Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Old October 17th 05, 04:08 PM
 
Posts: n/a
Default how to read AIS data from encapsulated NMEA VDO sentence

Hi all, I am a navigation software developer from Italy.

I need to understand how to read AIS data inside the "blob" contained
in VDO NMEA sentece. I have the ITU-R M.1371-1 document that should let
me understand such format. Unfortunately I really cannot understand how
to read the bits.

Someone, some times ago, posted such message:

The AIS sentences are in a strange format, encapsulated 6 bits. One
example he


'!AIVDO,1,1,,,13:r`R5P1orpG60JeHgRSj4l0000,0*56
' 0 1 2 34 5 6
'1 Total number of sentences needed to transfer the message, 1 to 9
'2 Message sentence number, 1 to 9
'3 Sequential message identifier, 0 to 9
'4 AIS channel number
'5 Encapsulateled 6-bit radio message
'6 Number of fill-bits, 0 to 5


'14eG=ch021rp4FTJdTGRRR0605q4


'VDO.Identifier : 1 000001
'Repeat indicator : 0 00
'MMSI : 316001711 010010110101011100110110101111
'VDO1.NavStatus : 0 0000
'VDO1.ROT : 0 00000000
'VDO1.SOG : 12.9 kns 0010000001
'VDO1.PosAcc : 1 1
'VDO1.Latitude : 46 39.3182'N 1101011100000010001011010010
'VDO1.Longitude : 71 38.2703'W
0011010101100100100010111100010100010100
'VDO1.COG : 065 010000000000
'VDO1.Heading : 064 110000000
'VDO1.TimeStamp : 3 sec. 000101
'VDO1.NavStatus : 0 1
'VDO1.CommStatus : 15:17 h 11001000100


I really cannot understand how he found such information inside
14eG=ch021rp4FTJdTGRRR0605q4. Please Help

Ciao
Cristiano


  #3   Report Post  
Old October 27th 05, 02:30 AM
Pascal
 
Posts: n/a
Default how to read AIS data from encapsulated NMEA VDO sentence


See this site:

http://www.navicon.dk/web/normal.php?pageid=95







escreveu:

Hi all, I am a navigation software developer from Italy.

I need to understand how to read AIS data inside the "blob" contained
in VDO NMEA sentece. I have the ITU-R M.1371-1 document that should let
me understand such format. Unfortunately I really cannot understand how
to read the bits.

Someone, some times ago, posted such message:

The AIS sentences are in a strange format, encapsulated 6 bits. One
example he


'!AIVDO,1,1,,,13:r`R5P1orpG60JeHgRSj4l0000,0*56
' 0 1 2 34 5 6
'1 Total number of sentences needed to transfer the message, 1 to 9
'2 Message sentence number, 1 to 9
'3 Sequential message identifier, 0 to 9
'4 AIS channel number
'5 Encapsulateled 6-bit radio message
'6 Number of fill-bits, 0 to 5


'14eG=ch021rp4FTJdTGRRR0605q4


'VDO.Identifier : 1 000001
'Repeat indicator : 0 00
'MMSI : 316001711 010010110101011100110110101111
'VDO1.NavStatus : 0 0000
'VDO1.ROT : 0 00000000
'VDO1.SOG : 12.9 kns 0010000001
'VDO1.PosAcc : 1 1
'VDO1.Latitude : 46 39.3182'N 1101011100000010001011010010
'VDO1.Longitude : 71 38.2703'W
0011010101100100100010111100010100010100
'VDO1.COG : 065 010000000000
'VDO1.Heading : 064 110000000
'VDO1.TimeStamp : 3 sec. 000101
'VDO1.NavStatus : 0 1
'VDO1.CommStatus : 15:17 h 11001000100


I really cannot understand how he found such information inside
14eG=ch021rp4FTJdTGRRR0605q4. Please Help

Ciao
Cristiano


  #4   Report Post  
Old October 27th 05, 04:50 AM
Terry Spragg
 
Posts: n/a
Default how to read AIS data from encapsulated NMEA VDO sentence

Pascal wrote:

See this site:

http://www.navicon.dk/web/normal.php?pageid=95







escreveu:


Hi all, I am a navigation software developer from Italy.

I need to understand how to read AIS data inside the "blob" contained
in VDO NMEA sentece. I have the ITU-R M.1371-1 document that should let
me understand such format. Unfortunately I really cannot understand how
to read the bits.

Someone, some times ago, posted such message:


The AIS sentences are in a strange format, encapsulated 6 bits. One
example he


'!AIVDO,1,1,,,13:r`R5P1orpG60JeHgRSj4l0000,0* 56
' 0 1 2 34 5 6
'1 Total number of sentences needed to transfer the message, 1 to 9
'2 Message sentence number, 1 to 9
'3 Sequential message identifier, 0 to 9
'4 AIS channel number
'5 Encapsulateled 6-bit radio message
'6 Number of fill-bits, 0 to 5


'14eG=ch021rp4FTJdTGRRR0605q4


'VDO.Identifier : 1 000001
'Repeat indicator : 0 00
'MMSI : 316001711 010010110101011100110110101111
'VDO1.NavStatus : 0 0000
'VDO1.ROT : 0 00000000
'VDO1.SOG : 12.9 kns 0010000001
'VDO1.PosAcc : 1 1
'VDO1.Latitude : 46 39.3182'N 1101011100000010001011010010
'VDO1.Longitude : 71 38.2703'W
0011010101100100100010111100010100010100
'VDO1.COG : 065 010000000000
'VDO1.Heading : 064 110000000
'VDO1.TimeStamp : 3 sec. 000101
'VDO1.NavStatus : 0 1
'VDO1.CommStatus : 15:17 h 11001000100


I really cannot understand how he found such information inside
14eG=ch021rp4FTJdTGRRR0605q4. Please Help

Ciao
Cristiano



Disassembling message protocols is tedious by hand. Automated
equipment does it with logic circuitry, which accomplishes tedious
tasks easily and knows the secret code.

We humans must parse manually.

The data is actually send as a bitstream and the protocol for every
device is proprietary.

The data presented above is actually encoded as hexadecimal or
ascii encode binary characters, so it actually looks more like
11110110010110101010101010101101100110011010101010 1010111010101...,
etc. These bits are sent as analogue levels, data encoded in one
long serial string. 100 character might be 800 bits, or 1600 bits,
in number base 10 notation. It might use another numerical base,
likely octal or hexadecimal.

The ports have their own protocols, and take each byte of 7 or 8
bits (as one example, I am not certain what this one actually uses,
perhaps 16 bit bytes) as a character. Each group of bits represents
one character in the string given above. The alphabet used by the
system may have many more than the 26 in our human alphabet, and
some of those characters are specific to the device in use. The
spacing and syntax is combined with error correcting parity (even -
odd) codes and checksum numbers which must also be decoded in logic
if you are writing an interface. Each receiving device must be up to
date with whatever protocol is used by all of the transmitters if it
is not to become discombobulated and confused, losing track of
character boundaries, etc, and those protocols seem to change with
each new generation of devices.

Different portions of the data string may use different schemes, ie,
the first 6 bits might be all ones, as a preamble and sync. The
next 4 bits might be a code to identify the remainder as a specific
type of statement, ie, sender I.D. Sometimes the info is bitsliced,
so that each bit represents a switch setting in the receiving parser
to cause it to understand that it is radar, weather, whatever.

I once independantly decoded the data sent by telco in it's caller
id string, sent between the ring tones to your phone. There are
padding bits, a preamble, and then each character is sent twice. It
was painful for a while, as I had to scope out the data without
benefit of knowing the protocol, even as simple as it was, using a
capture buffer to read each character out in ascii values, a sofware
protocol analyzer I wrote myself, which spit out gibberish ascii
codes until I learned to read it. If it had not been straight ascii,
normal in telephone modems, I would still be propping up my eyelids
with toothpicks.

You seem to have a protocol guide or data map, which you must now
come to know imtimately. Reverse engineering software data is fun
for some masochists, for a while.

400 page logic diagrams is where I cut my teeth, back in the 60's.
It's one way to burn out your brain so far as computers is
concerned. I got fed up with them, and retired years ago. Wouldn't
touch it, now, without cash up front and a clearly defined mission
statement engraved in titanium.

Good luck.

Terry K -SofDevCo-



  #5   Report Post  
Old October 27th 05, 06:00 PM
[email protected]
 
Posts: n/a
Default how to read AIS data from encapsulated NMEA VDO sentence

Terry, you're making this sound alot more complicated than it really
is.

The 6-bit encoded part of a VDM sentence is simply a bitstream, packed
using a special code. Each character represents 6 bits of data from
0x00 ot 0x3F. It starts with the message ID (6 bits), repeat (2 bits),
MMSI (30 bits), etc. This is all defined in m.1371 and 61993-2, even if
it is a bit confusing at first.

The first 6 bits being a character and the message ID make it easy to
tell that the example string above is a message 1.



  #6   Report Post  
Old May 25th 06, 12:52 AM
Junior Member
 
First recorded activity by BoatBanter: May 2006
Posts: 1
Default

Quote:
Originally Posted by
Terry, you're making this sound alot more complicated than it really
is.

The 6-bit encoded part of a VDM sentence is simply a bitstream, packed
using a special code. Each character represents 6 bits of data from
0x00 ot 0x3F. It starts with the message ID (6 bits), repeat (2 bits),
MMSI (30 bits), etc. This is all defined in m.1371 and 61993-2, even if
it is a bit confusing at first.

The first 6 bits being a character and the message ID make it easy to
tell that the example string above is a message 1.

Hi guys,

I'm actually working on a similar project right now except I have to do the ENCODE part of the VDM message. At first I thought every character represents 6-bit but that is not true. As you can see in the message there are lowered case alphabet characters (i.e. 'e') and the ascii value for 'e' is more than 6-bits. I'm a little stuck on my project right now too, can someone give me more info. Perhaps an example in C or C++. Thanks.
  #7   Report Post  
Old May 25th 06, 04:49 AM posted to rec.boats.electronics
Paul
 
Posts: n/a
Default how to read AIS data from encapsulated NMEA VDO sentence


"tcdang714" wrote in message
...

Wrote:
Terry, you're making this sound alot more complicated than it really
is.

The 6-bit encoded part of a VDM sentence is simply a bitstream, packed
using a special code. Each character represents 6 bits of data from
0x00 ot 0x3F. It starts with the message ID (6 bits), repeat (2 bits),
MMSI (30 bits), etc. This is all defined in m.1371 and 61993-2, even
if
it is a bit confusing at first.

The first 6 bits being a character and the message ID make it easy to
tell that the example string above is a message 1.



Hi guys,

I'm actually working on a similar project right now except I have to do
the ENCODE part of the VDM message. At first I thought every character
represents 6-bit but that is not true. As you can see in the message
there are lowered case alphabet characters (i.e. 'e') and the ascii
value for 'e' is more than 6-bits. I'm a little stuck on my project
right now too, can someone give me more info. Perhaps an example in C
or C++. Thanks.


--
tcdang714


I realize that you've asked about *encoding*, but here's an example of a
decoder written in visual basic, from a program I wrote for my pocket PC.
Perhaps you will be able to reverse-engineer something from this.
------------------
Function AsciiToHex6(AsciiIn)
AsciiToHex6 = A2H6(Asc(AsciiIn))
End Function

Dim A2H6(128)
Sub InitA2H6
Dim i, h
For i = 0 To 127
h = i
If i 48 Then
h = -1
ElseIf i 88 Then
h = h - 48
ElseIf i 119 Then
h = -1
ElseIf i 96 Then
h = -1
Else h = h - 56
End If
A2H6(i) = h
Next
End Sub

InitA2H6
------------------
The array A2H6() contains the hex6 value of the ascii input character, using
the character as an index. I put "-1" in the illegal positions, and ought
to do a little more defensive error-checking (but I probably won't ever get
around to it). The function AsciiToHex6(AsciiIn) just looks up the code and
returns it.

The IEC documents that describe the messages and coding aren't free, but
while I was googling around I did find this one:
http://www.gicoms.go.kr/knowledge/download.asp?filename=IEC%20standard%2061993(class %20A%20AIS).pdf&filepath=D:%5CGICOMS_FILE%5Cupdown %5CIEC%20standard%2061993(class%20A%20AIS).pdf

This contains a decent description of the 6-bit ascii encoding, as well as
the message structure. Note that the AIS data elements are not necessarily
six-bits long, so there will be an arbitrary alignment of the various
parameters across the 6-bit-ascii character stream. Also, some of the AIS
messages are longer than allowed in the NMEA sentence, so the messages have
to be split into multiple NMEA sentence. I can post my code for decoding
the arbitrary data if you think it would be instructive.

Be aware that some of the earlier discussions of 6-bit ascii encoding for
AIS show a different encoding (don't ask me how long I struggled with that
issue!). The one in my code sample, and the document above seem to be the
encoding actually used.

Good Luck,
Paul


  #8   Report Post  
Old June 7th 06, 04:21 AM posted to rec.boats.electronics
[email protected]
 
Posts: n/a
Default how to read AIS data from encapsulated NMEA VDO sentence

I too have been struggling with this.

I convert every 6 bits to a byte and convert to the approriate 8 bit
ascii character.

It all works for message type 1.

So, I thought I'd look at message type 5, which is

id 6 bits
repeat 2
user id 30
version 2
imo 30
call sign 42
name 120

It works up until imo, then for the call sign I get gibberish.

It seems characters don't like my approach ...

Does anyone have any code or suggestions?

Thanks

  #9   Report Post  
Old June 7th 06, 04:50 AM posted to rec.boats.electronics
[email protected]
 
Posts: n/a
Default how to read AIS data from encapsulated NMEA VDO sentence

Actually, I re-read what I posted and it didn't really make sense.

1. Start with the array of ascii characters
2. Convert to the 6 bit binary value.
3. Convert this byte array to a 6 bit bitstream.
4. Then just pull out the bits yuo want using the various bit
operators. I tried to use a bit-field, but it didn't seem to work in
Windows.

As I said, it all works until I get to character arrays (call sign etc)

  #10   Report Post  
Old June 9th 06, 09:03 AM posted to rec.boats.electronics
Paul
 
Posts: n/a
Default how to read AIS data from encapsulated NMEA VDO sentence


wrote in message
oups.com...
Actually, I re-read what I posted and it didn't really make sense.

1. Start with the array of ascii characters
2. Convert to the 6 bit binary value.
3. Convert this byte array to a 6 bit bitstream.
4. Then just pull out the bits yuo want using the various bit
operators. I tried to use a bit-field, but it didn't seem to work in
Windows.

As I said, it all works until I get to character arrays (call sign etc)


Here is a portion of the code i wrote to handle message type 5 (I'm only
handling the "Ship Static and Voyage Related Data
" variant for now). My comments follow the code. Pay attention to my
function H6StrToAsc, and how I manipulate the ascii.

-----------------
Case "5"
MMSI = H6StrToInt(AISString, 9, 38)
ndx = FindAISRecord(MMSI)
If ndx 0 Then Exit Sub 'not found and no room for new record --
discard report! (need to announce)

temp = H6StrToInt(AISString, 39, 40)
Select Case temp
Case 0 'Ship Static and Voyage Related Data
IMO = H6StrToInt(AISString, 41, 70)
ShipTypeStr = ShipType(H6StrToInt(AISString, 233, 240))
EtaMin = H6StrToInt(AISString, 289, 294)
EtaHour= H6StrToInt(AISString, 284, 288)
EtaDay = H6StrToInt(AISString, 279, 283)
EtaMonth= H6StrToInt(AISString, 275, 278)
Draft = H6StrToInt(AISString, 295, 302) / 10
Callsign = H6StrToAsc(AISString, 7, 71)
ShipName = H6StrToAsc(AISString, 20, 113)
Destination = H6StrToAsc(AISString, 20, 303)

Call CleanAt(Callsign) 'need to strip off any trailing '@'
characters some ships have
Call CleanAt(ShipName)
Call CleanAt(Destination)

AISArray(ndx, 0) = MMSI
AISArray(ndx, 15) = ShipName
AISArray(ndx, 16) = Callsign
AISArray(ndx, 17) = Destination
AISArray(ndx, 8) = CStr(EtaMonth) & ":" & CStr(EtaDay) & ":"
& CStr(EtaHour) & ":" & CStr(EtaMin)
AISArray(ndx, 11) = 0 '"Length val"
AISArray(ndx, 13) = Draft
AISArray(ndx, 14) = ShipTypeStr
AISArray(ndx, 18) = vbBLACK 'Plot Color

If AISArray(ndx, 29) = 1 Then Call DoAISLog(ndx) 'first time
a complete record
AISArray(ndx, 29) = AISArray(ndx, 29) Or 2
Call UpdateAISDisplayListElement(ndx)

Case 1 'Extended Ship Static and Voyage Related Data
ErrorReport("Extended Ship Static And Voyage Related Data: "
& AISString)

Case 2 'Aids to Navigation Data
ErrorReport("Aids to Navigation Data: " & AISString)

Case 3 'Regional Ship Static and Voyage Related Data
ErrorReport("Regional Ship Static and Voyage Related Data: "
& AISString)

End Select
-----------------
Sub CleanAt(str) 'used to strip off any trailing '@' characters some ships
leave in their strings
str = Left(str, InStr(str, "@")-1)
End Sub

-----------------
Function H6StrToAsc(str, characters, MSbit)
Dim tempstr, tempchar
Dim i
For i = 1 To characters
tempchar = H6StrToInt(str, MSbit, MSbit + 5)
If tempchar 32 Then tempchar = tempchar + 64
tempstr = tempstr & Chr(Tempchar)
MSbit = MSbit + 7
Next
H6StrToAsc = tempstr
End Function
-----------------

I decided to not convert the raw NMEA ascii string into an intermediate
form, but rather suck the bits out of the string as I need them. My
function H6StrToInt(AISString, starting_bit_position, ending_bit_position)
returns a the value of a particular bit-field.

The function H6StrToAsc(str, characters, MSbit) reads the encoded characters
from the NMEA ascii string, converting them from bits to numbers to
characters. I test the characters (char 32) and map those that are
between 0 and 31 (decimal) to the ascii characters between 64 and 95. I
don't remember exactly how I arrived at this manipulation, but I am pretty
sure I had to just look at the data and figure it out. There is a nice
application on the web that decodes some of the simple AIS strings, and I
probably used that to test my own decoding. Here is the URL:
http://rl.se/aivdm

Once you've got the decoder working, you can use the ITU "Search for Ship
Particulars" website to find out more about a specific ship, given it's
callsign or MMSI or name:
http://www.itu.int/cgi-bin/htsh/mars/ship_search.sh

Back to my code -- I just re-learned BASIC to do this application, after not
using it for over 30 years. I'm sure that my style is horrible, but I doubt
that I will attempt to improve it.

Oh yeah, in my code AISArray is where I keep the AIS records. I don't
remember why I used the intermediate variables instead of storing the data
directly in the array -- this is probably just how the code evolved and I
didn't want to go back and clean it up.

Good Luck!
-Paul







Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is Off
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT +1. The time now is 05:37 AM.

Powered by vBulletin® Copyright ©2000 - 2022, Jelsoft Enterprises Ltd.
Copyright 2004-2022 BoatBanter.com.
The comments are property of their posters.
 

About Us

"It's about Boats"

 

Copyright © 2017