Decodes the 12-bit altitude field of an ADS-B airborne position message
(type codes 9-18, barometric altitude). The field omits the M bit that
decodeAltitudeCode's 13-bit AC field carries, so it is
re-inserted (always 0 - the M encoding never appears in ADS-B position
messages) before delegating to the shared Gillham/linear decoder.
Parameters
positionAltitudeField: number
The raw 12-bit altitude field from bits 8-19 of an airborne position ME field.
Decodes the 12-bit altitude field of an ADS-B airborne position message (type codes 9-18, barometric altitude). The field omits the M bit that decodeAltitudeCode's 13-bit AC field carries, so it is re-inserted (always 0 - the M encoding never appears in ADS-B position messages) before delegating to the shared Gillham/linear decoder.