True when the transponder's Ident (SPI) pulse is active. Undefined for a reserved FS value (6 or 7), where Ident status is not defined.
True when the transponder is flagging a recent squawk code change. Undefined for a reserved FS value (6 or 7), where alert status is not defined.
Decoded 3-bit Flight Status (FS) subfield of a DF4/5/20/21 Mode-S surveillance reply - the Alert (recent squawk change) and Ident (SPI) flags. Ground/airborne status is also nominally packed into this field but is not decoded here - see decodeFlightStatus.