7/27/2019 Marine Automatic Identification System

http://slidepdf.com/reader/full/marine-automatic-identification-system 1/3

Marine Automatic Identification System (AIS)Short and sweet: AIS is composed of small bursts of data sent over normal marine VHF butusing GMSK modulation instead of FM to identify vessels, their position, and telemetry. Thedata is encoded into a bit pattern (bit vector) to make it as small as possible. For a much longerand drawn-out description see Wikipedia 

For practical (technical) information about AIS, it´s reception and decoding, you´ve come to theright place.

  Radio o  Frequencies o  Reception o  Purpose-built receivers 

  Data Format o  NMEA o  AIS Data Package (Bit Vector)o  Message Information 

AIS Radio TransportAIS was initially designed to use standard marine radio frequencies for data transmission. This

basically means that the existing set of antennas, cabling, and frequency allocation plans didn´tneed to be changed at all to see it implemented. All that was required was some specialequipment to encode and send/receive and decode the signals.

AIS over VHFAIS uses the high side of the duplex from two VHF radio "channels" 87B and 88B.

Channel A 161.975Mhz (87B)Channel B 162.025Mhz (88B)

The simplex channels 87A and 88A use a lower frequency so they are not even affected by thisallocation and can still be used as designated for the maritime mobile frequency plan.

AIS ReceptionThe data is encoded using GMSK modulation so you wont hear anything if you simply tune yourVHF radio to 88B - you need a special data output (found on some scanners) to access thedata. Many HAM radios that have been configured for high-speed packet radio communicationshave this output and others can be "hacked" to obtain it. You effectively need a Discriminator Tap This is an output from one of the chips in the radio that gives the raw signal before anyaudio filtering (the filtering destroys the GMSK data)

To find out about your radio or scanner just do a search something like "<radio model>Discriminator Output" and you should find what you need.

AIS ReceiversMost AIS communications are performed bypurpose-built hardware systems. There are severaltypes:

  Class A Transponders >$2000 Full implementation that can transmit avessel´s data on either AIS channel andreceive on both channels simultaneously.

  Class B Transponders ~$1000 For non-compulsorily fitted vessels withing tohave an inexpensive AIS system - same VHFspecs but fewer features and capabilities

 Dual Channel Receivers ~$400 Receives AIS on both channelssimultaneously - usually providing a NMEA

7/27/2019 Marine Automatic Identification System

http://slidepdf.com/reader/full/marine-automatic-identification-system 2/3

data output. No transmission.

  Scanning Receivers ~$200 Receives AIS on both one of the two channelsat a time (but scans between them) - usuallyproviding a NMEA data output. Notransmission.

AIS Data FormatThe AIS data format seems to be a fairly well kept secret. The following should help anyonewishing to explore the AIS data in more detail without shelling out the money and thenattempting to sift through the ITU M.1371-2 standard 

 NMEA AIS Message FormatThe NMEA standard uses two primary sentences to for AIS data !AIVDM (Received Data fromother vessels) and !AIVDO (Your own vessels information) Since you won´t be receiving AIVDOmessages we will focus on the AIVDM sentence.

Here is a typical NMEA standard AIS message:

!AIVDM,1,1,,A,14eG;o@034o8sd<L9i:a;WF>062D,0*7D 

In Order:

!AIVDM: The NMEA message type

1 Number of Sentences (some messages need more then one)

1 Sentence Number (1 unless it´s a multi-sentence

message)

The blank is the Sequential Message ID (for multi-

sentence messages)

A The AIS Channel (A or B)

14eG;... The Encoded AIS Data 

0* End of Data

7D NMEA Checksum

AIS Data EncodingIn the NMEA encoding for AIS - each ASCII character corresponds to 6 binary bits (unlikenormal ASCII which uses 8 bits) so you need to step through each character and subtract 48from the ASCII - then if it´s still a decimal number > 40 subtract another 8 - then convert tobinary: this guarantees a 6 bit number. Looking at our data (just the first few characters)

14eG

1 = 0000014 = 000100

e = 101101G = 010111and so on...

The complete data string decoded looks like this when strung back together.000001 000100 101101 010111 011100 001010 010000 000000 000000 000000 110111001000 110111 100001 101000 011100 011101 110010 011111 101011 110000 110101010111 010000 000000 001000 011000 011011

Now you start grabbing sets of bits from this and converting to decimal. Here are the key piecesof information assuming the first character is a ´1´ (the message type):

MMSI Number - starting from bit 8 for 30 bits

= 010010110101011101110000101001 = 316005417

HDG - bit 128 for 9 bits

7/27/2019 Marine Automatic Identification System

http://slidepdf.com/reader/full/marine-automatic-identification-system 3/3

COG - bit 116 for 12 bits (and divide by 10)SOG - bit 50 for 10 bits (and divide by 10)

Lat - bit 89 for 27 bits (a signed binary number, divide by 600000)Lon - bit 61 for 28 bits (a signed binary number, divide by 600000)

AIS Message TypesThe following are the individual messages that have been defined for AIS in the NMEA

standard. Each one that is linked contains the detailed bit vector binary message data

breakdown of the message contents. I will try to add all of them as quickly as possible.

01 - Position Report with SOTDMA 

02 - Position Report with SOTDMA 

03 - Position Report with ITDMA 

04 - Base Station Report

05 - Ship static and voyage related data 

06 - Addressed Binary Message

07 - Binary Acknowledge

08 - Binary Broadcast Message09 - Standard SAR Aircraft position report

10 - UTC and Date inquiry

11 - UTC and Date response

12 - Addressed safety related message

13 - Safety related Acknowledge

14 - Safety related Broadcast Message

15 - Interrogation

16 - Assigned Mode Command

17 - GNSS Binary Broadcast Message

18 - Standard Class B CS Position Report

19 - Extended Class B Equipment Position Report

20 - Data Link Management21 - Aids-to-navigation Report

22 - Channel Management

23 - Group Assignment Command

24 - Class B CS Static Data Report