BobE
regular
Reged: 15/01/2002
Posts: 542
Loc: Hampshire (West)
|
|
I've got a NASA engine coupled to an Easy splitter and back to the Standard Horizon plotter with a readout on a laptop (SOB v MAX) utilising the C-Map cartridge from the plotter when the laptops in use...(One of those expensive orange card readers from C-Map)
It all now works fine but I note that all the info., which I guess, is supposed to be broadcast by all the "biggies", is not always received...
Generally the vessels name is missing also destinations not shewn..
Of course there still seems to be a significant delay between the vessel sending the info. and me receiving it...
I try to check with the Marpa on the Radar, which is basically delayed by a maximum of one sweep, and there is really a large difference. Makes me wonder if I'd have benn better investing in a fluxgate for the Marpa!!
I'm hoping (weather permitting) to cruise some of North Brittany starting this Friday evening so when I get the wind vane going I'll try to be more specific as to time lags..
Comments???
Cheers Bob E...
|
rickp
regular
Reged: 10/11/2002
Posts: 5450
|
|
AIS has a number of message types, and some are sent more frequently than others. So the important, dynamic information (course, speed, rate of turn etc.) is sent quite often (up to every 30 seconds depending on speed).
The 'static' information, such as name, destination is sent every 6 minutes.
Rick
|
jonjo
regular
Reged: 27/02/2002
Posts: 2347
Loc: Cambridgeshire, UK
|
|
Quote:
there still seems to be a significant delay between the vessel sending the info. and me receiving it...
I doubt there is any radio/software propagation delay in the system. My PC based AIS software Yacht-AIS displays the time since the last AIS squawk for a selected target. Once at sea in shipping lanes I think you will find essential collision avoidance info is broadcast at a high rate i.e. better than once every 10 seconds.
|
Kevin_Mac
regular
Reged: 10/03/2006
Posts: 228
Loc: Poole
|
|
I had those problems with the NASA ais unit, and returned it for a replacement, which worked better but not perfectly, with boats sometimes just melting from view. Judging from my own experiences and by other threads on the subject, I suspect that the NASA units are subject to considerable QC issues, or perhaps design flaws. I suggest you try a replacement unit in the first instance
|
Fireball
regular
Reged: 15/11/2004
Posts: 7941
Loc: Chichester
|
|
The SH plotter shows only the basic info including name, the computer will have to show you the more detailed info like destination ....
You say there is a delay in the ship sending and you receiving - this is not possible - you will receive in the infinitesimally small time it takes for the signal to travel between the ship and your vessel - believe me, you will not be able to measure this with your setup. Are you suggesting that your display shows old information? Is this the case on the SH plotter? I believe the chart plotter is set to remove ships after either 10 or 15 minutes with no update received ...
|
ParaHandy
regular
Reged: 18/11/2001
Posts: 3652
|
|
"I try to check with the Marpa on the Radar, which is basically delayed by a maximum of one sweep, and there is really a large difference."
if you can still do a search on here, there was a post abt 2 years ago by TomE showing an E90 Raymarine display with AIS targets superimposed on radar/marpa acquired targets. The displacement of the two (radar & the same AIS target) was substantial. Tom had calibrated the radar's heading against a fixed navigation mark (Nab Tower if I remember correctly) but I never heard whether a further attempt at calibration corrected this. He was, however, convinced that a much faster compass input was required for MARPA.
Recently I used an E90 with AIS and there was a considerable position discepancy between MARPA and the corresponding AIS target and that, only after a struggle to get the MARPA to lock on. Clearly, the kit isn't happy on a boat gyrating from one side to another.
AIS data also includes the position of the GPS antenna on the transmitting ship. It is a displacement from one or other side and the bow. What your radar is picking up may well not be where the GPS antenna is. This does matter with a large container ship. Some software packages eg shipplotter do take account of this by painting a picture of the ship taken from its dimensions and the aerial offset and it is extremely useful.
|
BobE
regular
Reged: 15/01/2002
Posts: 542
Loc: Hampshire (West)
|
|
All points well taken gents,
I agree that delay in a radio signal from transmission to receipt is unmeasurable (by me anyway) However last trip I could see the big ship by eyeball and the AIS still said he was a long long way away.. The Radar said he was bl.....dy close tho'!!
I know that the Std Hor Plotter only gives basic info but it is supposed to give the vessels name ( as is the S o B program on the Laptop) I get say 20 ships out there on the screen and listed but only say 50% max are named either on plotter or laptop...
Means that if I did want to call them I'd have to use their MMSI i/o name maybe that's good but it takes me forever..
See you in Cherbourg !!
Cheers Bob E.....
|
Fireball
regular
Reged: 15/11/2004
Posts: 7941
Loc: Chichester
|
|
Quote:
However last trip I could see the big ship by eyeball and the AIS still said he was a long long way away.. The Radar said he was bl.....dy close tho'!!
Just goes to show - AIS isn't a reliable primary source for collision avoidance .... did both plotter and laptop agree with the long long way away position or was it not seen on the plotter? Was the ship underway?
It's not unusual for us not to have the name of the vessel - AIS uses 2 frequencies and the NASA AIS engine will receive on one at a time ... so you could quite happily miss the transmission of the ships name once every 6 minutes .... other engines receive both channels at the same time, but these are twice the price .... may be a little more reliable for data though!
|
BobE
regular
Reged: 15/01/2002
Posts: 542
Loc: Hampshire (West)
|
|
Oh and I agree about the Flugate master compas being desirable but sometimes in little wind one uses the autohelm which is controlled by its own fluxgate ( or am I misinformed) anyway I can identify the biggies on the radar fine and eyeball them but the AIS is I'm sure way behind eyes and radar.
It's certainly not direction error that concerns me, ( I can understand the cause of that) but Range error does matter after all they could be doing 25 kts to my 5 !!
Not a large problem but it niggles that the kit is not apparently doing what it's advertised to do..
Cheers Bob E......
|
BobE
regular
Reged: 15/01/2002
Posts: 542
Loc: Hampshire (West)
|
|
Plotter and Laptop usually agree and the ships between Christchurch and Alderney are almost certainly under way!!!
Cheers Bob E...
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
