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Capturing raw AIS data using rtl_fm and decoding using aisdecoder

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James Mead edited this page Apr 3, 2015 · 12 revisions

Capturing raw AIS data

I captured 4 x raw AIS data files by running the following commands on my Mac laptop while crossing the Solent on a RedJet ferry which departed West Cowes at 0915 on Monday, 30th March:

  • rtl_fm -f 162018500 -p 35 -s 48k > test-162018500-p35-s48k.raw
  • rtl_fm -f 162025000 -p 35 -s 48k > test-162025000-p35-s48k.raw
  • rtl_fm -f 161975000 -p 35 -s 48k > test-161975000-p35-s48k.raw
  • rtl_fm -f 161967400 -p 15 -s 48k > test-161967400-p15-s48k.raw

This resulted in the following raw data files:

Notes

  • I chose the frequency (-f option) values based on the signals I could see in GQRX running on my Mac laptop. The signals all seemed pretty weak.
  • I chose the value of the PPM error (-p option) based on the value (35) used in the example in this README. And that README suggests I also experimented by changing it to 15 in the last capture.

Decoding raw AIS data

I tried to decode the AIS NMEA sentences from the raw files captured above using aisdecoder on the Ubuntu Virtual Box VM running on my Mac laptop. I ran the following commands:

  • aisdecoder -h 127.0.0.1 -p 1234 -a file -c mono -d -f test-162018500-p35-s48k.raw
  • aisdecoder -h 127.0.0.1 -p 1234 -a file -c mono -d -f test-162025000-p35-s48k.raw
  • aisdecoder -h 127.0.0.1 -p 1234 -a file -c mono -d -f test-161975000-p35-s48k.raw
  • aisdecoder -h 127.0.0.1 -p 1234 -a file -c mono -d -f test-161967400-p15-s48k.raw

Unfortunately only a single NMEA sentence was decoded (from the last file: test-161967400-p15-s48k.raw):

!AIVDM,1,1,,A,13M@DV0002wqVHFM7gGEUmu00@D>,0*0F

I used AGG Software's Online AIS Decoder to decode this NMEA sentence:

Name Value Description
Packet Type AIVDM
CHANNEL A
Message Type 1 Scheduled Position Report
Repeat Indicator 0 Default
User ID 232002712
Navigation Status 0 Under way using engine
Rate of Turn (ROT) 0 Turning right at up to 708 degrees per minute or higher
Speed Over Ground (SOG) 0.2
Position Accuracy 1 A DGPS-quality fix with an accuracy of < 10 ms
Longitude -1.39816666666667 West
Latitude 50.8925283333333 North
Course Over Ground (COG) 143.1
True Heading (HDG) 190
Time Stamp 32
Reserved for regional 0 Not available (default)
RAIM flag 0 RAIM not in use (default)
Communication State 66830 Sync state: UTC Direct; Slot Timeout: 4 frames respectively are left until slot change; Slot number: 1294
Communication Sync State 0 Sync state: UTC Direct
Communication Slot Timeout 4 Slot Timeout: 4 frames respectively are left until slot change
Communication Sub Message 1294
Communication Utc Hour No value
Communication Utc Minute No value
Communication Time Stamp No value
Communication Slot Number 1294
Communication Received Stations No value
Communication Slot Offset No value
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