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Mode C vs. Mode S

Asked by: 9706 views , , ,
Aircraft Systems, General Aviation

Two questions:

  1. My understanding is that mode C transponders only transmit squawk, altitude, and position. So ATC has to assign a discrete code in order to attach a call sign, type, and flight plan to your aircraft. This system sounds a bit flimsy. What happens if two aircraft mistakenly squawk the same code? Or if a pilot accidentally changed their code during a critical time?
  2. Supposedly Mode S transponders now transmit their own unique aircraft identifier. Does this eliminate the need for squawk codes (other than the emergency codes)?

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1 Answers



  1. John D Collins on Mar 06, 2020

    1. Mode A/C transponders do not transmit position, they reply to secondary radar inquiries. Secondary radar uses 1030 MHz and uses two distinct pulse patterns, one that requests the transponder code and the other that requests the pressure altitude. When the transponder receives the queries, it responds after a set delay with 13 bits of data on 1090 MHz. The secondary radar uses the antenna position for the azimuth and the time between the query and the reply to determine distance from the site. The data is displayed on the controller’s screen as a target with the transponder code and altitude. The controller tags up the flight plan information with the target after going thru a verbal communication with the pilot, verifying the altitude and using an ident feature to highlight the position on their scope. The ATC system then tracks that transponder code as the tagged target. Multiple aircraft with the same transponder code can cause issues with tracking, that is why for most part the codes assigned are unique within a given sector. When that is not possible, one of the aircraft with a duplicate code will be reassigned a unique one. For aircraft outside of the sector, duplicate codes will always exist but are not of interest, for example all VFR traffic not receiving ATC services will be squawking the same code 1200.

    2. No Mode S does not eliminate the need for squawk codes. Mode S uses the same frequencies, but up to 112 bits of data may be exchanged. Mode S is much more efficient because each transponder is assigned a unique 24 bit address and individual transponders can be addressed by the secondary radar. Not all secondary radars are mode S capable and not all aircraft are equipped with mode S transponders, so mode S transponders also support the mode A/C queries. The mode S transponder supports the 4096 squawk codes, the ident feature, and altitude. However, the ATC system still relies on a common target identification system of the assigned transponder code. The extra data capability of mode S is used to exchange data between aircraft to aircraft and ground systems and if ES is supported, then ADS-B Out data.

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