Air Traffic Management Concept Baseline Definition - The Boeing ...

The 2.4 Kbps and 31.5 Kbps described above are the raw bit rates in the signal-in-space.
The available user bit rate is decreased by message overhead and is inversely proportional
to the number of stations sharing a VHF channel. A single channel (frequency) is used
across most of the U.S. and up to three additional channels may be available in high
density airport areas. Therefore, if 14 aircraft and two ground stations are within line-ofsight
of each other on one frequency, the average long term bit rate for each will be 2400 /
16 = 150 bps.
The ACARS specification has been expanded to provide SATCOM and HF media
connections. The VHF-unique protocol is stripped off and the remaining characters are
encapsulated in a SATCOM or HF protocol data unit for transmission. The ACARS MU,
the SATCOM data unit, and the HF data unit or radio are connected together with digital
data busses.
The raw bit rate of 10.5 Kbps has been mentioned for SATCOM. Although this bit rate is
available for providing a dedicated circuit for SATCOM voice, as described earlier, data
link protocols depend a packet service, which is multiplexed among multiple users. A bit
rate of 300 bps is a more reasonable value.
HF data radio has automatically-selected bit rates of 300, 600, 1200, and 1800 bps. The
bit rate is chosen based on the channel real time propagation characteristics, such as noise
and fading. Experience has shown that the bit rate is normally 600 bps. An estimated ten
aircraft can share a channel, providing an average bit rate of 60 bps per aircraft.
5.2.1.3 FANS-1
The set of data link services provided in a FANS-1 airplane is shown in Figure 5.4. The
ACARS protocol and the air/ground media are identical to those described above.
Communication between the controller and the pilot is provided by Two-Way Data Link
(TWDL), as described in RTCA Document DO-219 (RTCA,1993). This application has
been commonly called Controller-Pilot Data Link Communications (CPDLC) which, as
described below, is the formal name for the ATN application which provides the
equivalent functionality.
Position and intent reporting is provided by the Automatic Dependent Surveillance
function. Although there is an equivalent RTCA document, the specification produced by
AEEC, ARINC 745, was used for this implementation (ARINC, 1993). The ground
system requests a ‘contract’ with the aircraft, specifying the reporting period for basic and
supplemental data to be transmitted. The contract can also specify a set of events, such as
altitude deviation, which will also cause a report to be transmitted.
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