2.2.4. GENERIC EMERGENCY INDICATOR
The Generic Emergency Indicator is an element of ADS-B messages, coded on two bits. It provides the Surveillance Status that can be either:
- No change of the SQWK code, or
- Emergency condition (each time 7500, 7600, or 7700 is set on the transponder control panel), or
- Change in SQWK code (each time a new code different from 7500, 7600 or 7700 is set). This Surveillance Status is active for 18 seconds following the SQWK code change, or
- SPI condition (when the IDENT function is activated). This Surveillance Status is
active for 18 seconds following the IDENT function activation. The Generic Emergency Indicator does not indicate either the nature of the emergency or the SQWK code.

2.2.5. DISCRETE EMERGENCY CODES
The Discrete Emergency Code is an element of ADS-B messages, coded on three bits. It provides the nature of the emergency and/or urgency:
- Emergency modes:
o  General emergency

o  Communication failure

o  Unlawful interference

- Urgency modes:
o  Minimum fuel

o  Medical.

Note: At the time of writing this brochure, only the ACSS XS 950 (P/N 751-7800-10100) and T3CAS are able to transmit the Discrete Emergency Code.

2.2.6. DO-260 AND DO-260A
DO-260 “Minimum Operational Performance Standards for 1090 MHz Automatic Dependent Surveillance – Broadcast (ADS-B)” defines the standards for ADS-B application. DO-260A is a revision of DO-260.
From an operational view, the main differences are that DO-260 requires the
transmission of the Generic Emergency Indicator and Discrete Emergency Codes. And DO-260A requires, in addition, the transmission of the transponder SQWK code. This latter requirement would permit a smooth transition from SSR to ADS-B operations for ATC controllers as they are used to working with SQWK codes.

From a technical point of view, the main differences, in addition to the ones quoted
above, are that DO-260 requires the transmission of the Navigational Uncertainty Category (NUC), whereas DO-260A requires the Navigation Integrity Category (NIC), the Navigational Accuracy Category (NAC) and the Surveillance Integrity Level (SIL) instead of NUC. Appendix E – NUC, NAC, NIC, SIL provides the ranges
of these values.

2.2.7. GEOGRAPHICAL FILTERING OF SQWK CODE
When surveillance experts defined the ADS-B transmissions at the beginning, two philosophies stood out: the US American one supporting the need for the transmission of the SQWK code (for a some transition from SSR to ADS-B operations), and the European one seeing no need for SQWK code.
Consequently, the industry had decided at that time the SQWK code to be transmitted through ADS-B only over the USA territory. Outside USA airspaces, the SQWK code was not transmitted. This was the Geographic Filtering of SQWK code defined in DO-260A.
　
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