Marinetraffic.com & AIS: two boats side by side, reported differently?

I just noticed (what I think is) an odd thing about some of the Marinetraffic.com AIS reporting. I've seen instances where a boat in a given harbor is reported via satellite (more info available for $$$), whereas several other AIS-equipped boats in the same harbor are reported by a local Marinetraffic receive/forward station (vessel name and nationality and so forth immediately available).

The boats reported via sateliite are at least as close to the local receive/forward stations as some of the other nearby vessels, in fact sometimes apparently adjacent (within the limits of available display scale). Near as I can tell, it doesn't appear to be a Class A vs/ Class B thing...

Thought maybe somebody here might know how/why that can happen, when I reminded myself of points made in this thread:
http://www.trawlerforum.com/forums/s3/closest-point-approach-46431.html

Points like this, for example:

ranger42c said:

Sometimes it takes a while for our system to resolve an MMSI into a name. Dunno why... Maybe the MMSI transmits every time, name only every other sometimes?

Click to expand...

twistedtree said:

Yes, that's exactly how it works. You "dynamic" data is transmitted regularly updating you position, COG, and SOG. And "static" data is transmitted much less frequently, so the boat's name, dimensions, call sign, etc. can take a while after first contact to populate.

Click to expand...

Any clues?

-Chris

I wonder if the satellite data takes priority and it's only picking up some of the boats?

I dunno. Imagine one boat in a harbor reported by satellite, and 12 other boats being reported by the local receive/forward stations. Looks like the currency is about the same.

-Chris

My guess is that satellite data takes priority and not all AIS traffic reaches the satellite.

From this link (https://www.navcen.uscg.gov/?pageName=AISMessage27), it is unclear if *only* Class A and B/SO transmit this message, but that’s my read. If so, the variance in AIS equipment might explain what you are seeing.

James

Even though there is an algorithm in the transmit system, quite often signals from different boats conflict with each other, especially in crowded locations. The shore receiver can only handle a set amount of data. In the instance you describe the data transmitted may be conflicted going to the local receiver but not to the satellite. Just a guess on my part. You might send your question to the gurus at Marine Traffic and see what they say. In the past when I have had a question they were quick to answer.

Would have thought to do that, but it looks like their "Contact Us" links only go to Facebook and Twitter... and I don't do either of those...

-Chris

Marinetraffic.com & AIS: two boats side by side, reported differently?

I thought all AIS data are sent on the VHF band, not satellite. Are there changes to AIS that are new and exciting that I am unaware of? My AIS is sent by VHF. I know that for sure.

Jim

Try marine-traffic@googlegroups.com

And yes, jdcave, there have been many innovations. All vhf signals go vertical as well as horizontal. Newer satellites have been equipped with receivers that capture the AIS broadcasts. The big issues was figuring out how to separate and differentiate all the signals coming from anywhere in the satellites horizon, which is large. Because of the cost, data retrieved using that method was by subscription only. The main benefit was it allowed vessels that were away from the coast and in areas with no costal stations to be tracked. Great thing if you happen to own some big boats and want to know where they are.

wwestman said:

Try marine-traffic@googlegroups.com

Click to expand...

Thanks, giving that a shot...

-Chris

I think it has to do with whether the shore based station can receive vs whether the AIS is being received by satellite.

It would depend on the power of the AIS transmitter(power - some class B are 2 watt, some are 5) and the antenna (quality/height maybe?) VS the quality of the shore based receiver (which are generally voluntary contributors to the network that providers like marine traffic.com use.)

For example, the nearest shore based receiver to where I keep my boat (south of Anacortes, WA) is located on shore in Anacortes. It loses track of me in the extreme north end of Swinomish Channel. Other boats are lost at different points/distance from the station, indicating different signal strengths/receiving distances from that station affect how or whether it is able to resolve them. Additionally, there are some other boats with AIS on all the time in La Conner (one across the dock from me) that do show up on a shore based receiver. My boat does not, but I think it is showing up on satellite AIS.

By contrast, the satellite receivers have a better chance of picking up all the AIS signals, even if they are weaker, because there are some of the VHF AIS transmissions that go straight up from your antenna to the AIS satellites and are picked up. Of course, those satellites cost $ and they want you to pay to see those. I'm pretty sure they are picking up my boat, because there is an AIS satellite target there on my dock when I leave my AIS on, and not when I turn it off...

I THINK that is what you are seeing.

One thing I've noticed is the time stamp differs. One may say 1' ago and another says 5 hours ago where the signal was lost and the marker just sits there.

When you are up a hundred miles in space, the slot collisions of AIS random traffic in dense areas is significant. Data is not getting through.
You will find it much more tolerant in the wide open spaces at sea.

Microsats at low orbit are being promoted to get the sats lower, and thereby cover less area per sat, thus minimizing time slot collisions.

here is a read: http://cdn2.hubspot.net/hub/183611/file-967076079-pdf/Collateral_for_Download/Satellite_AIS_For_Dummies_E-book.pdf

ranger42c said:

Thanks, giving that a shot...

Click to expand...

No help from that address:

Subject: Delivery Status Notification (Failure)

Hello

We're writing to let you know that the group you tried to contact (marine-traffic) may not exist, or you may not have permission to post messages to the group. A few more details on why you weren't able to post:

* You might have spelled or formatted the group name incorrectly.
* The owner of the group may have removed this group.
* You may need to join the group before receiving permission to post.
* This group may not be open to posting.

If you have questions related to this or any other Google Group, visit the Help Center at https://groups.google.com/support/.

Thanks,

Google Groups

I see instances where there are 8-10 pleasure craft, a couple tugs, sometimes 4-6 passenger vessels, the occasional cargo vessel or tanker... all named and presumably forwarded by one of the local shore stations

And one pleasure craft with "position received by satellite" -- adjacent to some of those others, sometimes apparently within feet of another.

I used to know a bit about radio frequency propagation and so forth... and at these power levels its difficult to imagine a case where a satellite can receive a given signal and a shore station can't -- at these distances -- can't, unless there's some kind of blockage, like maybe a building or something up close and personal to the transmitter. Doesn't seem like a possibility in this case, though, given other boats in near enough proximity to suggest they're at the same dock.

Does Marine Traffic maybe have some kind of "opt out" option, where people can pay to NOT have their vessel info/location repeated on the net?

-Chris

Ranger, that was an old address that they obviously have stopped using. I think your only option would be to try them via Facebook as much as hate having that as an only option. When it first cranked up it was a university project run by a bunch of students. Over the years it has changed and is not as user friendly as it used to be.

Too bad; I expect you're correct they could answer easily enough.

I don't care enough to start doing Facebook, though.

This just started out of curiosity, as we think ahead about our next island vacation. We've been to Nassau, Aruba, Barbados, several of the Leeward Islands, several of the Windward Islands, a couple of the BVIs, and a few places on the Pacific side... so I've been looking at places like Puerto Rico, USVIs, Bermuda, other Bahamas locations as possible destinations... and just noticed the phenomenon along the way.

-Chris

ranger42c said:

I see instances where there are 8-10 pleasure craft, a couple tugs, sometimes 4-6 passenger vessels, the occasional cargo vessel or tanker... all named and presumably forwarded by one of the local shore stations

And one pleasure craft with "position received by satellite" -- adjacent to some of those others, sometimes apparently within feet of another.

I used to know a bit about radio frequency propagation and so forth... and at these power levels its difficult to imagine a case where a satellite can receive a given signal and a shore station can't -- at these distances -- can't, unless there's some kind of blockage, like maybe a building or something up close and personal to the transmitter. Doesn't seem like a possibility in this case, though, given other boats in near enough proximity to suggest they're at the same dock.

Does Marine Traffic maybe have some kind of "opt out" option, where people can pay to NOT have their vessel info/location repeated on the net?

-Chris

Click to expand...

I have been paying closer attention to my home dock - Shelter Bay Marina in La Conner, since you posted this thread. I had intended to turn off my transponder yesterday to verify what I thought was my Sat footprint disappeared from MarineTraffic. I found that I was actually being received by a terrestrial station and was showing up proper, and a boat that also leaves their AIS on at the Marina (True Grit) which is almost always showing up on Marine Traffic as a terrestrial contact was not; it did appear in its spot as a satellite contact though. The terrestrial signal that picks us up in Shelter Bay is somewhere out in the Strait of Juan De Fuca. (I'm not sure where, the Marine Traffic website plots the shore station at 170 meters elevation and in the middle of the channel!) I really think it is due to, like you said, obstructions, differences in antenna/transmitter quality, etc.

Regarding why satellites pick us up, the satellites have a completely unobstructed view from the sky when they pass over (they are in near earth orbit, so they can be close enough to receive signals that escape UP). I'm not sure if they are receiving signals that escape directly 90 degrees up, or at an angle to the horizon. Marine Traffic reports on their website they have received AIS signals from 200 nm away from a high altitude on an island. https://help.marinetraffic.com/hc/en-us/articles/203990918--What-is-the-typical-range-of-the-AIS-

I don't think there is any way to opt out of AIS networks. On Marine Traffic, there appears to be ways that people who are part of AIS reporting networks can opt out (or rather, decline to send their data to) marinetraffic specifically. I have a free user account on Marinetraffic, and have found no way to claim vessel ownership with that account level. Keep in mind too, that most of marinetraffic's contributor's are a wider network of AIS receiver stations that contribute to multiple similar platforms, so trying to opt out of one won't necessarily get you opted out of the others...I've found my vessel on over half a dozen AIS websites/apps. I've chosen to take ownership of the ones that let me build a profile/upload photos so I can at least control that.

The only other way I know of is the little silent switch I didn't wire in when I installed my transceiver....