Minimum Aviation System Performance Standards for Aircraft ...

3.5.1.3.12 Heading While on the Surface Field
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Heading (§3.3.7) indicates the orientation of an A/V, that is, the direction in which the
nose of an aircraft is pointing. ADS-B Participants are not required to broadcast heading
if their length/width code (part of the aircraft size code, Table 3.3.4.1a) is Zero (0).
However, each ADS B participant that reports a length code of 2 or greater shall
(242AR3.76) transmit messages to support the heading element of the SV report when
that participant is on the surface and has a source of heading available to its ADS-B
transmitting subsystem.
Heading shall (242AR3.77-A) be reported for the full range of possible headings (the full
circle, from 0° to nearly 360°). The heading of surface participants shall (242AR3.77-B)
be communicated and reported with a resolution of 6 degrees of arc or finer.
Notes:
1. If heading is encoded as a binary fraction of a circle, a resolution of 6° of arc or
finer would require at least 6 binary bits.
2. The reference direction for heading (true north or magnetic north) is communicated
in the True/Magnetic Heading Flag (§3.2.7) of the Mode Status report.
3. For operations at some airports, heading may be required to enable proper
orientation and depiction of an A/V by applications supporting those surface
operations.
3.5.1.3.13 Heading Valid Field
The “heading valid” field in the SV report shall (242AR3.78-A) be ONE if a valid
heading is provided in the “heading while on the surface” field of the SV report;
otherwise, it shall (242AR3.78-B) be ZERO.
3.5.1.3.14 Pressure Altitude Field
Barometric pressure altitude shall (242AR3.79) be reported referenced to standard
temperature and pressure (1013.25 hPa or mB, or 29.92 in Hg). Barometric pressure
altitude shall (242AR3.80) be reported over the range of -1,000 feet to +100,000 feet.
If a pressure altitude source with 25 foot or better resolution is available to the ADS-B
transmitting subsystem, then pressure altitude from that source shall (242AR3.81-A) be
communicated and reported with 25 foot or finer resolution. Otherwise, if a pressure
altitude source with 100 foot or better resolution is available, pressure altitude from that
source shall (242AR3.81-B) be communicated and reported with 100 foot or finer
resolution.