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The /data folder contains raw AIS files collected in a beautiful city of Normandy: Vernon

Fortunatly Vernon is crossed by the Seine, therefore we can test reception of signals from ships with AIS receiver like dAISy

Ships data are critical in case of tsunami because some of them are carrying people or hazardous materials.

We've deployed an AIS station based on:

• em-trak R100
• Raspberry Pi
• Raspbian
• Kplex
• OpenCPN
• libais
• gpsd

You can look at the station dashboard's via Marine Traffic

This work is part of the CaribeWaveFWI Project

Each file ais_vernon_yyyymmdd.log contains the raw AIS data of the day with a timestamp at local time of Paris

YYYY-MM-DD HH:MM:SS, o---------------AIS SENTENCE -----------------o

2016-03-28 22:12:12, !AIVDM,1,1,,A,23GR0u0P0006kshL5qQ00?vH0@7:,0*44

The file named log_ais_1243_vernon.log.xz contains compressed version of all the data collected by the station since 2016-03-28 22:12:12

This file have the same format than the others files. and can be uncompress with unxz utilities.

If you want a pure NMEA AIS file without timestamp you can use this GNU/Awk command line:

awk '{print $3}' ais_vernon_yyyymmdd.log

Decode AIS

The GPSD Project has documented the AIS protocol on this page

Small extract of this great decoding manual from Eric S. Raymond @esrtweet Don't hesitate to help the project on Gratipay

Fast explanation about what the fields mean:

!AIVDM,1,1,,B,177KQJ5000G?tO``K>RA1wUbN0TKH,0*5C

• Field 1, !AIVDM, identifies this as an AIVDM packet.

• Field 2 (1 in this example) is the count of fragments in the currently accumulating message. The payload size of each sentence is limited by NMEA 0183’s 82-character maximum, so it is sometimes required to split a payload over several fragment sentences.

• Field 3 (1 in this example) is the fragment number of this sentence. It will be one-based. A sentence with a fragment count of 1 and a fragment number of 1 is complete in itself.

• Field 4 (empty in this example) is a sequential message ID for multi-sentence messages.

• Field 5 (B in this example) is a radio channel code. AIS uses the high side of the duplex from two VHF radio channels: AIS Channel A is 161.975Mhz (87B); AIS Channel B is 162.025Mhz (88B). In the wild, channel codes 1 and 2 may also be encountered; the standards do not prescribe an interpretation of these but it’s obvious enough..

• Field 6 (177KQJ5000G?tO`K>RA1wUbN0TKH in this example) is the data payload. We’ll describe how to decode this in later sections.

• Field 7 (0) is the number of fill bits requires to pad the data payload to a 6 bit boundary, ranging from 0 to 5. Equivalently, subtracting 5 from this tells how many least significant bits of the last 6-bit nibble in the data payload should be ignored. Note that this pad byte has a tricky interaction with the <[ITU-1371]> requirement for byte alignment in over-the-air AIS messages; see the detailed discussion of message lengths and alignment in a later section. The *-separated suffix (5C) is the NMEA 0183 data-integrity checksum for the sentence, preceded by "". It is computed on the entire sentence including the AIVDM tag but excluding the leading "!".

For comparison, here is an example of a multifragment sentence with a nonempty message ID field:

!AIVDM,2,1,3,B,55P5TL01VIaAL@7WKO@mBplU@<PDhh000000001S;AJ::4A80?4i@E53,0*3E

!AIVDM,2,2,3,B,1@0000000000000,2*55

Technically, NMEA0183 does not actually require that a !-led sentence be AIS. This format can be used for any encapsulated data. The syntax and semantics of fields 1-4 are fixed, and the fill-bit field and NEA checksum are required, but the payload fields may contain any encapsulated data. It is, however, a safe bet that any such sentence containing an A or B channel code in field 5 is AIVDM/AIVDO.

You can decode AIS NMEA message with many tools:

libais is a good start in Python there are references to other tools on the project page.

Two examples in Python

One line message:

2016-04-01 00:09:59, !AIVDM,1,1,,A,240UuRhP1RP716JL4:sLlOwnb6hT,0*3C

Extract Only the data payload

import ais

ais.decode('240UuRhP1RP716JL4:sLlOwnb6hT',0)

{
u'slot_timeout': 1L,
u'sync_state': 0L,
u'true_heading': 511L,
u'utc_spare': 0L,
u'sog': 9.800000190734863,
u'rot': -731.386474609375,
u'nav_status': 0L,
u'repeat_indicator': 0L,
u'raim': True,
u'id': 2L,
u'utc_min': 9L,
u'spare': 2L,
u'cog': 328.1000061035156,
u'timestamp': 59L,
u'y': 49.04743194580078,
u'x': 1.5329283475875854,
u'position_accuracy': 1L,
u'utc_hour': 22L,
u'rot_over_range': True,
u'mmsi': 269057419L,
u'special_manoeuvre': 1L
}

Two lines message:

2016-04-01 10:46:06, !AIVDM,2,1,4,A,53GR=4400000Ho??C40@D9E==T0000000000001?90>27t@PJ08888888888,0*61

2016-04-01 10:46:06, !AIVDM,2,2,4,A,88888888880,2*20

TODO: Concatenate the two data payload below and don't forget to replace the 0 args at the end by 2

import ais

ais.decode('540UuRl00000PF3OC7UHTdTpN18Tp@622222220t4iQ7651<04TSmAC```888888888888880',2)

{
u'destination': u'ROUEN               ',
u'dim_d': 6L,
u'name': u'VIKING RINDA        ',
u'eta_hour': 12L,
u'ais_version': 1L,
u'draught': 1.7999999523162842,
u'mmsi': 269057419L,
u'repeat_indicator': 0L,
u'dim_b': 97L,
u'dim_c': 7L,
u'dte': 0L,
u'dim_a': 38L,
u'eta_day': 2L,
u'eta_minute': 0L,
u'callsign': u'HE 7419',
u'spare': 0L,
u'eta_month': 4L,
u'type_and_cargo': 60L,
u'fix_type': 1L,
u'id': 5L,
u'imo_num': 0L
}`

Licence

The data are published under the licence Creative Commons 4 By

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You do not have to comply with the license for elements of the material in the public domain or where your use is permitted by an applicable exception or limitation. No warranties are given. The license may not give you all of the permissions necessary for your intended use. For example, other rights such as publicity, privacy, or moral rights may limit how you use the material