Definition of Discrete Code
A discrete code is a four-digit SSR code where the last two digits are not “00”.
Description of Discrete and Non-Discrete Codes
Discrete SSR codes are typically assigned by air traffic controllers. In contrast, non-discrete codes are often used at the pilot’s discretion, especially in emergencies or when no discrete code has been assigned. It’s important to note that exceptions exist in both cases. Controllers may occasionally assign non-discrete codes, such as the conspicuity code 1000 in the European Union for Mode S equipped aircraft that remain within Mode S airspace until landing. However, this code may have different meanings in other countries. Conversely, pilots may use a discrete code based on specific regulations, like how gliders in the United States not in contact with an ATC facility should squawk 1202.
Discrete Codes
Discrete SSR codes are assigned to ensure that no two aircraft in the same airspace share the same code simultaneously. This unique assignment allows for proper identification, enabling controllers to match targets on their displays with actual aircraft. To maintain this system, a method for assigning discrete codes is essential. Ideally, each aircraft retains its SSR code throughout the flight. Unfortunately, this isn’t always possible in busy airspaces, necessitating changes to avoid duplication. Such changes can increase workload for both pilots and controllers, leading to busier communication frequencies and a higher risk of human error. To address these challenges, various support tools have been introduced. For example, centralized systems like the Centralized Code Assignment & Management System (CCAMS) in Europe analyze flight data across large airspace volumes to optimize SSR code assignments. These automated systems also alert controllers when a code change is needed, often using Controller-Pilot Data Link Communications (CPDLC) to ease communication and reduce errors.
Non-Discrete Codes
There are 64 non-discrete codes, which are never used for identification. Many of these codes are reserved for special situations either worldwide or in specific airspaces, like the European Union or the USA. Three key international non-discrete codes are: 7500, indicating unlawful interference; 7600, signaling communication failure; and 7700, denoting an emergency. Notably, the same non-discrete code may have different meanings in various countries, as outlined in national documents like Aeronautical Information Publications (AIPs). For instance, the code 1000 in the EU is used for Mode S equipped aircraft that will remain in Mode S airspace until they reach their destination. This helps reduce the number of SSR code changes required. However, in Canada, code 1000 is assigned to IFR flights in low-level airspace when no other ATC instruction is provided.
National documents often specify procedures for assigning non-discrete codes based on criteria such as VFR flights, IFR flights, special operations like SAR, military flights, and flights in uncontrolled airspace. Automated ATS systems handle non-discrete code tracks in unique ways, including not filtering track labels for codes 7500, 7600, and 7700, avoiding duplicated SSR code warnings for non-discrete codes, and using specific text strings or colors to indicate these codes.
