Technical Provisions for Mode S Services and Extended Squitter

C-14 Technical Provisions for Mode S Services and Extended Squitter
Note.— If a source of A/V Heading is not available to the ADS-B Transmitting Subsystem, but a source
of Ground Track Angle is available, then Ground Track Angle may be used instead of Heading, provided that
the STATUS BIT FOR HEADING subfield is set to ZERO (0) whenever the Ground Track Angle is not a
reliable indication of the A/V’s Heading. (The Ground Track Angle is not a reliable indication of the A/V’s
Heading when the A/V’s Ground Speed is low.)
C.2.3.3.2.2 Heading/Ground Track Value
This 7-bit field (“ME” bits 14 – 20, Message bits 46 – 52) shall define the direction (in degrees clockwise from
true or magnetic north) of aircraft motion on the surface. The Heading/Ground Track shall be encoded as an
unsigned Angular Weighted Binary numeral, with an MSB of 180 degrees and an LSB of 360/128 degrees,
with ZERO (binary 000 0000) indicating a value of ZERO degrees. The data in the field shall be rounded to
the nearest multiple of 360/128 degrees.
Note.— The reference direction for Heading (whether True North or Magnetic North) is indicated in the
Horizontal Reference Direction (HRD) field of the Aircraft Operational Status Message (§C.2.3.10.13).
C.2.3.3.3 Compact Position Reporting (CPR) Format (F)
The one-bit (“ME” bit 22, Message bit 54) CPR Format (F) field for the Surface Position Message shall be
encoded as specified for the Airborne Position Message. That is, F=0 shall denote an “even” format coding,
while F=1 shall denote an “odd” format coding (§C.2.6.7).
C.2.3.3.4 Time Synchronization (T)
This one-bit field (“ME” bit 21, Message bit 53) shall indicate whether or not the Time of Applicability of the
message is synchronized with UTC time. “T” equal to ZERO (0) shall denote that the time is not
synchronized to UTC. “T” equal to ONE (1) shall denote that Time of Applicability is synchronized to UTC
time.
When T=1, the time of validity in the Airborne Message format shall be encoded in the 1-bit “F” field that (in
addition to CPR format type) shall indicate the 0.2 second time tick for UTC time of position validity. The “F”
bit shall alternate between ZERO (0) and ONE (1) for successive 0.2 second time ticks, beginning with F=0
when the Time of Applicability is an exact even-numbered UTC second.
C.2.3.3.5 CPR Encoded Latitude/Longitude
Draft
The CPR Encoded Latitude/Longitude field in the Surface Position Message shall be a 34-bit field (“ME” bits
23 – 56, Message bits 55 – 88) containing the Latitude and Longitude coding of the Aircraft's Surface
Position. The Latitude (Y) and Longitude (X) shall each occupy 17 bits. The Surface Latitude and Longitude
encoding shall contain Surface CPR-encoded values in accordance with §C.2.6. The unambiguous range
for local decoding of Surface Messages shall be 166.5 km (90 NM). The positional accuracy maintained by
the Surface CPR encoding shall be approximately 1.25 meters.
Notes.—
DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
1. The Latitude/Longitude encoding is also a function of the CPR format value (the “F” bit).
2. Although the positional accuracy of the surface CPR encoding is approximately 1.25 meters in
most cases, implementers should be aware that the longitude position accuracy may only be
approximately 3.0 meters when the latitude is either –87.0 ±1.0 degrees, or +87 ±1.0 degrees.
C.2.3.3.5.1 Extrapolating Position (When T=1)
This extrapolation shall conform to §C.2.3.2.3.1 (Substitute "surface" for "airborne" where appropriate).
C.2.3.3.5.2 Extrapolating Position (When T=0)
This extrapolation shall conform to §C.2.3.2.3.2 (Substitute "surface" for "airborne" where appropriate).
DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris