ACARS VHF
ACARS Categoria A e B / SITA in conformità ARINC 597
ACARS è l'acronimo di Aircraft Communications
Addressing and Reporting System. Si tratta di un sistema di trasmissione
dati a 2400 Baud sviluppato da diverso tempo, ma solo in questi ultimi anni
si è diffuso in larga scala tra le varie compagnie aeree. L'introduzione
di questo sistema consentirà di ridurre fino a far scomparire le comunicazioni
in voce tra il controllo del traffico aereo e i piloti aumentando così la
sicurezza e la velocità delle comunicazioni, che saranno mantenute esclusivamente
tra il computer dell'aereo e il computer del centro di controllo. I messaggi
inviati e ricevuti tra aereo e terra spaziano dalle informazioni meteorologiche,
alla posizione del velivolo, dall'altitudine, ai livelli di carburante e
pressione dell'olio, alle richieste dei passeggeri, e tanto altro ancora. La frequenza primaria in Europa è 131.725
MHz.
ACARS Category A&B (SITA) (according to ARINC 597-5)
2400 bps PSK packet-like system used by Civilian Aircraft for onboard flight-deck
computer interconnections into ground stations.
Struttura del sistema ACARS
| 128 | 16 | 16 | 8 | 8 | 56 | 8 | 16 | 8 | 8 | 32 | 48 | 8 | 128 | 8 | |
| pre-key | bit sync | char sync | SOH | mode | address | ack/nak | label | block ID | STX | Seq No. | Flight No. | Text | ETX | CRC | BCS |
| pre-key | A maximum of 128 bits is transmitted to allow for the transmitter to key. If the transmitter keys up and settles on frequency quickly, it is possible to see most of the 128 bits, if the transmitter is slow then much less than 128 bits is seen. The received signals therefore does not always contain all 128 bits during this pre-key sequence. The sequence is also designed to compensate for variable response times of receiver squelches and AGC settling times. At 2400bps, 128 bits equates to about a 20th of a second (53.333mS to be precise). |
| bit sync | 16 bits of alternate bits to allow the decoder to lock into the bit speed. |
| char sync | 2 bytes with a unique bit pattern allows the decoder to precisely character syncronise to the following 8 bit bytes. |
| SOH | Start of header marker byte. |
| mode | Type of transmission. (AGCS-1, AGCS-D etc). |
| Address | Aircraft Registration number. Filled with a special sequence if a squitter burst. |
| ack/nak | Acknowledged/Not acknowledged. Advises other station if a re-send is necessary. |
| label | Label identifies type of message being sent. Usually a letter and a number. See labels table. |
| block ID | Single byte which increments on each successful transmission. |
| STX | If STX present, then text message follows. |
| Seq. No. | Message sequence number. Usually made from the current time (minutes/seconds). Only transmitted in downlink data blocks. |
| Flight No. | Aircraft carriers international flight number. Only transmitted in downlink data blocks. |
| Text | The text message itself. Just as with AX25 packet, the actual length is variable but limited to a maximum of 220 characters. |
| ETX | End of text marker. If ETB is transmitted instead then more text follows in a subsequent data packet. |
| CRC | Checksum. |
| BCS | End of packet block marker byte. |
Ulteriori informazioni sul sistema ACARS le potrete trovare su questi siti,
