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epoch, I00 ms before that next subsequent 0.2 s epoch.
Note 1.- In this way, thc time of transmission of an uirborne position message would never differ by more than 100 m from
the time of upplicability ofthe data in that message. By specifving "100 ms + 50 ms" rather than 100 ms exactly, some tolerance
is allowed for variations in implemetztation.
Note 2.- The position may be estimated by extrapolating the positionfrom the time of validity of the@ (included in the position
fix) to the time of applicability of the data in the register (which, if T = I, is an exact 0.2 s UTC time tick). This m y he dotae by
a sirnple linear extrapolution using the velocity provided with the position@ and the time diference between the position fix validity
time and the time of applicability of the transmined data. Alternatively, other methods of estimating the position, such as alphabeta
trackers or Kalmanfilters, may be used.
Every 200 ms, the contents of the position registers shall be updated by estimating the AN position at the next subsequent 0.2 s
UTC epoch. This process shall continue with new position fixes as they become available from the source of navigation data.
2.3.2.3.2 Extrapolating position (when T = 0)
T shall be set to zero if the timc of applicability of the data being loaded into the position register is not synchronized to any
particular UTC epoch. In that case, the position register shall be reloaded with position data at intervals that are no more than
200 ms apart. The position being loaded into thc register shall have a time of applicability that is never more than 200 ms different
from any time during which the register holds that data.
Note.- This may be accomplished by loading the azrborne position register at intervals that are, on average, no more than
200 ms apart, with data for which the trme oj'applicability is between the time the register is loaded and the time that it zs loaded
again. (Shorter intervals than 200 nas are permitted, but not required.)
If T = 0, receiving ADS-B equipment shall accept airborne position messages as being current as of the time of reccipt. The
transmitting ADS-B equipmenr shall reload the airborne position register with updated estimates of the AN position, at intervals
that are no more than 200 ms apart. The process shall continuc with new position reporls as they become available.
2811 1102
No. 77
Annex 10 - Aeronautical Telecommunicatiuns Volume 111
2.3.2.3.3 Timeout when new position data are unavailable
In the event that the navigation input ceases, the extrapolation described in 2.3.2.3.1 and 2.3.2.3.2 shall be limited to no more than
two seconds. At the end of this tirneout of two seconds, all fields of the airborne position register, except the altitude field, shall
be cleared (set to zero). When the appropriate register fields are cleared, the zero type code field shall serve to notify ADS-B
receiving equipment that the data in the latitude and longitude fields are invalid.
2.3.2.4 ALTITUDE
This 12-bit field shall provide the aircraft altitude. Depending on the type code, this field shall contain either:
1) Barometric alritude encoded in 25 or 100 foot increments (as indicated by the Q bit) or,
2) GNSS height above ellipsoid ( H a ) .
"Baro-Altitude" shall be interpreted as barometric pressure altitude, relative to a standard pressure of 1013.25 hectopascals (29.92
in Hg). It shall not be interpretcd as baro corrected altitude.
Format type code 20 to 22 shall be reserved for the reporting of GNSS height (HAE) which represents the height above the surface
of the WGS-84 ellipsoid and may be used when bar0 altitude is not available.
Note.- GNSS altitude (MSL)is not accurate enough for use in the position report
2.3.2.5 SINGLE ANTENNA FLAG (SAF)
This I-bit field shall indicate the type of antenna system that is being used to transmit extended squitters. SAF = 1 shall signify a
single transmit antenna. SAF = 0 shall signify a dual transmit antenna system.
At any time that the diversity configuration cannot guarantee that both antenna channels are functional, then the single antenna
subfield shall be set to ONE.
The surveillance status field in the airborne position message format shall encode information from the aircraft's Mode A code and
SPI condition indication as specified in Annex 10, Volume IV, 3.1.2.8.6.3.1.1.
2.3.3 SURFACEPO SITION FORMAT
The surface position squitter shall be formatted as specified in the definition of register number 0616 in the following paragraphs.
This 7-bit field shall provide information on the ground speed of the aircraft. A non-linear scale shall be used as defined in the
following table where speeds are givcn in kmlh (kt).
Encoding Meaning Quantiration
0 no information available
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