If you've been following along for a while, you may have noticed that I have some various strange combinations of interests, of which one seem to involve things that deal with radios and things that fly.
On that subject, I've made posts about listening in on things like weather balloons, gliders and commercial aircraft. It's pretty amazing what sorts of rabbit holes you could go down when you are talking about all the various ways that stuff in the air communicates with stuff on the ground.
Over Christmas, I came across a couple of SDR dongles in a desk drawer while doing a bit of a reorganization.
Those who don't know, SDR (or Software Defined Radio) are neat little devices that are small radio receivers that can be plugged into the USB port of a computer. While by itself the SDR is just a small brick that doesn't do a lot by itself, but when plugged into a computer that's loaded up with the appropriate software that controls the SDR, you can literally receive any radio signal that's out there.
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| SDR Dongle |
So, having found my lost stash of SDR dongles, I started to think of what sort of project I could do - after all there's no sense in just keeping them in the desk drawer.
I wanted to do something that was a bit unique as a project, something that would take more advantage of the processing power of the computer to decipher what was coming down the wire. So obviously I wanted to focus on something was a bit obscure.
After some snooping around the web, I came across this image
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| Flight Management Computer |
If you ever seen any photos of the flight deck of any commercial airliner, you may see this little screen sitting between the pilots. This is the Flight Management Computer and is typically used as a navigation database. This database is used to store route information which the autopilot in turn will use to fly the aircraft to the proper destination. It also reads in data from the aircraft's systems to give a gross weight and determine the best speeds for climb, cruise, descent, holding, approach, etc. It will also compute the aircrafts position based upon inputs from the IRSs, GPS and radio position updating. On top of all that, this little box also monitors the aircrafts systems to keep any eye out for any potential issues.
So there is a lot going on with this little gizmo. However, the most important thing, at least from my perspective, is that the FMC also transmits data about what's going on with the airplane down to ground stations through either VHF or HF radio frequencies using the ACARS transmission standard
ACARS, which stands for Aircraft Communications Addressing and Reporting System, is a digital data link standard that allows aircraft to communicate with ground stations, enabling the exchange of information for operational efficiency and flight safety.
The type of stuff that is communicated through ACARS include:
- Air Traffic Control (ATC) requests for clearances and instructions from air traffic controllers.
- Communication between an aircraft and its base for operational matters.
- Communication between an aircraft and its base for administrative purposes.
- Weather reporting
- Transmit aircraft health and maintenance information.
As you can see, there is a lot of stuff that could go into an ACARS transmission. Since I am relatively close to a number of large airports, I figure that I should be able to listen in some action.
But first I needed to understand what exactly was being transmitted.
From what I can gather, ACARS transmit in a standardized message format that typically, among some other details, contains the following information in the message:
- Date and Time
- Label - which indicates the nature of the message
- Aircraft Registration Number
- Flight Number
- Message details
So for example, a typical ACARS message could look something like this:
[#4 (F:130.025 L:-34.5 E:1) 18/02/2025 00:03:54.852 --------------------------------
Mode : 2 Label : H1 Id : 0 Nak
Aircraft reg: N27722 Flight id: UA0630
No: D26K
#DFB9 94.1316CLMBAIR -26.3 -9.50.584 94.59 94.0621CLMBAIR -26.8 -9.
80.584 94.59 94.0926CLMBAIR -27.0 -10.30.585 94.59 94.1331CLMBAIR -27
.5 -10.50.590 94.59 94.0936CLMBAIR -28.0 -10.50.597 94.59 94.13
Obviously it's rather cryptic, but sometimes you do see some stuff in plain text, usually of a pretty mundane nature:
[#3 (F:131.725 L:-19.1 E:0) 17/02/2025 01:58:21.571 --------------------------------
Mode : 2 Label : 84 Id : 1 Nak
Aircraft reg: C-GDMP Flight id: WS1224
No: M24A
HI
ITS LOOKING CLOSE FOR MI
N REST FOR MY DH TMW.
ANY CHANCE OF CHANGING T
O SHERATON TO IMPROVE MY
TURN AROUND TIME.
The key to figuring out the meaning of the messages can be found in the Label value. It took a bit of digging around but I did manage to source out a list of what those labels meant - which once you know what the labels meant, you are able to roughly decipher the contents of the message.
Since I needed to spend a fair amount of time doing the research on what those labels actually meant, I figured I would be helpful and provide the definitions that I uncovered for the ACARS Message labels for anyone that's interested (a bit of a warning, it's a pretty long list!):
Label Message Type
00 Emergency SitRep, Hijack Situation Report
10 British Airways ETA Report, American Airlines Freq Change
11 Inrange Arrival Report, Fedex In Range Arrival Report
13 CMU loadsheet uplink
14 General Aviation Free Text
15 General Aviation Position Report
16 Fedex Position Report-AUTPOS, General Aviation Weather Request
17 Northwest Airlines Position/Weather Report
18 General aviation weather report
19 ATIS report
1C Flight plan request
1L Atlantic Coast Airlines/United Express QF OFF message
1M Estimated Time of Arrival report
20 SAS Initialization
21 SAS Takeoff Data calculation
23 SAS Loadsheet Uplink
24 Northwest Airlines Position/Weather Request
25 SAS Special Load Notification
26 SAS SIGMET message
27 SA Weather Request
28 Weather Request
29 SAS Flow Message
2S Weather Request
2U Weather
30 UPS Position Report
31 Northwest Airlines Weather Report
36 Northwest Airlines In Range Message
39 Northwest Airlines arrival/free text message
3F UPS ETA Downlink Message
3G UPS Free Text Message Format
3J UPS Downlink Message
3S UPS Downlink Message
3U UPS Uplink acknowledgement
41 SAS Flow Message
46 Pirep
47 747-400 Airway Position Downlink
48 Misc Msgs
49 Air Canada Status Reports
4A Air Canada latest new format
4M Cargo Info, Air Canada Uplink to Cockpit Printer-specific message
4P Weather report/forecast
4Q Air Canada departure format
4R OFF Air Canada-Specific Message Format
4S Weather Request
4T ETA Report
4X Air Canada Frequency List
50 Possible HFDL Msg router
51 Ground GMT Request/Update
52 Ground UTC Request/Update
53 Reserved Spare
54 ACARS Frequency Uplink, Aircrew Initiated Voice Contact Request
57 Alternate Aircrew Initiated Position Report
58 Aircraft Tracking Control, Aircraft Tracking Periodic Position Report
5D ATIS Request
5P Temporary Suspension of ACARS Service (reponse to Q6)
5R Aircraft initiated position report
5U Downlink Weather Request
5V VDL Switch Advisory
5Y ETA Revision, Diversion
5Z Airline designated downlink
7A Aircraft Initiated Engine Data, Takeoff Thrust Report
7B Misc Aircrew Initiated Msgs
80 Delta/Canadian specific 3C03 position report format, Airline Defined
81 Airline Defined
82 Airline Defined
83 Airline Defined
84 S.A.S. Free Text Message, Airline Defined
85 Airline Defined
86 Airline Defined
87 Airline Defined
88 Airline Defined
89 Airline Defined
8A SAS Out report
8B SAS Off report
8C SAS On report
8D SAS In report
8E SAS Out/return in report
8G SAS Take off data calculation request
8H SAS Loadsheet request/confirmation
8I SAS Flightplan request
8J SAS Crewlist request
8X Uplink ATIS Information
8Z Avionics unable to process data
99 Emergency Situation Report
A0 ATIS Facilities Notification
A1 Deliver Oceanic Clearance
A2 Unassigned
A3 Deliver Departure Clearance
A4 Acknowledge Departure Clearance
A5 Request Position Report
A6 Request ADS Report
A7 Forward Free Text to Aircraft
A8 Deliver Departure Slot
A9 Deliver ATIS Information
AA ATC Communications
AB Terminal Weather Information for Pilots
AC Pushback Clearance Uplink
AD Expected Taxi Clearance
AE Unassigned
AF CPC Command Response
AG Unassigned
AW ATS Wind Data
B0 ATS Facility Notification
B1 Flight Plan Oceanic Clearance Request
B2 Oceanic Clearance Readback
B3 Request Departure Clearance
B4 Acknowledge Departure Clearance
B5 Provide Position Report
B6 Provide ADS Report
B7 Forward Free Text to ADS
B8 Request Departure Slot
B9 Request ATIS information, Flight Plan Information Receipt
BA ATC Communications
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
BW ATS Wind Response
C# Message to Cockpit Printer #
C0 Uplink message to all printers
C1 Uplink message to printer 1
C2 Uplink message to printer 2
C3 Uplink message to printer 3
CA Printer Error
CB Printer Busy
CC Printer Local, Printer in Test Mode
CD Printer out of Paper, Printer No Paper
CE Printer Buffer Overrun
CF Printer Reserved Status
DL Reserved for Data Loading
D De-Icing Downlinks
DI De-Icing Uplinks
EI Internet Email Message
E1 Internet Email Message
E2 Internet
EL Engine Monitoring Unit Message Downlink
ER Engine Monitoring Unit Message Downlink
F3 Dedicated Transceiver Advisory
H1 Message to/from terminal, Messages from Avionic Subsystem, OAT, OAX
H2 Meteorological Report
H3 Icing Report
H4 Meteorological Report Configuration Downlink
HF HF Report
HX Undelivered Uplink report
KB Loopback Request/Response
LB Cabin E-Logbook
LC Cabin E-Logbook
LS Technical (Cockpit) E-Logbook
LT Technical (Cockpit) E-Logbook
M1 IATA Departure Msg, IATA Departure Message
M2 IATA Arrival Msg, USR, User Defined Message
M3 IATA Return to Ramp Message
M4 IATA Return from Airborne Message
P0 AMS-Protected Downlink Message, Test System Destination
P6 Unassigned
P7 AMS-Protected Uplink ATS Message
P8 AMS-Protected Uplink AMS-Specific Message
P9 AMS-Protected Uplink AOC Message
PA AMS-Protected Downlink ATS Message
PB AMS-Protected Downlink AMS-Specific Message
PC AMS-Protected Downlink AOC Message
Q0 ACARS Link Test
Q1 ETA Departure/Arrival Report
Q2 ETA Report
Q3 GMT Clock Update
Q4 Voice Circuit Busy - Reply to 54
Q5 Unable to process uplinked messages
Q6 Voice-to-ACARS change over-Joining ACARS
Q7 Delay message
Q8 Reserved
Q9 Reserved
QA Out/fuel report, Departure Report
QB Off report
QC On Report
QD Arrival Report / In Fuel
QE Out/fuel destination report
QF OFF (Off/destination report) / Out Fuel
QG Returning to Gate Report
QH Out Report
QK Landing report
QL Arrival Report
QM Arrival Info Report
QN Diversion Report
QP OUT report
QQ OFF report
QR ON report
QS IN report
QT OUT/Return IN report
QU Unassigned
QV Autotune Reject
QW Unassigned
QX Intercept, airborne subsystem unable to process
QY Unassigned
QZ Unassigned
RA Command Acft terminal to TX data
RB Response of aircraft terminal to RA uplink message
RE Refuel
S1 VHF Network Statistics Report
S2 VHF Performance Report
S3 LRU Configuration Report
SA Media Advisory
SQ Uplink Squitter
UP Message Acknowledgement
WP Potable Water Remote Pre-selection
_D General Response
_J No info to TX
Whew!
So, armed with this knowledge, looking at the 2 messages I used as an example:
- I see that the first message has a label of "H1" which means that it's a message from the aircraft's avionic subsystem - likely a status report on aircraft position and activity - by looking at the message contents I can make out GPS co-ordinates and altitude.
- The second message, the one with the plain text, had a label of "84" - which indicated that it was a SAS Free Text Message - which makes sense when you look at the message contents
Now that I am armed with this new found knowledge, my next step is to step up my SDR dongle and computer to monitor ACARS frequencies and to capture the messages that are received.
My ultimate plan is to only parse out the free text messages, since they seem to provide a bit of a behind the scenes peek of the working lives of aircrew, and probably far more interesting than position reports.
Once I've got a process sorted out on how to capture these messages, I would like to perhaps post them as a live feed somewhere. Now since I believe there are some rules out there that prohibit posting "live" transmissions, I would need to create something that won't post until at least 48 hours after the transmission.
In my next post I'll work on getting my SDR up and look at starting to gather my messages.
