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

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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. 70

TWDL = Two-Way Data Link CPDLC = Controller Pilot Data Link Comm. TWDL (CPDLC) ADS AOC ADS = Automatic Dependent Surveillance ACF = ACARS Convergence Function ACF AFN = ATS Facilities Notification AFN ACARS SATCOM HF VHF Figure 5.4 FANS-1 Communication The AOC functionality is generally the same as described above. It includes the normal collection of airline-defined functions and messages. The FANS-1 installation also includes direct interface to the FMC to support uplink and downlink of a large number of FMC-hosted parameters, including flight plans. Although this functionality was previously available, this is the first time it has been installed in an entire fleet of airplanes. ADS and TWDL were both intended to be used over the ATN, so they were designed as bit-oriented applications. Since ACARS can only accept character-oriented messages, an ACARS Convergence Function has been specified to convert bit-oriented messaged to character-oriented format for transmission and to convert received messages back to bitoriented format. This is done by taking each nibble (four bits) of the bit string and expressing it as a hexadecimal character (0...9, A...F). A 16-bit cyclic redundancy check is calculated on the original bit string and the four characters representing the result are appended to the character string calculated for the message. The reverse procedure is performed at the receiving end. An additional function was required to send and receive messages, in ACARS character format, to find the necessary addresses for communicating in the FANS-1 environment and to communicate function availability at the two ends (e.g., an ATS ground station that can send and receive TWDL but not ADS). This function is called the ATS Facilities Notification (AFN) function. The FANS-1 functionality was first demonstrated in the South Pacific, for flights between the Los Angeles and San Francisco to Sydney, Australia and Auckland, New Zealand. Air traffic control uses TWDL replaces HF voice communication for regular pilot/controller dialog. Position reports by HF voice (about every 40 minutes) have been replaced by periodic (about every 15 minutes) ADS reports or by position reports using TWDL for 71

TWDL = Two-Way<br />

Data Link<br />

CPDLC = Controller<br />

Pilot Data Link Comm.<br />

TWDL<br />

(CPDLC)<br />

ADS<br />

AOC<br />

ADS = Automatic<br />

Dependent Surveillance<br />

ACF = ACARS<br />

Convergence Function<br />

ACF<br />

AFN = ATS Facilities<br />

Notification<br />

AFN<br />

ACARS<br />

SATCOM HF VHF<br />

Figure 5.4 FANS-1 Communication<br />

<strong>The</strong> AOC functionality is generally the same as described above. It includes the normal<br />

collection of airline-defined functions and messages. <strong>The</strong> FANS-1 installation also<br />

includes direct interface to the FMC to support uplink and downlink of a large number of<br />

FMC-hosted parameters, including flight plans. Although this functionality was previously<br />

available, this is the first time it has been installed in an entire fleet of airplanes.<br />

ADS and TWDL were both intended to be used over the ATN, so they were designed as<br />

bit-oriented applications. Since ACARS can only accept character-oriented messages, an<br />

ACARS Convergence Function has been specified to convert bit-oriented messaged to<br />

character-oriented format for transmission and to convert received messages back to bitoriented<br />

format. This is done by taking each nibble (four bits) of the bit string and<br />

expressing it as a hexadecimal character (0...9, A...F). A 16-bit cyclic redundancy check is<br />

calculated on the original bit string and the four characters representing the result are<br />

appended to the character string calculated for the message. <strong>The</strong> reverse procedure is<br />

performed at the receiving end.<br />

An additional function was required to send and receive messages, in ACARS character<br />

format, to find the necessary addresses for communicating in the FANS-1 environment<br />

and to communicate function availability at the two ends (e.g., an ATS ground station that<br />

can send and receive TWDL but not ADS). This function is called the ATS Facilities<br />

Notification (AFN) function.<br />

<strong>The</strong> FANS-1 functionality was first demonstrated in the South Pacific, for flights between<br />

the Los Angeles and San Francisco to Sydney, Australia and Auckland, New Zealand. <strong>Air</strong><br />

traffic control uses TWDL replaces HF voice communication for regular pilot/controller<br />

dialog. Position reports by HF voice (about every 40 minutes) have been replaced by<br />

periodic (about every 15 minutes) ADS reports or by position reports using TWDL for<br />

71

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