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Introduction
What is ACARS? It’s SMS for Civil Aviation to put it simply.
It is a digital data link for short messages between ground stations and aircraft. It was first developed by ARINC and was originally called ARINC Communication Addressing and Reporting System (ACARS). Nowadays, the ARINC portion of the name has been replaced and now is called Aircraft Communication Addressing and Reporting System (ACARS).
ACARS is used by both the Airline companies and ATC to talk to active civil aircraft to communicate important information like load sheets, weather data, flight plan clearance, wheels off time and the like. This reduces reliance on extremely long voice messages over the radio. Aircraft performance data are also sent over ACARS for proactive maintenance and health monitoring. Quite interesting when you think of it really.
Well, let’s not get too deep in to what ACARS is as I’m sure you already know what it is.
Let’s focus on how to receive and decode ACARS. Quite obviously, you’ll need some way to receive the radio signals before they can be decoded. Here, we shall use the ubiquitous RTL-SDR dongle.
The RTL-SDR dongle is also available as a kit which includes an antenna capable of receiving ACARS packets. If you are starting out, I highly recommend buying it as a kit. That’s the easiest way to get into the hobby.
The Setup
To receive ACARS packets, we need to choose the antenna configuration for optimum reception of ACARS packets.
To determine the antenna configuration, we first need to know the frequency at which ACARS packets are usually sent. As per this website, following are the commonly used ACARS frequencies in MHz.
129.125 129.350
129.525 130.025
130.450 130.825
131.125 131.425
131.550 131.725
131.850 136.575
136.650 136.700
136.750 136.800
136.850 136.975
Now that we know the frequency range, we need to determine the size of the dipole antenna that we will be requiring. For that we shall use the following formula.
0.95 x 3 x 10^8
Length = _______________ metres
f x 10^6
The commonly used frequencies are around 130 MHz. Putting 130 in the above formula gives us about ~ 1 metre length of the dipole antenna required for receiving ACARS packets. That means you need to use the longer pair of antenna that comes with the kit to their full extension.
You need to keep in mind that ACARS packets are sent vertically polarised. This means that you have to place your dipole antenna vertically like below. Just ensure that the overall length of the antenna is about 1 metre. As you can see, I’ve just used the flexible tri-pod that comes with the kit to attach the antenna to a chair by the window.
Determining the Frequencies
You will need a way to first determine the frequency at which ACARS are being transmitted in your area. We can use a program called gqrx which is a popular SDR software, if you are on Linux.
Install it with pacman and run it. Once open, you need to focus the spectrum somewhere close to 130 MHz. Wait for a few minutes until you see activity like the below screenshot. You can see short bursts of activity at 131.725 and 131.825 MHz. This is how you can determine the active frequencies for ACARS in your local area.
Decoding ACARS
Decoding ACARS in Linux is quite easy with this software called acarsdec which is available on the AUR.
Simply use the following command to start decoding ACARS packets. Here, 131.725 and 131.825 are the frequency channels that I found that were active in my area. You can give up to 16 different channels for the software to decode simultaneously. The -r 0 indicates the ID number of the RTL SDR device to be used. If you have only one connected to the computer, then it is usually 0.
acarsdec -r 0 131.725 131.825Example ACARS Packets
The following are some ACARS Packets that I was able to decode.
Weather Data
Aircraft reg: xx-xxx
QUDELOZ6E~1WX UPLINK~
- WX RESPONSE
VOGO 301230Z 28016KT 6000 FEW010 SCT020 29/24 Q1005 NOSIG=
VOGA 301330Z 28010KT 3000 BR SCT012 BKN080 29/26 Q1006 NOSIG=
END
ATIS
Aircraft reg: xx-xxx
/BOMCDYA.TI2/VABB DEP ATIS X
1400Z ATIS VABB 301403
VABB ATIS X
1400ZRWY27
TRL FL80
/ EXPECT ILS APPROACH.
WIND 280/10KT MAX20KT
VIS 2500M
FBL RA
CLD FEW 1200 SCT 1800 FEW 2500 TCU
T28 DP24
QNH 1003HPA
TEMPO
ETB
Aircraft reg: xx-xxx
1500 SHRA
RWY 27 CONDITION REPORT. AT TIME 1345.
RUNWAY CONDITION CODE, 5, 5, 5, FIRST PART
HUNDRED PERCENT WET, SECOND PART HUINDRED
PERCENT WET, THIRD PART HUNDRED PERCENT WET.
DEPARTURES PASSING 800 FT CONTACT
ETB
ATC
Aircraft reg: xx-xxx Flight id: 6E124P
No: M38A
/BOMCFYA.DC1/CDA
1407 250630 VABB
PDC 322
IGO124P CLRD TO VEBN
OFF 27 VIA ANOLI2A
KAKPO TRANSITION
SQUAWK 0354 ADT 1355
NEXT FREQ 121.850
ATIS V
CRUISE FLT LEVEL 080
REPORT POB THROUGH
SECURIT
ETB
Aircraft reg: xx-xxx
/BOMCFYA.FS1/FSM
1439 250630 VABB
IGO924W RCD RECEIVED
REQUEST BEING
PROCESSED
STANDBY
Aircraft reg: xx-xxx
/BOMTFYA.FS1/FSM 1440 250630 VABB
IGO924W RCD REJECTED
FLIGHT PLAN NOT HELD
REVERT TO VOICE
PROCEDURES
AIRCRAFT TYPE MISMATCH B86E
Airline Communication
Aircraft reg: xx-xxx
MSG FROM ASS SEQ00094
AIRCRAFT INIT MESSAGE
LX0155/30JUN25 HB-JHE BOMZRH
LAST FLIGHTLEG LX0154/30JUN25 ZRHBOM
NEXT FLIGHTLEG LX0154/01JUL25 ZRHBOM
AIRCRAFT SECURITY SEARCH AT BOM REQUIRED
AIRCRAFT SECURITY SEARCH A
ETB
Aircraft reg: x-xxx
LOADSHEET PRELIM 01
BOM HKG CX0044/30 B-LIA
F0Y0J0 4/0/0 30JUN25
ZFW ACT 236823 MAX 288031
TO FUEL 73500
TOW ACT 310323 MAX 412769
TRIP FUEL
ETB
Aircraft reg: x-xxx
55500
LAW ACT 254823 MAX 302092 L
BALANCE AND SEATING
BW 160393 DOW 160944
BI 720.00 DOI 702.10
LIZFW 680.24 MACZFW 26.33
LITOW 757.14 MACTOW 26.89
LIL
ETB
Aircraft reg: x-xxx
.11P/CAO.11P/RMD.11P/ROX.12P/RMD.32P/TDP.
41P/CAO.41P/RCM.41P/RFL.42P/CAO.42P/RCM.42P/RFL.42P/RRY
TTL PAX 000 UNDERLOAD 47269
***** NOTOC REQUIRED: YES *****
CMDR NAME WHITE B
SI
PANTRY CODE: 7ERF-A
P
ETB
Aircraft reg: x-xxx
ANTRY EFFECT: 143 / -5
NORMAL MACTOW LIMITS:
FWD MACTOW LIMIT 16.57
AFT MACTOW LIMIT 31.41
Aircraft reg: xx-xxx Flight id: AI0675
No: M30A
VABBVECC1637/FI AI0675/OT 1612/FB 121
##########################
Destination Airport : VECC
Departure Airport : VABB
Wheels off Tme : 1637
Maintenance Data
Aircraft reg: xx-xxx Flight id: EK007Y
No: D18D
#DFB..,..:..:.. ..,..:..:.. ..,..:..:..
SEC GRD PWR NOT AVAIL:30,16:38:29 ..,..:..:.. ..,..:..:.. ..,..:..:.. ..,..:..:..
PRI GRD PWR ON :30,15:32:34 ..,..:..:.. ..,..:..:.. ..,..:..:.. ..,..:..:..
P
ETB
Aircraft reg: xx-xxx Flight id: EK007Y
No: D18E
#DFBRI GRD PWR OFF :30,16:37:06 ..,..:..:.. ..,..:..:.. ..,..:..:.. ..,..:..:..
SEC GRD PWR ON :30,15:32:46 ..,..:..:.. ..,..:..:.. ..,..:..:.. ..,..:..:..
SEC GRD PWR OFF :30,16:34:31 ..,..:.
ETB
Aircraft reg: xx-xxx Flight id: EK007Y
No: D18G
#DFB..
RIGHT ENGINE ON :..,..:..:.. ..,..:..:.. ..,..:..:.. ..,..:..:.. ..,..:..:..
RIGHT ENGINE OFF :30,15:26:40 ..,..:..:.. ..,..:..:.. ..,..:..:.. ..,..:..:..
PARKING BRAKE ON :30,15:26:30
ETB
Aircraft reg: xx-xxx Flight id: EK007Y
No: D18H
#DFB30,17:06:11 30,17:15:19 30,17:17:10 30,17:20:24
PARKING BRAKE OFF :30,17:02:04 30,17:09:27 30,17:15:49 30,17:19:08 30,17:22:03
APU RUNNING :30,15:26:21 30,16:34:30 ..,..:..:.. ..,..:..:.. ..
ETB
Aircraft reg: xx-xxx Flight id: EK007Y
No: D18I
#DFB,..:..:..
APU NOT RUNNING :30,15:35:13 30,17:07:14 ..,..:..:.. ..,..:..:.. ..,..:..:..
APU BLEED VLV ON :30,15:25:29 30,15:26:41 30,15:42:45 30,16:33:29 30,17:05:54
APU BLEED VLV OFF :30,15:2
ETB
Aircraft reg: xx-xxx Flight id: EK007Y
No: D18J
#DFB6:10 30,15:42:44 30,16:33:28 30,17:05:51 30,17:07:02
REFUELING MODE ON :..,..:..:.. ..,..:..:.. ..,..:..:.. ..,..:..:.. ..,..:..:..
REFUELING MODE OFF :..,..:..:.. ..,..:..:.. ..,..:..:.. ..,..:..:
ETB
Aircraft reg: x-xxx Flight id: CX0044
No: C27A
#CFB/.
A RPT33 PG1 L-FUEL REAL
B .B-LIA 30JUN25 1726 CPA044 VABB/VHHH 685-2270-012 PW-508 TA
1 46.7 0.792 73.9 +23
2 13.2 19.8 20.2 13.3
3 0.794 0.793 0.794 0.792
4 68.09
ETB
Aircraft reg: x-xxx Flight id: CX0044
No: C27B
#CFB 86.11 88.31 69.97
5 76.09 66.19 66.11 77.89
6 76.82 89.22 91.63 76.87
7 93.23 120.06 121.07 93.27
8 98.29 88.05 89.98 98.26
9 74.86 204.86
ETB
Aircraft reg: x-xxx Flight id: CX0044
No: C27C
#CFB 204.71 76.13
10 95.49 161.87 162.41 96.06
11 85.22 124.38 125.73 85.26
12 97.08 191.32 191.16 97.10
13 96.31 226.89 226.38 96.32
14 100.35 250.92 250.5
ETB
Aircraft reg: x-xxx Flight id: CX0044
No: C27D
#CFB6 100.24
15 120.27 120.07
16 110.63 110.71
17 109.33 109.78
18 106.85 106.34 106.34 106.80
19 13408 10664 10629 13387
20 16969 47933 47918 16969
21 0.0 3.8 3.7
22
ETB
Aircraft reg: x-xxx Flight id: CX0044
No: C27E
#CFB 0.806 0.786 0.786
23 128.13 60.51 59.30
24 128.02 74.71 73.12
25 169.46 67.09 65.80
26 169.48 76.51 76.50
27 145.55 52.40 51.89
28 55.08 54.
ETB
