Technical Provisions for Mode S Services and Extended Squitter

DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
C-12 Technical Provisions for Mode S Services and Extended Squitter
The position data that is loaded into the Airborne Position register shall be an estimate of the A/V position at
the Time of Applicability.
Note 3.— The position may be estimated by extrapolating the position from the time of validity of the fix
(included in the position fix) to the Time of Applicability of the data in the register (which, if T=1, is an exact
0.2 UTC time tick). This may be done by a simple linear extrapolation using the velocity provided with the
position fix and the time difference between the position fix validity time and the Time of Applicability of the
transmitted data. Alternatively, other methods of estimating the position, such as alpha-beta trackers or
Kalman filters, may be used.
Every 200 ms, the contents of the position registers shall be updated by estimating the A/V position at the
next subsequent 0.2 second UTC epoch. This process shall continue with new position fixes as they
become available from the source of navigation data.
C.2.3.2.3.2 Extrapolating Position (When T=0)
“T” shall be set to ZERO (0) if the Time of Applicability of the data being loaded into the position register is
not synchronized to any particular UTC epoch. The position being transmitted must have a Time of
Applicability that is no greater than 100 milliseconds from the time of transmission. Additionally, the position
register shall be re-loaded with position data at intervals that are no more than 200 ms apart. This ensures
that the position contained in the position registers will have a Time of Applicability that is never more than
200 ms different from any time during which the register holds that data. If the transmitted position data is
loaded from the position register, the position register shall be updated such that the 100 millisecond
performance is achieved.
Note.— This may be accomplished by loading the Airborne Position register at intervals that are no
more than 200 ms apart, with data for which the Time of Applicability is between the time the register is
loaded and the time that it is loaded again. For example, loading the register at 200 millisecond intervals
would require that the time of applicability at register load time is exactly 100 milliseconds ahead of the
register load time. Greater flexibility in the time of applicability at register load time is provided by increasing
the update rate.
If “T” is ZERO (0), then ADS-B Receiving Subsystems shall accept Airborne Position Messages as being
current as of the Time of Receipt. Updating the position data as above ensures that the ADS-B Transmitting
Subsystem does not induce more than 100 milliseconds of timing error into the transmitted position data.
C.2.3.2.3.3 Time-Out When New Position Data is Unavailable
Draft
In the event that the navigation input ceases, the extrapolation described in §C.2.3.2.3.1 and §C.2.3.2.3.2
shall be limited to no more than two (2) seconds. At the end of this timeout of two seconds, all fields of the
Airborne Position register, except the altitude field, shall be cleared (set to ZERO).
Note.— The altitude field, bits 9 to 20 of the register, would only be cleared if current altitude data were
no longer available.
With the appropriate register fields cleared, the broadcast of the ZERO TYPE Code field shall serve to notify
ADS-B Receiving Subsystems that the data in the Latitude and Longitude fields are invalid.
C.2.3.2.4 Altitude
This 12-bit field (“ME” bits 9 – 20, Message bits 41 – 52) shall provide the aircraft altitude. Depending on the
TYPE Code, this field shall contain either:
1. Barometric altitude encoded in 25 or 100 foot increments (as indicated by the Q Bit) or,
2. GNSS height above ellipsoid (HAE).
Note.— GNSS altitude MSL is not accurate enough for use in the position report.
DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris