03.17.2014 10:45
AIS talkers for NMEA VDM/VDO messages
I got an email with this question:
Back before ITU 1371 was opened up, I helped Eric Raymond (ESR) a bit with his writing of AIVDM/AIVDO protocol decoding. I still think that his document is drastically better written than the crap that is in the ITU documents. ESR also has NMEA Revealed with a list of talker ID.
Let's start with a sample message:
Following the talker are 3 letters that define the "sentence type." In the world of NMEA, this is the type of message that you are about to parse. VDO is a type that not many people know about. It says that the message that follows is an AIS message that is about "own ship." This really means that the device is talking about itself. The more commonly observed VDM means "AIS message that the device received over a VHF radio channel." The VDO in the above sample message makes sense as this is an AIS basestation telling us the message 4 (basestation report) for itself. The message has two extra fields on the right: ",b003669730,1249053583". This is the older USCG logging format and has a receiver identifier and UNIX UTC timestamp. The b at the beginning of the receiver name says that the message was logged from a basestation. If we decode that message, we should find that the MMSI of the basestation report should match the receiver name.
This next sample message has "BSVDM". The talker is "BS". This means that we are hearing from a basestation. The logger has added ",r003669717" with an "r" for receiver, but it might just be missconfigured. This message is from my 2006 notes and I was just figuring out AIS and what the USCG was doing back then. I did have a basestation on my desk for a while.
Please, please, please: If you have the opportunity to talk to people at NMEA, IMO, IALA, IHO, ITU, or IEC, tell them that closed / paywalled specifications are the work of the devil (and contradictory to the mission of SOLAS). It's bad enough that people aren't willing to put the discussion leading up to these specs online with their names attached. Can we please at least get ALL specs for these public data standards (because they are mandated by law in many cases) out there for free distribution licenses?!?!?
In my data feed m getting some sentences with packet types I do not recognize. Could you tell me what they mean? !BSVDM !ABVDM I'm also getting multi-sentences with different packet types. Is that allowed? First part can be ABVDM and second part AIVDM.I was just asked in email to explain the first field in the CSV that is the AIS NMEA armored messages. Sadly, the NMEA specification is paywalled by the annoying folks at NMEA so I can't share with you where talkers are defined. The same goes for the IEC standards that define AIS device messages and testing procedures. And I only have draft versions of a few of those that people on the commitees have given me over the years. For example, I have "Maritime navigation and radiocommunication equipment and systems - Digital interfaces - Part 100: Single talker and multiple listeners - Extra requirements to IEC 61162-1 for the UAIS".
Back before ITU 1371 was opened up, I helped Eric Raymond (ESR) a bit with his writing of AIVDM/AIVDO protocol decoding. I still think that his document is drastically better written than the crap that is in the ITU documents. ESR also has NMEA Revealed with a list of talker ID.
Let's start with a sample message:
!AIVDO,1,1,,B,403OvpQuUwgBMJRe6VE5Aai005q8,0*6F,b003669730,1249053583The first two letters after the bang ("!") are the "talker." In NMEA speak, this is the device that generated the text in your local environment. It has nothing to do with any remote AIS devices and what they sent over the VHF radio. Above, we have "AI". This is the classic "AIS" device talker. It is what you get with most AIS receivers and transceivers (they are NOT transponders).
Following the talker are 3 letters that define the "sentence type." In the world of NMEA, this is the type of message that you are about to parse. VDO is a type that not many people know about. It says that the message that follows is an AIS message that is about "own ship." This really means that the device is talking about itself. The more commonly observed VDM means "AIS message that the device received over a VHF radio channel." The VDO in the above sample message makes sense as this is an AIS basestation telling us the message 4 (basestation report) for itself. The message has two extra fields on the right: ",b003669730,1249053583". This is the older USCG logging format and has a receiver identifier and UNIX UTC timestamp. The b at the beginning of the receiver name says that the message was logged from a basestation. If we decode that message, we should find that the MMSI of the basestation report should match the receiver name.
This next sample message has "BSVDM". The talker is "BS". This means that we are hearing from a basestation. The logger has added ",r003669717" with an "r" for receiver, but it might just be missconfigured. This message is from my 2006 notes and I was just figuring out AIS and what the USCG was doing back then. I did have a basestation on my desk for a while.
!BSVDM,1,1,,A,85MwpwQKf7sgdeioePgqI0@Q;F=0q3va1sdLFPCS4wB7PtDMD:62,0*34,r003669717,1165850344And a final sample that has "ANVDM". This was an AIS ATON (aid-to-navigation) device that uses "AN" as the talker:
!ANVDM,1,1,,B,35MnbiPP@PJutdlH2M1sIq:80000,0*10,r003669947,1294012804As for "AB", I was unsure. I did receive some sample data with an AB talker, but I do not know what kind of device was doing the logging. AB might stand for Ais Basestation in someones mind. Looking at NMEA's closed documentation (yes, I paid rediculous amount for version 4.0 of the spec), I found:
AB Independent AIS Base Station AD Dependent AIS Base Station AI Mobile Class A or B AIS Station AN AIS Aids to Navigation Station AR AIS Receiving Station AS Limited Base Station AT AIS Transmitting Station AX AIS Simplex Repeater Station SA Physical Shore AIS StationThere was no "BS" talker in there. Don't ask me to explain the differences between all those codes.
Please, please, please: If you have the opportunity to talk to people at NMEA, IMO, IALA, IHO, ITU, or IEC, tell them that closed / paywalled specifications are the work of the devil (and contradictory to the mission of SOLAS). It's bad enough that people aren't willing to put the discussion leading up to these specs online with their names attached. Can we please at least get ALL specs for these public data standards (because they are mandated by law in many cases) out there for free distribution licenses?!?!?
03.06.2014 21:43
Software Defined Radios
Last time I went looking for Software
Defined Radios (SDR) to play with, the price was in the $300-$500
and up. That is quite different than some of the options we have
today. I just ordered at
NooElec NESDR Mini SDR & DVB-T USB Stick (R820T) w/ Antenna and
Remote Control. It's a R820T tuner IC made by Rafael Micro with
a frequency range of ~ 25MHz to 1750MHz. It apparently can output
2.5M S/s
(million samples per second).