Martin_J
Well-Known Member
ah.. intriguing because A & B presumably refer to the two different frequencies..
Ships transmit so frequently anyway...
Ships transmit so frequently anyway...
Changing AIS transmitted information.
- Thread starter SpottyDog5
- Start date
prv
Well-Known Member
Not the static data they don't, that's only every six minutes. If you miss it a time or two due to your cheap-ass receiver being on the wrong channel when it's sent, that's ten or fifteen minutes of just seeing a worrying MMSI-only target on your display, with no indication of what it is.
For a difference of £20 or whatever it might be, I can't see why anyone would buy the Nasa black box.
(I've just checked, and both Digital Yacht and Comar's basic units appear to be true two-channel receivers (Comar definitely is, Digital Yacht's description is a little vague))
Pete
ShinyShoe
Well-Known Member
So if you aren't fussed about transmitting you should be able to switch of transmission in your existing unit and plug it into the existing aerial and plotter to see what else is out there.
Worth doing as a one off. See how useful it is (at 25kts while dodging ships, yachts and waves), then if it is useful either set it up to easily movable and decide if you want to flick MMSIs each time using software (sounds a pain to me!) or look at a second receiving unit...
Not sure of its value - but i know ribbers who say they've been out and suddenly ended up in fog and have felt it useful. Trouble being it only tells you you are about to hit a 300GT vessel not a little yacht who doesn't have AIS...
Worth doing as a one off. See how useful it is (at 25kts while dodging ships, yachts and waves), then if it is useful either set it up to easily movable and decide if you want to flick MMSIs each time using software (sounds a pain to me!) or look at a second receiving unit...
Not sure of its value - but i know ribbers who say they've been out and suddenly ended up in fog and have felt it useful. Trouble being it only tells you you are about to hit a 300GT vessel not a little yacht who doesn't have AIS...
testmonkey
Well-Known Member
you can download from Internet the ProAIS software and easily change the name, type, length, beam and call sign of the vessel which gets transmitted. What you cannot change is the MMSI. Technically it would be easy to take the transponder and plug it into RIBs antenna and plotter and change the name to be "RIB whatever". But then before proceeding further need to check legalities with OfCom. You could try to fill ship fixed radio license amendment form http://www.ofcom.org.uk/static/archive/ra/publication/ra_info/ra367.htm and update the vessel name to be the name of the RIB. Assuming for a moment that the RIB has no VHF it also does not need call sign so you can update the form accordingly and enter blank in the transponder for call sign bit. If the RIB has handheld VHF that can be operated indendently, maye ship portable radio license can be applied from ofcom.
Now - surely someone will soon point out that it isn't meant to be possible to transfer MMSI's between vessels but that wasn't explicitly mentioned in the ofcom instructions...
Anyhow - the use case brought up in this thread might be something for authorities to consider to be supported. As AIS transponders become cheaper they will soon be like handheld DSC radios - people will want to take them along with them on different boats. Telecoms regulators should probably support - and encourage that as it can increase safety at sea. Maybe Ofcom needs to create a new "Ship portable AIS-transponder" license class, possibly allocate a range of MMSI numbers, just like Ship portable radio license, on file would be licensee contact information, emergency contact. Or maybe the existing ship portable radio license class needs to incorporate use of portable AIS transponders?
For the primary AIS transponder use i.e. collision avoidance at sea vessel name, size, speed, course is what counts right?
Now - surely someone will soon point out that it isn't meant to be possible to transfer MMSI's between vessels but that wasn't explicitly mentioned in the ofcom instructions...
Anyhow - the use case brought up in this thread might be something for authorities to consider to be supported. As AIS transponders become cheaper they will soon be like handheld DSC radios - people will want to take them along with them on different boats. Telecoms regulators should probably support - and encourage that as it can increase safety at sea. Maybe Ofcom needs to create a new "Ship portable AIS-transponder" license class, possibly allocate a range of MMSI numbers, just like Ship portable radio license, on file would be licensee contact information, emergency contact. Or maybe the existing ship portable radio license class needs to incorporate use of portable AIS transponders?
For the primary AIS transponder use i.e. collision avoidance at sea vessel name, size, speed, course is what counts right?
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SpottyDog5
Well-Known Member
Thanks for your comments guys, you've got me thinking...
PilotWolf
Well-Known Member
BrianH
Well-Known Member
Not mistaken at all. Most units now have two actual receivers simultaneously listening on the two frequencies. Many older units (and all NASA ones, even the latest 'Ais Engine 3') were not, they had a single receiver with a dual frequency front end tuner that alternated between the A and B channels (nothing to do with AIS Class A and B transponders), it saved on circuitry. Many of the one-at-a-time alternating channel units were very coy about admitting this fact and merely quoted 'dual channel reception' hoping the potential customer would infer a genuine parallel receiver.
For 7 years I too operated an alternating single channel AIS receiver and sometimes had to wait a very long time to fill in a target's static data. This year I updated to a genuine parallel receiver and the difference was immediately apparent.
On another thread I once commented on the advantages and received enough appreciative PMs to prompt me to repeat:
Both Class A and B transponders automatically alternate transmissions between the two AIS frequencies (VHF 161.975 Mhz and 162.025 Mhz), which doubles the system bandwidth. An alternating frequency receiver may not be listening on the frequency carrying a transmission and will consequently miss it. For the Class A dynamic data sentence this does not matter as the frequency (time) of transmission is only a matter of seconds for a sender that is moving and the next transmission will be received when the sender changes to the alternate channel frequency - provided the receiver has not as well changed the listening frequency, in which case he must wait for the next ... or the next.
But Class B dynamic sentences are only sent every 30 seconds*, so missing one (or even two with worst case synchronisation) can cause Class B targets to jump across the screen. Even worse is the case of the static data sentence sent every six minutes for both Class A and B. It can often take up to 12 minutes or even longer (by the 6 minute multiple) to be included in the target data. Not good for wanting to call an impending threat by name on VHF to ask intentions ... but then, one shouldn't do that - as I have been reprimanded for so doing on this forum.
* For Class B dynamic data transmissions, the 30 second frequency applies to vessels with a speed of 2-14 knots. However, for vessels moving at 14-23 knots the frequency is halved to 15 seconds and at greater than 23 knots reduced to every 5 seconds. Even so, a fast planing motor yacht target will, with an unsynchronised receiver, jump spasmodically along its screen track.
But Class B dynamic sentences are only sent every 30 seconds*, so missing one (or even two with worst case synchronisation) can cause Class B targets to jump across the screen. Even worse is the case of the static data sentence sent every six minutes for both Class A and B. It can often take up to 12 minutes or even longer (by the 6 minute multiple) to be included in the target data. Not good for wanting to call an impending threat by name on VHF to ask intentions ... but then, one shouldn't do that - as I have been reprimanded for so doing on this forum.
* For Class B dynamic data transmissions, the 30 second frequency applies to vessels with a speed of 2-14 knots. However, for vessels moving at 14-23 knots the frequency is halved to 15 seconds and at greater than 23 knots reduced to every 5 seconds. Even so, a fast planing motor yacht target will, with an unsynchronised receiver, jump spasmodically along its screen track.
