ADS-B Call Sign DO-260B 2.2.8.2.6
The ADS-B Report Assembly Function shall first extract the Aircraft Identification Character subfields (§2.2.3.2.5.3) from the ADS-B Flight Identification and Category Message (§2.2.3.2.5.3) for further processing.
Each of the eight characters extracted is encoded in a subset of International Alphabet No. 5 (IA-5) in accordance with §2.2.3.2.5.3. The encoding of each character is 6 bits long with bit_6 being the most significant and bit_1 being the least significant. IA-5 is a seven bit encoding with bit_7 being the most significant and bit_1 being the least significant.
In order to provide an IA-5 encoding in an eight bit format; the Report Assembly Function shall:
(1). Retain bit_6 through bit_1 of the character encoding.
(2). If bit_6 is ―ZERO, set bit_7 to ―ONE
(3). If bit_6 is ―ONE, set bit_7 to ―ZERO
(4). Set bit_8 to ―ZERO
(5). Format bit_8 through bit_6 into “0MdddddL”.
When valid Flight Identity data is not available, the Call Sign data sent to the user application shall be set to ALL ZEROs.