Missing AIS targets -sometimes

D

daibachsail

Well-Known Member
Joined
28 May 2008
Messages
72
Location
Wimborne Dorset
Visit site
We have noticed on recent cross channel trips that sometimes there are no targets showing (despite many large ships in near proximity) while at others there are large numbers eg 20. This is with same range and target settings and in a short period of time when they could not have moved far. This is with EasyAIS interfaced to Standard Horizon running C-Map.
Recently seen the same phenomena on a yacht in Sweden with exactly the same set-up- some days we had AIS other times none at all!
There apears to be little logic but it is worrying as previously in poor vis we had assumed all were showing!! and were more relaxed than usual with that knowledge.
Any ideas about these phantoms?
 
Missing AIS targets 2

Furthermore you will, I am sure, find that the target on the AIS screen is very often "behind" where it should be ...
I'm not sure of the frequency of transmissions by class "A" transponders..
I use the AIS for advance warnings and the RADAR when the things get to within about 5 miles...
Also think the MARPA plotting on the Radar is entirely under my own electronic kit's control...
Doesn't rely on someone else switching whatever is needed on!!
Cheers, Good sailing, Bob E...
 
daibachsail said:
we had assumed all were showing!!?
Click to expand...

You should never assume the electronics show everything. AIS certainly doesn't show any smaller boats which don't have a transmitter, and even on larger boats you're assuming that their box, wiring and aerial are working and that your box, wiring and aerial are working and also that you are receiving a strong signal from them. Given how crackly voice VHF can be, I wouldn't put too much trust in this being the case - use the AIS by all means but don't look at that until after you've looked and listened up on deck.
 
I find the signal from B class transponders to be a bit intermittent sometimes because of their low power/low antennae but I've never had a problem with A class. For me normal reception is at 20 to 25 miles which can reduce to 15 to 20 miles in poor conditions - heavy rain etc - but in the 4 years I've had the system I've never yet seen a big ship visually that I had not already picked up ages before on AIS and allways exactly where AIS said it was. The exception of course being grey funnel liners and the e-border cutters.

You occasionally hear traffic control telling a ship to turn their AIS on so I guess some ships do leave it off occasionally but the real missing sector are fishing boats who rarely seem to appear.
 
daibachsail said:
We have noticed on recent cross channel trips that sometimes there are no targets showing (despite many large ships in near proximity)
Click to expand...

Have you checked your AIS input? I had a similar problem with a Raymarine C series which intermittently lost AIS sync.
 
haydude said:
Have you checked your AIS input? I had a similar problem with a Raymarine C series which intermittently lost AIS sync.
Click to expand...

What do you mean by the AIS input, it is coming in on the VHF aerial via a proper splitter and the AIS box is showing red flashes which I believe it means that it has detected AIS signals but it is not showing it, sometimes despite showing it and hour earlier!!

What was your conclusion about how it can be intermittent?
 
PeterR said:
I find the signal from B class transponders to be a bit intermittent sometimes because of their low power/low antennae but I've never had a problem with A class. For me normal reception is at 20 to 25 miles which can reduce to 15 to 20 miles in poor conditions - heavy rain etc - but in the 4 years I've had the system I've never yet seen a big ship visually that I had not already picked up ages before on AIS and allways exactly where AIS said it was. The exception of course being grey funnel liners and the e-border cutters.

You occasionally hear traffic control telling a ship to turn their AIS on so I guess some ships do leave it off occasionally but the real missing sector are fishing boats who rarely seem to appear.
Click to expand...

Last week we were within 3 miles of 5 cruise ships and large ferries but none were showing on the AIS!
 
This is interesting. I find targets appear and disappear in a random way from the screen of my NASA. Aerial checks out OK, so I was on the point of ditching the unit and buying another. I might hang on to see if others are getting this problem.
Certainly, some vessels switch off their AIS but probably not many. But that wouldn't explain the random appearance and disappearance, often at quite close range - less than five miles.


(NASA AIS and dedicated aerial at spreader height)
 
oldvarnish said:
This is interesting. I find targets appear and disappear in a random way from the screen of my NASA. Aerial checks out OK, so I was on the point of ditching the unit and buying another. I might hang on to see if others are getting this problem.
Certainly, some vessels switch off their AIS but probably not many. But that wouldn't explain the random appearance and disappearance, often at quite close range - less than five miles.


(NASA AIS and dedicated aerial at spreader height)
Click to expand...

Never had a problem with my nasa. Does what it says every time. Though must admit I don't stare at it continuosly.
 
oldvarnish said:
This is interesting. I find targets appear and disappear in a random way from the screen of my NASA. Aerial checks out OK, so I was on the point of ditching the unit and buying another. I might hang on to see if others are getting this problem.
Certainly, some vessels switch off their AIS but probably not many. But that wouldn't explain the random appearance and disappearance, often at quite close range - less than five miles.


(NASA AIS and dedicated aerial at spreader height)
Click to expand...

I get the same as you describe above, on a Raymarine C80, possibly on about 1 in 10 targets appear and disappear for a few seconds to a few minutes, it can be over both short and long distances, ie 1 to 20 mile, it didn’t do this 6 years ago when first installed.
I also now find that many ships are not inputting all their info; ships name, destination etc. are missing. MMSI and call sign are usually shown and of course the heading and speed which are electronic inputs.
Possibly the novelty of manually setting it all up before leaving port, is now wearing a bit thin.
 
daibachsail said:
What do you mean by the AIS input, it is coming in on the VHF aerial via a proper splitter and the AIS box is showing red flashes which I believe it means that it has detected AIS signals but it is not showing it, sometimes despite showing it and hour earlier!!

What was your conclusion about how it can be intermittent?
Click to expand...
We had 'fun' with our AIS on the last boat - until I diagnosed a fault AIS engine - having tested it when static alongside a known good display - swapping over power and antenna ...

The first port of call for you will be to see if you can see data coming into the unit - the Standard Horizon allows you to look at the data coming into the port - if that is corresponding with the flashes on the AIS engine then it is receiving everything that the engine is passing out ...

I would also do a test bypassing the VHF splitter - either just remove the VHF set and splitter from the loop or put up an emergency antenna and test with that. From a dedicated pushpit mounted antenna I easily get 14Nm range.

The other one to do is to do a factory reset on the SH - you loose all your waypoints and settings, but I did find once that I had to do this as I could see the input from AIS on the SH - interpreted with a laptop (split data), located the transmitter on the laptop but not shown on the SH when it should have been - did a factory reset and then setup the AIS port again and all was fine - it was a bit of a pain having lost the rest of the settings though so don't do unless you've got an identifiable problem with the SH.
 
ksutton said:
I also now find that many ships are not inputting all their info; ships name, destination etc. are missing. MMSI and call sign are usually shown and of course the heading and speed which are electronic inputs.
Click to expand...

I had a similar problem, except that it applied to ALL Class A transmissions (interestingly all data was received on Class B transmissions).
(NASA engine with PC plotting software).

I eventually tracked the problem down to the serial-to-USB adapter I was using. It seems it was not able to cope with the 'long' AIS sentences which contain the static data such as ships name/destination/cargo etc.

It's also the case that the 'long' sentences are transmitted much less frequently than the basic data, so if reception is a bit weak this information could seem to be intermittent. This can be compounded with the NASA because it switches sequentially between the two AIS channels increasing the chance of missing the static data transmissions. Many programs have a setting somewhere to adjust how long the static data should be retained if not refreshed.

Vic
 
Sorry for delay in getting back - I have been locked out!

I would think you have a VHF aerial problem. When we got a new boat 3 years ago we had the same problem as you with our new AIS - intermittent and unpredictable. Turned out it was a fault in the brand new Vitronics aerial at the top of the mast. Once replaced worked fine, and no problems with our AIS since. And we have used it everywhere from Poole / Scilly / SW Ireland / East Coast Ireland / West Scotland / Round Ireland / and right up to Skye.
 
You must log in or register to reply here.

Share this page

Top