AIS Marine traffic v. what I am receiving aboard - beginner expectations

F

FairweatherDave

Well-Known Member
Joined
28 Sep 2009
Messages
2,089
Location
Solent
Visit site
Having had reception problems with my antenna which I hope are now sorted I am trying to work out what I should expect to see on my tablet.
Q 1. If I go onto Marine Traffic of course I see tons of boat movements. What surprises me is they continue to show positions that have not been updated for 22 hours, next to other boats transmitting a couple of hours or minutes ago. I can't believe that the boats suddenly stopped transmitting mid passage but maybe they do?
Q 2. I have a rail mounted antenna (approx a meter long) for the moment. Presuming line of sight is required I'm not going to see much until I leave Chichester and get into open water. I'm looking forward to seeing what range I get. If anyone wants to suggest what I might see I am interested. Obviously boats that were transmitting 22 hours ago won't be visible so the picture won't be as exciting as Marine Traffic. I'm curious if I'll be able to pick up the Bembridge entrance bouys but I don't expect to.

Equipment is a Quark A 026 receiver with a shiny new Metz antenna
 
"Q 1. If I go onto Marine Traffic of course I see tons of boat movements. What surprises me is they continue to show positions that have not been updated for 22 hours, next to other boats transmitting a couple of hours or minutes ago. I can't believe that the boats suddenly stopped transmitting mid passage but maybe they do?
Read more at http://www.ybw.com/forums/showthrea...ard-beginner-expectations#SQhTTbdmtoqDIS87.99"

AFAIK Marine Traffic use amateur shore based stations to receive AIS signals - so what they show depends on someone ashore picking up the signal. So the ship would still be transmitting, it's just that Marine Traffic's contributors aren't receiving the signal. I often see boats apparently sailing right through the middle of Anglesey on Marine Traffic!
 
In the Solent you will rapidly discover to switch the AIS off as so many leisure boats feel the need to transmit in such a limited expanse of water that the system becomes useless.
Get further afield, say mid channel, and I would hazard you will pick up commercial shipping at 10 miles or so.
I am assuming your data is overlaid onto a plotter, and you are not trying to use MarineTraffic?
 
Absolutely!! That's the stupid thing. I have barely used my VHF ever, 99 % of my sailing is Solent based and I thought getting AIS in advance of a potential first channel crossing via my tablet would be fun and a bit of security. But my mast head aerial has clearly had a problem, and so has my old VHF and I have ended up with a new aerial on the rail and a new VHF. I'm desperate just to get it all checked out in real life, see my AIS works properly and then switch it off and go sailing! No expectation of using Marine Traffic out at sea.
 
FairweatherDave said:
Q 1. If I go onto Marine Traffic of course I see tons of boat movements. What surprises me is they continue to show positions that have not been updated for 22 hours, next to other boats transmitting a couple of hours or minutes ago. I can't believe that the boats suddenly stopped transmitting mid passage but maybe they do?
Click to expand...

On Marine Traffic it's also possible to report your position via their OnCourse app which reports your position to their service using your mobile device GPS and mobile data network. These will of course only show up online though and not on other proper antenna based AIS receivers on ships etc and obviously only works when the user has mobile coverage. I suspect these users are also responsible for the reported positions on land etc where they've forgotten to turn it off while driving home or similar!

Marine Traffic online does seem to take a while to age out targets but it's fairly easy to see how long ago the position was reported.
 
I fitted a new Metz antennae to my pushpit for my Quark receiver last winter. In James Watt Dock I was receiving AIS transmissions from 6-7 miles away, out near Holy Loch and up Gareloch. This range was as good as it got on the passage from JWD to Skye in April. Generally well pleased with the setup. (Though crossing from Ballycastle to Islay I failed to pick up anything from two big ships in the traffic separation zone. )
 
Thanks Blue_mischief that's what I am expecting for the rail mounted aerial (Salty John calculates just over 8 miles for two yachts with rail mounted antenna). Were the two big ships in the separation zone over seven miles away? I was hoping the extra height of those ships should allow greater range for the AIS.( My plan at some point is to remount the aerial at the top of the mast)
 
FairweatherDave said:
Thanks Blue_mischief that's what I am expecting for the rail mounted aerial (Salty John calculates just over 8 miles for two yachts with rail mounted antenna). Were the two big ships in the separation zone over seven miles away? I was hoping the extra height of those ships should allow greater range for the AIS.( My plan at some point is to remount the aerial at the top of the mast)
Click to expand...
That's the strange thing, one was probably less than a mile away, the other probably three or four. On all other occasions the Quark+ipad performed well.
 
Marine traffic can often give misleading info. For example I have just entered our boat name and it shows we left port 1st Oct. 2017, but also shows an icon heading for port with last info recd. by them 30th May 2018, and we have been in port since 30th. (AIS switched off).��
M.
 
Some examples from the northern Adriatic.

My Metz antenna is also mounted on the aft rail next to the outboard bracket but too close to a wind generator mast that shields my AIS transmissions to receivers on that bearing line.

Aft Config.jpg

The northern Adriatic is limited with contributor receivers to Marine Traffic and the nearest to my marina in NE Italy is some 25nm distant at Koper in Slovenia, shown bottom cente-right in the OpenCPN screen-shot below:

AIS OCPN 04-07-2017.jpg

The following two track reports for the same day that I made that screen-shot, are from Marinetraffic and Vesselfinder respectively. Note that reports are missing from Marinetraffic while on starboard tack (wind 180° 15kts) with the port side significantly lowered by heeling, as well as shielded by the wind-generator mast. Vesselfinder also missed most, although not all.

Vesselfinder, unlike Marinetraffic, does not report where the contributor receivers are located but in my experience is consistently better at detecting and displaying target reports throughout the Mediterranean.

AIS MT 04-07-2017.jpg


AIS VF 04-07-2017.jpg
 
Last edited:
Thanks Barnac1e. Those are interesting screenshots along with your interpretations. I've learnt a fair bit from this thread about internet AIS. Hadn't thought much about it previously so I'm glad I mentioned the Bembridge entrance bouys. Thanks Blue_mischief for the extra bit, really looking forward to seeing how my Quark performs with the big ships between Chichester and Bembridge.
 
I get 7 miles range easily and some details at 15 miles on occasion with a rail mounted VHF aerial. Very very useful in crossing shipping lanes in fog and saved the navigator from complete panic. Its also useful when assessing car transporters 5 miles off that cannot swerve whatever as it enables you to judge their course better. At close quarters (as in Solent) I drop range to 5 or even 1 mile to kill the clutter as the nearer boats are going to hit me first.
 
Marine traffic is useful for playing with in your study at home and finding out where your mates have got to. Its useless for navigation as it cannot be relied on. As to the range of real AIS I reliably receive ships at 20 to 25 miles range with a masthead arial. Yachts I normally pick up at 7 to 10 miles. You can of course get freak conditions when these distances increase 10 fold or more. A friend of mine who has just got a new set claims to reliably pick up ships at 100 miles or more. I am not sure if he just has a much more sensitive set or he is picking up signals from relay stations.
 
I used to have the AIS antenna mounted on the pushpit but it suffered from shielding in several directions. Put in a splitter (its rx only) to use the masthead antenna and the range, number and speed of reception for targets increased about ten fold.

Also worthy of note that, in the Med, a tropospheric boundary effect often extends the range out to 200+ miles especially in the evening.
 
RobbieW said:
I used to have the AIS antenna mounted on the pushpit but it suffered from shielding in several directions. Put in a splitter (its rx only) to use the masthead antenna and the range, number and speed of reception for targets increased about ten fold. .
Click to expand...
That shielding has got me thinking. One would hope at some point a boat AIS signal will get picked up when the antenna is no longer shielded. My question is how often do transponders ping out their messages? Apologies for the basic question but why not ask:)
 
FairweatherDave said:
That shielding has got me thinking. One would hope at some point a boat AIS signal will get picked up when the antenna is no longer shielded. My question is how often do transponders ping out their messages? Apologies for the basic question but why not ask:)
Click to expand...

It varies but in principle 10 secs (Class A) to 30 secs (Class B) but see this more detailed summary, http://arundale.com/docs/ais/ais_reporting_rates.html (which may or may not be up to date, it was just the first hit in Google search)
 
Thanks for the link Robbie, that's interesting stuff. I do plan to move my antenna to the top of the mast, the RG8 cable I purchased is long enough for when the time comes, and I already have a Glomex Active Splitter for the reasons you have given. It is organising getting up the mast that is the issue (Plus a little anxious that the fatter cable might be hard to get moused inside the mast.)
Will google "tropospheric boundary effect " at some point! :)
 
You must log in or register to reply here.

Share this page

Top