Cruisers Forum
 


Join CruisersForum Today

Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 22-12-2018, 13:01   #106
cruiser

Join Date: Feb 2012
Location: Half Moon Bay, CA, USA
Boat: 1963 Pearson Ariel, Hull 75
Posts: 1,197
Re: "A" versus "B" AIS - worth transmitting?

Quote:
Originally Posted by tanglewood View Post
Re A vs B power levels, something said earlier bears repeating. Antenna height, and quality of installation will have WAY more impact on your ability to see and be seen, than the power difference between A & B. Like with radar, the horizon will get you way before the power runs out.
Totally true. As long as the signal is a few dB above the noise, power has very little impact on range. Here's a good calculator: Radio Line of Sight Calculator for use on VHF/UHF Ham Bands.

That having been said, a boat can be a very noisy place in terms of radio frequency interference - especially if there are any switching power supplies on board the other vessel (it can be very severe if the other vessel has noisy LED lights). Since you have no control over the RF noise environment at the other (receiving) vessel - more power can make a difference in receivable range on the fringes. Your signal strength drops by the square of the distance (twice as far = 25% the signal strength). But only effects like atmospheric ducting and sporadic meteor trail reflections (ever notice really incredible receiving ranges on some days?) will get you over the VHF horizon.

The better the quality of the installation, the better. The higher the antenna, the better. Once you've run out of height capability, the higher the power, the better (but at this stage you've reached a point of diminishing returns).

Don't expect consistent range beyond the VHF horizon, in any case.
__________________

Cpt Pat is offline   Reply With Quote
Old 04-02-2020, 05:41   #107
Registered User

Join Date: Feb 2020
Posts: 9
Re: "A" versus "B" AIS - worth transmitting?

Hi all!
Sorry for digging out the subject, but I've just stumbled upon a very disconcerting statement in the FAQ section of ONWA, major maker of cheap AIS product :

"Why is it that I cannot see the ship nearby in the Automatic Identification System (AIS) screen, though I see it on the sea?
A: There are two classes of Automatic Identification System (AIS) devices: Class “A” Transponder and Class “B” Receiver. Class “A” transponders are those with built-in VHF transmitter and receiver. This means that the device can transmit and receive vessel information to and from its own vessel. On the other hand, Class “B” receivers are only able to receive vessel information from other vessels which has AIS transmitters. Thus, vessels with Class “B” AIS devices cannot be seen by other vessels, it can only receive information from other AIS devices.
Another scenario would be if the other vessel has no AIS device at all. Then, only the radar can detect its presence on navigational equipment."

...

So according to them, class B is only a receiver? Not a transmitter?

And we're talking about professional AIS makers... but that would explain why their Black Box type B transponder is below $500 NZD! No more than the usual receivers!

Scary though... how many buy this product, convinced they can be seen?

Your opinion?
__________________

Joka is offline   Reply With Quote
Old 04-02-2020, 06:00   #108
Registered User

Join Date: Jul 2005
Location: Bellingham
Boat: Outbound 44
Posts: 8,250
Re: "A" versus "B" AIS - worth transmitting?

Quote:
Originally Posted by Joka View Post
Hi all!
Sorry for digging out the subject, but I've just stumbled upon a very disconcerting statement in the FAQ section of ONWA, major maker of cheap AIS product :

"Why is it that I cannot see the ship nearby in the Automatic Identification System (AIS) screen, though I see it on the sea?
A: There are two classes of Automatic Identification System (AIS) devices: Class A Transponder and Class B Receiver. Class A transponders are those with built-in VHF transmitter and receiver. This means that the device can transmit and receive vessel information to and from its own vessel. On the other hand, Class B receivers are only able to receive vessel information from other vessels which has AIS transmitters. Thus, vessels with Class B AIS devices cannot be seen by other vessels, it can only receive information from other AIS devices.
Another scenario would be if the other vessel has no AIS device at all. Then, only the radar can detect its presence on navigational equipment."

...

So according to them, class B is only a receiver? Not a transmitter?

And we're talking about professional AIS makers... but that would explain why their Black Box type B transponder is below $500 NZD! No more than the usual receivers!

Scary though... how many buy this product, convinced they can be seen?

Your opinion?
Utter nonsense. They clearly have no clue as to the difference between class A and class B AIS. Both are transponders, or if you like transmitter receivers. An AIS unit that is receive only is neither class A or B, it is a recieve only radio.
Paul L is offline   Reply With Quote
Old 04-02-2020, 06:05   #109
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Denmark (Winter), Helsinki (Summer); Cruising the Baltic Sea this year!
Boat: Cutter-Rigged Moody 54
Posts: 28,097
Re: "A" versus "B" AIS - worth transmitting?

Quote:
Originally Posted by Paul L View Post
Utter nonsense. They clearly have no clue as to the difference between class A and class B AIS. Both are transponders, or if you like transmitter receivers. An AIS unit that is receive only is neither class A or B, it is a recieve only radio.

I agree; utter nonsense.


You will be fine with one of the new Class B+ SOTDMA transponders. Does everything you need.


Don't waste your money on a legacy Class B one.


Make sure and install it really well with a dedicated antenna tuned for AIS and really well executed cabling. Don't use a splitter. And you'll be fine.


The one comment in that quote which is correct, however, is that not all vessels transmit AIS, and so AIS is no substitute for radar.
__________________
"Parce que je suis heureux en mer, et peut-tre pour sauver mon ame. . . "
Dockhead is offline   Reply With Quote
Old 04-02-2020, 06:48   #110
cruiser

Join Date: Feb 2012
Location: Half Moon Bay, CA, USA
Boat: 1963 Pearson Ariel, Hull 75
Posts: 1,197
Re: "A" versus "B" AIS - worth transmitting?

Yes, nonsense. Dockhead and Paul L above are entirely correct. Give a wide berth to the source of that statement. Whoever wrote that silliness doesn't know the first thing about AIS.
Cpt Pat is offline   Reply With Quote
Old 04-02-2020, 08:17   #111
Registered User
 
Sailmonkey's Avatar

Join Date: Sep 2009
Location: Houston
Boat: '76 Allied Seawind II, 32'
Posts: 7,930
Re: "A" versus "B" AIS - worth transmitting?

Its utter nonsense.

Ill admit to having purchased an inexpensive plotter from them to replace a backup antique garmin that had died. The plotter does what its supposed to do, but they could never grasp the problem I was having seeing my own vessel in the AIS receiver display. I kept telling them Id like a software update to disallow my own MMSI. They were unable to wrap their heads around me having an independent transponder.
Sailmonkey is offline   Reply With Quote
Old 04-02-2020, 09:07   #112
Registered User

Join Date: Aug 2015
Location: Marina del Rey
Boat: Hunter 31
Posts: 1,141
Re: "A" versus "B" AIS - worth transmitting?

My understanding is that class B transmits every 30 sec, while class A every 2 sec or so. It makes a difference when Im close quarters such as in a marina. Class B vessels lag by 20-30 sec on average, so their position or radar and the AIS display is not the same. If you are in the open sea it makes little difference.
Pizzazz is offline   Reply With Quote
Old 04-02-2020, 09:11   #113
Registered User

Join Date: Jun 2019
Location: Rochester, NY
Boat: Chris Craft Catalina 381
Posts: 947
Re: "A" versus "B" AIS - worth transmitting?

Quote:
Originally Posted by Pizzazz View Post
My understanding is that class B transmits every 30 sec, while class A every 2 sec or so. It makes a difference when Im close quarters such as in a marina. Class B vessels lag by 20-30 sec on average, so their position or radar and the AIS display is not the same. If you are in the open sea it makes little difference.
Keep in mind, those transmit times are the most frequently it'll transmit. Depending on boat speed, it may be less often (transmit gets more frequent as you go faster). Class B SOTDMA (B+) can transmit as often as every 5 seconds (at 23 kts or faster). So not as fast as Class A, but a lot closer.
rslifkin is online now   Reply With Quote
Old 04-02-2020, 14:43   #114
Moderator
 
Jim Cate's Avatar

Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 16,542
Re: "A" versus "B" AIS - worth transmitting?

Quote:
Originally Posted by Pizzazz View Post
My understanding is that class B transmits every 30 sec, while class A every 2 sec or so. It makes a difference when Im close quarters such as in a marina. Class B vessels lag by 20-30 sec on average, so their position or radar and the AIS display is not the same. If you are in the open sea it makes little difference.
Surely you are not using AIS for collision avoidance in a marina, are you? In such situations, visual observation is the only prudent method, so frequency of broadcast is not a factor.

Jim
__________________
Jim and Ann s/v Insatiable II lying Port Cygnet again, freezing our bums off.
Jim Cate is offline   Reply With Quote
Old 04-02-2020, 19:47   #115
Registered User

Join Date: Aug 2015
Location: Marina del Rey
Boat: Hunter 31
Posts: 1,141
Re: "A" versus "B" AIS - worth transmitting?

Actually, the way I discovered how bad the AIS lag was in the marina is when I was trying to dock the boat from inside the cabin, just using cameras. I failed of course (my wheel pilot does not turn fast enough) but learned to always trust the radar first. Sometimes (very rarely in SoCal) you do get dense fog where you cant see to the other side of Marina Del Rey.
Pizzazz is offline   Reply With Quote
Old 04-02-2020, 20:57   #116
Registered User

Join Date: Jul 2005
Location: Bellingham
Boat: Outbound 44
Posts: 8,250
Re: "A" versus "B" AIS - worth transmitting?

Class B AIS transmit their position every 3 mins when traveling 0-2kts.
__________________
Paul
Paul L is offline   Reply With Quote
Old 09-02-2020, 04:15   #117
cruiser

Join Date: Feb 2012
Location: Half Moon Bay, CA, USA
Boat: 1963 Pearson Ariel, Hull 75
Posts: 1,197
Re: "A" versus "B" AIS - worth transmitting?

Quote:
Originally Posted by Pizzazz View Post
Actually, the way I discovered how bad the AIS lag was in the marina is when I was trying to dock the boat from inside the cabin, just using cameras. I failed of course (my wheel pilot does not turn fast enough) but learned to always trust the radar first. Sometimes (very rarely in SoCal) you do get dense fog where you can’t see to the other side of Marina Del Rey.
What? Does your marina have some sort of AIS markers set up? Did they set up AIS AtoNs at your slip? Otherwise, I don't see any application for AIS in docking.

Docking in the fog from inside the cabin using your autopilot and cameras?

Are you an airline pilot by any chance?
Cpt Pat is offline   Reply With Quote
Old 09-02-2020, 05:34   #118
Registered User

Join Date: Jan 2011
Posts: 281
Re: "A" versus "B" AIS - worth transmitting?

Quote:
Originally Posted by tanglewood View Post
Re A vs B power levels, something said earlier bears repeating. Antenna height, and quality of installation will have WAY more impact on your ability to see and be seen, than the power difference between A & B. Like with radar, the horizon will get you way before the power runs out.


Can confirm that. Had a dedicated AIS antenna about 10 feet over deck. Later changed to a splitter and used the VHF antenna 65 feet over deck. The extra height was a big improvment even even with the use of a splitter.
__________________

Oceansailor is offline   Reply With Quote
Reply

Tags
ais

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

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


Similar Threads
Thread Thread Starter Forum Replies Last Post
AIS GME Transmitting not receiving the dude abides Navigation 0 29-10-2018 21:52
Boston Whaler versus Albury versus ??? Magor Powered Boats 3 26-02-2014 12:43
AIS Not Transmitting RProulx Marine Electronics 31 06-05-2011 05:35
AIS - Can I Recieve Permanently but Choose to Switch On / Off Transmitting ? impi OpenCPN 35 17-11-2010 05:38

Advertise Here


All times are GMT -7. The time now is 12:36.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2020, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.