Monday, April 5, 2010

Decode AIS String using PHP

This is for all who may want to decode an AIS String but never knew how to do.

For all who don't know what is AIS.. try this link
http://www.imo.org/Safety/mainframe.asp?topic_id=754
http://en.wikipedia.org/wiki/Automatic_ ... ion_System



this code is tested for type 1 messages and further development has been stalled. I had just coded it in my free time on ship. It is scripted in PHP. but you may adapt for c++ or VB.

100% original
//code to decode ais messages
/*
? Receives a broadcast message,
? Organises the binary bits of the Message Data into 6-bit strings,
? Converts the 6-bit strings into their representative "valid characters" – see IEC 61162-1,
table 7,
? Assembles the valid characters into an encapsulation string, and
? Transfers the encapsulation string using the VDM sentence formatter.

sample message
!AIVDM,1,1,,A,1P000Oh1IT1svTP2r:43grwb0Eq4,0*01

*/
$ais = "1P000Oh1IT1svTP2r:43grwb0Eq4";
$aisdata168=NULL;//six bit array of ascii characters
$ais_nmea_array = str_split($ais);
foreach ($ais_nmea_array as $value)
{
$dec=ascii_2_dec($value);
$bit8=asciidec_2_8bit($dec);
$bit6=dec_2_6bit($bit8);
echo $value ."-" .$bit6 ."
";
$aisdata168 .=$bit6;
}
echo $aisdata168 . "
";

echo "mmsi= " . bindec(substr($aisdata168,9,29)) . "
";
echo "cog= " . bindec(substr($aisdata168,116,12))/10 . "
";
echo "sog= " . bindec(substr($aisdata168,50,10))/10 . "
";

function ascii_2_dec($chr)
{
$dec=ord($chr);//get decimal ascii code
$hex=dechex($dec);//convert decimal to hex
return ($dec);
}

function asciidec_2_8bit($ascii)
{
if ($ascii<48){}
else
{
if($ascii>119){}
else
{
if($ascii>87 && $ascii <96){}
else
{
$ascii=$ascii+40;
if ($ascii>128){$ascii=$ascii+32;}
else{$ascii=$ascii+40;}
}
}
}
return ($ascii);
}
function dec_2_6bit($dec)
{
$bin=decbin($dec);
return(substr($bin, -6));
}

5 comments:

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    Cheers
    Christian, Satellite Direct Tv

    ReplyDelete
  2. Really nice and impressive blog i found today.

    ReplyDelete
  3. @ Christian.. Thanks for your words of appreciation

    ReplyDelete
  4. good share, great article, very usefull for us...thanks!

    ReplyDelete