ACARS (Aircraft Communications Addressing and Reporting System)

ACARS is one of the RF protocol used by planes, it was first deployed in 1978. ACARS is a 2400 bps packet-like system. It is a Telex format used for the transmission of short, simple messages between aircraft and ground. The protocol is also used to provide temperature data to weather satellites (e.g. NOAA).

The above is an ACARS AM signal in the time domain. The 16 byte preamble with all 128 bits set to one which generates a 2400 Hz tone for 0.05 seconds is clearly visible at the start of the packet. And there is almost enough detail to visually detect the  two individual tones, even though at 48KSPS there is only at most 20 samples per tone.

ACARS VHF Frequencies (MHz)	Location
129.125	USA and Canada Additional
130.025	USA and Canada Secondary
130.425	USA Additional
130.450	USA and Canada Additional
131.125	USA Additional
131.450	Japan Primary
131.450	NZ ARINC Channel
131.475	Air Canada Company Channel
131.525	European Secondary Channel
131.550	World Primary
131.725	Europe Primary
131.825	Lufthansa Company Channel
131.825	Air France Company Channel
131.850	New European Channel
136.700	USA Additional
136.750	USA Additional
136.750	New European Channel
136.800	USA Additional
136.850	SITA North America
136.900	European and USA Secondary
136.925	ARINC European Channel

Spectral occupancy: ~5kHz AM modulation

The data is transmitted as fast frequency shift keying (FFSK) using two tones, 1200Hz and 2400 Hz. The presence of 1200 Hz indicates a bit toggle from the previous state, the presence of 2400 Hz indicates that the bit is the same.

The code used is 7 bit ASCII 7 + 1 even parity bit.

(In even parity, the parity bit is 1 if the number of 1s in the binary string is odd, thereby making the number of 1s in the full binary string even. In odd parity, the parity bit is 1 if the number of 1s in the binary string is even, thereby making the number of 1s in the full binary string odd.)


So to transmit the letter Y (with an even parity bit) this would be 01011001

Using the two above information, the following would be the transmission of the letter Y using the above FFSK and coding.

	0	1	0	1	1	0	0	1
1200 Hz	-	on	on	on	-	on	-	on
2400 Hz	on	-	-	-	on	-	on	-

ACARS information is sent in (one or more) packet frames.

Each message frame consists of at least 50, and up to a maximum
of 272 characters or bytes. Each character uses a 7 bit ACSII code
with an additional eighth parity bit. This results in a total message
transmission duration of between 0.17 and 0.91 seconds.

Message Frame Format

	size(bytes)	Description
Preamble	16	All bits set to 1, to help AGC, and warm up TX/RX hardware 0xFF
Bit sync	2	Characters "+" and "*" to synchronise bits 0x2B, 0x2A
SYNC	2	Character "SYN" is sent to synchronise bytes 0x16, 0x16
Start of heading	1	Character "SOH" is sent 0x01
Mode	1	System interface selection (Ref: ARINC Characteristic 597-4)
Address	7	Aircraft registration/identification number
Acknowledgement	1	One Character is sent ACK (0x06) or NAK (0x15)
Label	2	Type of message (message routine and message type)
Block	1	Message block identifier number
STX	1	STX (0x02), or if no text ETX (0x03) and jump down to BCS
Flight Number	6	Airline flight number
Message Text	0<->220	Printable characters only
Suffix	1	ETX (0x03)or ETB (0x17)
BCS	2	Block Check Sequence, Polynomial code for the detection of errors
BCS Suffix	1	ETB (0x17) or DEL (0x7F)

A few ACARS EXAMPLES (with the Pre-key, BCS and ETB removed)
+*<SYN><SYN><SOH>2..GXONE<NAK>_<DEL>7<STX>M56AES0001<ETX>
+*<SYN><SYN><SOH>X.5Y-KQX<NAK>Q06<STX>S57AKQ01-*<ETX>

+*<SYN><SYN><SOH>2.VH-OGE<NAK> 80<STX>4026QF0039/MEL.<CR><LF>MVA<CR><LF>QF39/13.VHOGE.MEL<CR><LF>AD0829/0840 EA1129 AKL<ETX>
+*<SYN><SYN><SOH>2.VH-OGE<NAK> H1<STX>D029QF0039#1BBTKO<CR><LF>0840 13JAN04 MEL AKL P207 P165 00480 276 145727 04<CR><LF>L 1022 1035 872 8975 1197 61 095 132 05H 2000<CR><LF>R 1025 1043 858 8964 1199 68 098 127 09B 200<CR><LF>1078 1023 096 032P430<CR><LF>1079 1025 093<ETX>
+*<SYN><SYN><SOH>2.VH-OGE<NAK> 86<STX>5208QF0039/AKL.<CR><LF>ARI<CR><LF>AKL ETAB 11:40<CR><LF>WH/CH 01 UN/MNR 02<CR><LF>GIDAY<ETX>
+*<SYN><SYN><SOH>2.VH-OGE<NAK> H1<STX>D063QF0034#1FBFCP<CR><LF>2107 14JAN04 AKL MEL P010 M272 24972 759 129461 09<CR><LF>R 0235<CR><LF>NO 087 098 011 000 000 000 000 000 <CR><LF>PC 085 013 000 000 000 000 000 000 <ETX>
+*<SYN><SYN><SOH>2.VH-OGE<ACK> _ <STX>1243QF0033<ETX>

This information can be better presented by trimming out the fat and displaying something like this:
ACARS Mode: 2
Identification: ..GXONE
Acknowledgement: NAK
Message Routine: 7B
Block number: 0x37
Message: One of the toilets in coach is flooded.

A few example ACARS Modes (codes vary from country to country)

Country	Code	Ground Station
Worldwide	2	to free Groundstations
UK	[	East Midlands
UK	R	Manchester
Ireland	D	Dublin
Ireland	O	Cork
Ireland	S	Shannon
Belgium	R	Brussels
Canada	R	Ottawa
Australia	A	Adelaide

ACARS Message Types

00	Emergency Situation Report
51	Ground GMT report uplink
52	Ground UTC report uplink
54	Voice Go-ahead
57	Alternative Aircrew Initiated position
5D	ATIS Report
5P	Temporary Suspension
5R	Aircrew Initiated position report
5U	Weather Request
5Y	Aircrew Revision to previous
5Z	Airline Designated
7A	Aircrew Initiated Engine Data/Takeoff
7B	Aircrew Entered Miscellaneous Message
80-89	Airline Defined Message
A0	ATS Facility Notification
A1	Oceanic Clearance
A2	Unassigned
A3	Departure Clearance
A4	Flight Systems Message
A5	Unassigned
A6	Request ADS Reports
A7	Free text from ATC
A8	Deliver Departure Slot
A9	ATIS Report
AA	ATC Communications
AB	Terminal Weather Information for Pilots
AC	Pushback Clearance uplink
AD	Expected Taxi Clearance
AE	Unassigned
AF	CPC Command Response
B0	ATS Facility Notification
B1	Request Oceanic Clearance
B2	Oceanic Clearance Readback
B3	Request Departure Clearance
B4	Departure Clearance Readback
B5	Waypoint position report
B6	Provide ADS Report
B7	Free text to ATC
B8	Request Departure slot
B9	Request ATIS Report
BA	ATC Communication
BB	Terminal Weather Information for Pilots
BC	Pushback Clearance Request
BD	Expected Taxi Clearance Request
BE	CPC Log-on/Log-off Request
BF	CPC WILCO
BG	Unassigned
C0-C9	Cockpit Printer Message
CA	Cockpit Printer Message
CB	Printer Busy
CC	Printer in local or test mode
CD	Printer out of paper
CE	Printer buffer Overrun
CF	Printer Initialization before completion
EI	Internet Email Message
F3	Dedicated transceiver advisory
H1	General Message
H2	Meteorological Report
H3	Icing Report
HF	HFDL Message
HX	Undelivered Uplink report
M2	User Defined Message
Q0	Link Test
Q1	Departure Report
Q2	Estimated Time of Arrival Report
Q3	Clock Update Advisory
Q4	Voice Circuit Busy (Not Supported)
Q5	Unable to deliver uplink message
Q6	Voice to Data Changeover advisory
Q7	Delay Message
QA	OUT Fuel Report
QB	OFF Report
QC	ON Report
QD	IN / Fuel Report
QE	OUT / Fuel/Destination Report
QF	OFF Destination Report
QG	OUT / Return IN Report
QH	OUT Report
QK	Landing Report
QL	Arrival Report
QM	Arrival Information Report
QN	Diversion Report
QP	OUT Report
QQ	OFF Report
QR	ON Report
QS	IN Report
QT	OUT / Return IN Report
QX	Intercept / Unable to Process
RA	Command/Response
RB	Command/Response
REG	Registration
S1	VHF Satellite Report
S2	VHF Performance Report
S3	LRU Configuration Report
SA	Media Advisory
SQ	Squitter Report
UP	Message Acknowledgement
:;	Aircraft Transceiver Auto-Tune
_j	Reserved
_<DEL>	General Response (Demand Mode)

7-bit ASCII (American Standard Code for Information Interchange)

(BIN)	(DEC)	(HEX)	(OCT)	Symbol (SYM)	Description
0000000	000	00	000	NUL	NULL
0000001	001	01	001	SOH	Start of Header
0000010	002	02	002	STX	Start of Text
0000011	003	03	003	EXT	End of Text
0000100	004	04	004	EOT	End of Transmission
0000101	005	05	005	ENQ	Enquiry
0000110	006	06	006	ACK	Acknowledge
0000111	007	07	007	BEL	Bell
0001000	008	08	010	BS	Backspace
0001001	009	09	011	HT (TAB)	Horizontal Tab
0001010	010	0A	012	NL (LF)	New Line (Line Feed)
0001011	011	0B	013	VT	Vertical Tab
0001100	012	0C	014	NP (FF)	New Page (Form Feed)
0001101	013	0D	015	CR	Carriage Return
0001110	014	0E	016	SO	Shift Out
0001111	015	0F	017	SI	Shift In
0010000	016	10	020	DLE	Data Link Escape
0010001	017	11	021	DC1	Device Control 1 (XON)
0010010	018	12	022	DC2	Device Control 2
0010011	019	13	023	DC3	Device Control 3 (XOFF)
0010100	020	14	024	DC4	Device Control 4
0010101	021	15	025	NAK	Negative Acknowledge
0010110	022	16	026	SYN	Synchronous Idle
0010111	023	17	027	ETB	End of Transmission Block
0011000	024	18	030	CAN	Cancel
0011001	025	19	031	EM	End of Medium
0011010	026	1A	032	SUB	Substitute
0011011	027	1B	033	ESC	Escape
0011100	028	1C	034	FS	File Separator
0011101	029	1D	035	GS	Group Separator
0011110	030	1E	036	RS	Record Separator
0011111	031	1F	037	US	Unit Separator
0100000	032	20	040	SP	Space
0100001	033	21	041	!	Exclamation mark
0100010	034	22	042	"	Quotation mark
0100011	035	23	043	#	Number sign
0100100	036	24	044	$	Dollar sign
0100101	037	25	045	%	Percentage sign
0100110	038	26	046	&	Ampersand
0100111	039	27	047	'	Apostrophe
0101000	040	28	050	(	Opening parenthesis
0101001	041	29	051	)	Closing parenthesis
0101010	042	2A	052	*	Asterisk
0101011	043	2B	053	+	Plus sign
0101100	044	2C	054	,	comma
0101101	045	2D	055	-	Hyphen (minus)
0101110	046	2E	056	.	Full Stop
0101111	047	2F	057	/	Slash
0110000	048	30	060	0	Zero
0110001	049	31	061	1	One
0110010	050	32	062	2	Two
0110011	051	33	063	3	Three
0110100	052	34	064	4	Four
0110101	053	35	065	5	Five
0110110	054	36	066	6	Six
0110111	055	37	067	7	Seven
0111000	056	38	070	8	Eight
0111001	057	39	071	9	Nine
0111010	058	3A	072	:	Colon
0111011	059	3B	073	;	Semicolon
0111100	060	3C	074	<	Less than sign
0111101	061	3D	075	=	Equal sign
0111110	062	3E	076	>	Greater than sign
0111111	063	3F	077	?	Question Mark
1000000	064	40	100	@	At sign
1000001	065	41	101	A
1000010	066	42	102	B
1000011	067	43	103	C
1000100	068	44	104	D
1000101	069	45	105	E
1000110	070	46	106	F
1000111	071	47	107	G
1001000	072	48	110	H
1001001	073	49	111	I
1001010	074	4A	112	J
1001011	075	4B	113	K
1001100	076	4C	114	L
1001101	077	4D	115	M
1001110	078	4E	116	N
1001111	079	4F	117	O
1010000	080	50	120	P
1010001	081	51	121	Q
1010010	082	52	122	R
1010011	083	53	123	S
1010100	084	54	124	T
1010101	085	55	125	U
1010110	086	56	126	V
1010111	087	57	127	W
1011000	088	58	130	X
1011001	089	59	131	Y
1011010	090	5A	132	Z
1011011	091	5B	133	[	Open square bracket
1011100	092	5C	134	\	Backslash
1011101	093	5D	135	]	Close square bracket
1011110	094	5E	136	^	Caret
1011111	095	5F	137	_	Underscore
1100000	096	60	140	`	Grave accent
1100001	097	61	141	a
1100010	098	62	142	b
1100011	099	63	143	c
1100100	100	64	144	d
1100101	101	65	145	e
1100110	102	66	146	f
1100111	103	67	147	g
1101000	104	68	150	h
1101001	105	69	151	i
1101010	106	6A	152	j
1101011	107	6B	153	k
1101100	108	6C	154	l
1101101	109	6D	155	m
1101110	110	6E	156	n
1101111	111	6F	157	o
1110000	112	70	160	p
1110001	113	71	161	q
1110010	114	72	162	r
1110011	115	73	163	s
1110100	116	74	164	t
1110101	117	75	165	u
1110110	118	76	166	v
1110111	119	77	167	w
1111000	120	78	170	x
1111001	121	79	171	y
1111010	122	7A	172	z
1111011	123	7B	173	{	Open Curly Bracket
1111100	124	7C	174	|	Vertical Bar
1111101	125	7D	175	}	Close Curly Bracket
1111110	126	7E	176	~	Tilde
1111111	127	7F	177	DEL	Delete Character

Ok some texe goes here

(ref: http://www.pervisell.com/ham/raftmode.htm#I76 )
(ref:  http://www.tapr.org/aprsdoc/ACARS.TXT )
(ref: https://github.com/antoinet/gr-acars2 )
(ref: http://www.acarsd.org/docu.html )
(ref: https://books.google.com/books?id=sS3pAQAAQBAJ )
(ref: http://www.sigidwiki.com/wiki/ACARS )
(ref: https://en.wikipedia.org/wiki/Aircraft_Communications_Addressing_and_Reporting_System )