AIS Class-B Transponder and GPS Receiver

[continuouswave]

I notice that at least two manufacturers of AIS Class-B transponders participate in these discussion, and they would be an excellent source of information on my question:

Must an AIS Class-B transponder have its own dedicated GNSS or GPS receiver? Or, can it use input from an external GNSS or GPS receiver provided by some serial-data connection like NMEA-0183 or NMEA-2000?

[nigel1]

Simple answer is that a Class B Transponder must have a dedicated GPS antenna.
Without a dedicated GPS antenna, the AIS will not be compliant. The antenna is a must if the unit is to comply with its type approval certificate.

[continuouswave]

Originally Posted by nigel1

Simple answer is that a Class B Transponder must have a dedicated GPS antenna.

I am making the inference that the GPS ANTENNA, as distinct from a GPS receiver, must be dedicated for AIS because the antenna location is described in relation to the boat's fore-and-aft and athwartship dimensions as part of the data being sent by the vessel in its AIS transmission.

It seems that most (perhaps all) AIS Class-B transponders contain an internal GPS receiver and provision to connect an external antenna. My corollary question is thus:

If I have an AIS Class-B transponder, can I configure it to use the position data from another GPS receiver and use that other GPS receiver's antenna location as my "dedicated antenna"?

[goboatingnow]

Originally Posted by continuouswave

I am making the inference that the GPS ANTENNA, as distinct from a GPS receiver, must be dedicated for AIS because the antenna location is described in relation to the boat's fore-and-aft and athwartship dimensions as part of the data being sent by the vessel in its AIS transmission.



It seems that most (perhaps all) AIS Class-B transponders contain an internal GPS receiver and provision to connect an external antenna. My corollary question is thus:



If I have an AIS Class-B transponder, can I configure it to use the position data from another GPS receiver and use that other GPS receiver's antenna location as my "dedicated antenna"?

No, it's a requirement of the type approval and ITU/IMO specification that the unit has a dedicated GPS receiver.

Dave


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[jeffrobbins]

The GPS receiver is a included in the type approval and Class B units are required to have their own dedicated GPS receiver.

As part of the certification process, the GPS system, including both GPS receiver and manufacturer's matching external antenna (except like Vesper Marine that have an internal antenna) is tested against accuracy and immunity to interference specifications, among a number of other things.

As a result, tests are also performed to verify the unit does not transmit position data received from external NMEA GPS sentences.

However, you can use the GPS data generated by the AIS with other equipment via NMEA 0183 or 2000 interfaces, if the AIS unit provides this capability.

[goboatingnow]

It's of course worth pointing out and to some extent crossing my own post, that non compulsory fit vessels dot have to have a type approved AIS. Hence there's technically no reason to not have one with no internal gps. Bit the cost these days of adding a GPS receiver is negligible

Dave


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[Paul_DY]

The technical reason why a Class B transceiver/transponder must have its own GPS receiver is for timing purposes. It uses the high accuracy GPS clock to synchronise its detection of the AIS time slots which are only 26mS in length and so timing is very critical.

Strangely enough the Class A transponders can be connected to a separate GPS unit as they organise and synchronise the timeslots but the 'carrier sense' system used by Class B requires that they listen for a free time slot before transmitting and therefore they need the more accurate timing.

This technical requirement for its own GPS was then written in to the type approval specification and that is why you will always find that a Class B transponder has its own GPS receiver and either an external or internal antenna.

[Paul_DY]

Hi Dave, in answer to your post, any AIS that transmits in the VHF range will run foul of international and national radio agencies if it does not meet the approval standards, so you will not find any manufacturer developing a Class B transponder without a GPS.

[jeffrobbins]

Originally Posted by Paul_DY

The technical reason why a Class B transceiver/transponder must have its own GPS receiver is for timing purposes. It uses the high accuracy GPS clock to synchronise its detection of the AIS time slots which are only 26mS in length and so timing is very critical.

Strangely enough the Class A transponders can be connected to a separate GPS unit as they organise and synchronise the timeslots but the 'carrier sense' system used by Class B requires that they listen for a free time slot before transmitting and therefore they need the more accurate timing.

Indeed timing is critical Paul, but, class B's sync to class A's, taking into account propagation delay, so they don't require their own GPS time pulse. Class A's have a similar way they can sync to other class A's or base stations if they don't have their own source of precise GPS time sync.

[goboatingnow]

An internal gps is not required for a " functional " class B transponder. In fact IEC 62287-1 clearly lays out the allowed degradation path which allows operation with external supplied gnss information.

The internal gps is required to build a IEC 62287-1 compliant transponder , not to supply internal timing

Dave


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[El Pinguino]

I think they now have to comply with 'IEC 62287-2:2013(E) ( which) specifies operational and performance requirements, methods of testing and required test results for Class B "SO" shipborne AIS equipment using Self-organised TDMA (SOTDMA) techniques as described in Recommendation ITU-R M.1371. This standard takes into account other associated IEC International Standards and existing national standards, as applicable. The main differences between Class B "CS" (IEC 62287-1) and Class B "SO" units are that the Class B "SO":
- covers all 25 kHz channels listed in Recommendation ITU-R M.1084-5;
- only uses the internal GNSS, no position sensor input is allowed;
- requires use of VDL Message 17 for correction of the internal GNSS;
- has a presentation interface;
- has additional reporting intervals, down to 5 s;
- has two power settings, with a high level of 5 W;
- has the capability to transmit binary messages. It is applicable for AIS equipment used on craft that are not covered by a mandatory carriage requirement of AIS under SOLAS Chapter V.'

[El Pinguino]

The way I read the above is that prior to IEC 62287-1 being replaced by -2 in the middle of 2013 they could have external input.

[El Pinguino]

More stuff.

I have an AMEC Camino 101 , 1 year old, the brochure AMEC CAMINO-101 Class B AIS transponder states :-
'Designed to Internationally-recognized Standards
IEC62287-1 Class B, IEC61162-1, EC61162-2
This Class B transponder is approved for use in the USA by the US Coast Guard and the FCC.'

and

'Ability to use an external GPS source instead of the internal GPS system
The CAMINO-101 can now use an external NMEA GPS data feed instead of using its internal GPS. This allows the user to interface the CAMINO to a standard chartplotter or NMEA GPS and use the GPS signal from the external device. Simply connect the NMEA output from the other GPS device to the NMEA input on the CAMINO and make sure both devices are set to use the same baud rate. An external antenna must not be connected when the CAMINO is powered up in order to use the external GPS function. For more information see our blog post here.'

The manual states...

NOTE: 1. ........

3. If user wants to use additional external GPS source the following points shall be
assured: (i) the external position source is using WGS-84 datum; (ii) the accuracy of the external GPS source should be shorter than 10 m in LAT or LON; (iii) the external position is within 26 m of the internal position.'

I don't know why you would bother though as the optional GPS Ant only costs $10

[goboatingnow]

Originally Posted by El Pinguino

I think they now have to comply with 'IEC 62287-2:2013(E) ( which) specifies operational and performance requirements, methods of testing and required test results for Class B "SO" shipborne AIS equipment using Self-organised TDMA (SOTDMA) techniques as described in Recommendation ITU-R M.1371. This standard takes into account other associated IEC International Standards and existing national standards, as applicable. The main differences between Class B "CS" (IEC 62287-1) and Class B "SO" units are that the Class B "SO":
- covers all 25 kHz channels listed in Recommendation ITU-R M.1084-5;
- only uses the internal GNSS, no position sensor input is allowed;
- requires use of VDL Message 17 for correction of the internal GNSS;
- has a presentation interface;
- has additional reporting intervals, down to 5 s;
- has two power settings, with a high level of 5 W;
- has the capability to transmit binary messages. It is applicable for AIS equipment used on craft that are not covered by a mandatory carriage requirement of AIS under SOLAS Chapter V.'

No to my knowledge there are no class b SOTDMA units on the market , all are csma , 62287. -2 does not supersede 62287-1 one is SOTDMA the other is csma


Dave


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[El Pinguino]

Ahh... understood, ( I think) . I just looked through the specs of the 6 that Milltech sell and all still appear to l be -1. That now makes sense.