guess is to prevent double target identification in radar systems that are capable of both Mode-A/C/S and ADS-B

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    $\begingroup$ @ymb1 i have reworded a bit to clarify $\endgroup$ – Dave Mar 25 '19 at 15:33
  • $\begingroup$ Thanks. I'm trying to understand it still. If 1000 blocks the 4-digit code, but doesn't block the hex, why not use 1000 since the US is ADS-B covered? And only assign the 4-digit one to those flying internationally. $\endgroup$ – ymb1 Mar 25 '19 at 15:39
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    $\begingroup$ @ymb1 I belive its because ADS-B is not mandated until Jan 1. 2020 so the legacy systems are still up and running and the primary mode of identification. All of the previous SSR systems were backwards compatible and somewhat overlaid on each other aside from being "denied airspace entry after 1/1/2020" I don't think the FAA has made a hard plan for shutting down Mode S $\endgroup$ – Dave Mar 25 '19 at 16:16
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My last flight, 3/20/2019, discrete codes were being given. For me, one was assigned as I departed a local airport and contacted Burlington (VT) approach, when they thought I was landing, then changed to another when they realized I was not landing there and handed me off to Boston Center for flight following for the rest of the flight. I have not heard 1000 assigned in my flights around/thru the Boston (MA) Class B, down to the Cape area, or out to middle of NY state.

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