AIS..how accurate is yours???

[DotDun]

Originally Posted by AedanC

How far it wanders from the mooring on MarineTraffic when it's at home. Not a judge of accuracy at all really, just an uncalibrated judge of repeatability.

That fair, but I'm wondering if using the internal GPS antenna is susceptible to electronic noise when you are underway, i.e., other gear turned on, radio transmissions, radar, etc.

My AIS/GPS antenna is rail mounted, but I'm re-working that area of the boat and was thinking about bringing it inside. Hence, this is a topic of interest to me.

[Paul Elliott]

Originally Posted by DotDun

That fair, but I'm wondering if using the internal GPS antenna is susceptible to electronic noise when you are underway, i.e., other gear turned on, radio transmissions, radar, etc.

My AIS/GPS antenna is rail mounted, but I'm re-working that area of the boat and was thinking about bringing it inside. Hence, this is a topic of interest to me.

I've got a powerboat with a Garmin Class-B transponder, sharing the VHF antenna with a built-in splitter. I mounted the GPS antenna under the fiberglass deck, and it has been working quite reliably. I've left it running while tied up, and have captured my track while underway, and I've seeing no evidence of abnormal position jitter. There's always some, even on my sailboat with an externally-mounted AIS GPS, but it's on the order of +/- 20 feet (approximate guess).

Obviously your results will depend on any deck coring, interfering metal topside, and nearby noisy electronics. For a while I had a GPS antenna mounted belowdeck on my sailboat that gave me less than perfect results. There was thick plywood coring, an aluminum track nearby on the deck, and stanchions close by. All this probably didn't help.

[AedanC]

Originally Posted by DotDun

That fair, but I'm wondering if using the internal GPS antenna is susceptible to electronic noise when you are underway, i.e., other gear turned on, radio transmissions, radar, etc.

My AIS/GPS antenna is rail mounted, but I'm re-working that area of the boat and was thinking about bringing it inside. Hence, this is a topic of interest to me.

I can't really add a lot to that as we rarely transmit on our radio and our radar only comes on when visibility is poor. The only things regularly transmitting apart from the AIS would be our mobile phones trying to check in with masts, which would be intermittent and probably not that frequent. When sailing if it was out by 10m I probably wouldn't notice. I can say that with all the other nav instruments on and a tablet PC working about 30cm away from the Vesper I don't notice our recorded tracks jumping around. The Vesper feeds one of the plotters and it's position is nice and stable. It's a fibreglass boat by the way and there is nothing metal above the Vesper.

We have another, 2001 vintage, plotter and it's position is derived from a rail mounted antenna, I don't notice any difference in stability between the two.

Some day if I'm bored on the mooring I might connect up the GPS antenna that came with the Vesper and see if it makes any difference, but I'd have to be quite bored!

[Jim Cate]

In answer to several above posts;

The Vesper Vision AIS we use has an internal GPS antenna and circuitry. OUr unit is mounted on the coaming under the dodger. Sensitivity seems quite acceptable,but as mentioned above, one can attach an external antenna if required, as perhaps in the case of a below decks installation on a metal boat. I do not know if it receives any other than the US GPS satellites... should ask them!

I judge the consistency of position by observing the track displayed in the anchor alarm mode. The patterns are quite consistent with the boat's movement around the anchor, even over several day's time. Any position that was in error by more than the alarm radius would cause an alarm, so this is evidence of long term repeatability. In places with the appropriate scale charts, we are shown as being in the correct pen in a marina,, so I reckon the absolute accuracy is within a few meters under normal conditions.

The SOG/COG info we broadcast should be identical to that shown on the instrument screen, and that seems accurate compared to other instrumentation. this is not a fool proof test, but the best that I can do.

As to range, we normally see ship class A at around 30 mile minimum, frequently 45-60 miles, and occasionally at extreme ranges: one day saw all the traffic around Cape Leeuwin whilst at anchor near Hobart, Tasmania, a distance of around 1500 miles! We use our masthead normal VHF antenna with a Vesper splitter. As Ann said, performance with other class B vessels is spotty, likely due to poor installations. Good ones show up at 8-10 miles. We've never done a rigorous test on our receivability, but have been seen by friends at similar distances.

So, in answer to Ping's original question, I think our AIS is pretty damn accurate, certainly accurate enough to enable decent CPA calculations by a receiving vessel. Our ability to maintain course and speed is a far bigger source of error IMO!

Jim

[Telemark43]

I'm using a Vesper xb 8000 and their splitter to my antenna atop the mast at about the same height. I receive (and can be seen) for fifty (50 Miles) and can get fixed objects transmitting at about 75 miles. Your system via the antenna or otherwise is faulty. I would expect more than 30 miles on a regular basis.

[DotDun]

Originally Posted by Jim Cate

In answer to several above posts;

The Vesper Vision AIS we use has an internal GPS antenna and circuitry. OUr unit is mounted on the coaming under the dodger. Sensitivity seems quite acceptable,but as mentioned above, one can attach an external antenna if required, as perhaps in the case of a below decks installation on a metal boat. I do not know if it receives any other than the US GPS satellites... should ask them!

I judge the consistency of position by observing the track displayed in the anchor alarm mode. The patterns are quite consistent with the boat's movement around the anchor, even over several day's time. Any position that was in error by more than the alarm radius would cause an alarm, so this is evidence of long term repeatability. In places with the appropriate scale charts, we are shown as being in the correct pen in a marina,, so I reckon the absolute accuracy is within a few meters under normal conditions.

The SOG/COG info we broadcast should be identical to that shown on the instrument screen, and that seems accurate compared to other instrumentation. this is not a fool proof test, but the best that I can do.

As to range, we normally see ship class A at around 30 mile minimum, frequently 45-60 miles, and occasionally at extreme ranges: one day saw all the traffic around Cape Leeuwin whilst at anchor near Hobart, Tasmania, a distance of around 1500 miles! We use our masthead normal VHF antenna with a Vesper splitter. As Ann said, performance with other class B vessels is spotty, likely due to poor installations. Good ones show up at 8-10 miles. We've never done a rigorous test on our receivability, but have been seen by friends at similar distances.

So, in answer to Ping's original question, I think our AIS is pretty damn accurate, certainly accurate enough to enable decent CPA calculations by a receiving vessel. Our ability to maintain course and speed is a far bigger source of error IMO!

Jim

Thanks!

You gave me a good idea. I can run anchor alarm on a Furuno plotters, I'll switch it to use the AIS GPS from the N2k network and see it there is any dancing at anchor overnight.

[cal20dennis]

My AIS receive only is found inside a Standard Horizon VHF radio. We depend on the small screen, several years older technology as an invaluable source of "more help" while making passages across the Sea of Cortez as well as along the California coastline. Imagine receiving a readout detailing other vessels underway in your "possible collision" area clearly indicating courses, speed, closest point of approach....Who really cares how far away a neighbor boat is located in a marina slip?
I urge all cruising sailors (especially for night passages) to buy and learn how to use AIS.. This technology is fantastic! We also believe in radar and chartplotters, as well as deptsounders, radios and eyesight as on watch gear. Best wishes and be careful!

[El Pinguino]

Originally Posted by cal20dennis

My AIS receive only is found inside a Standard Horizon VHF radio. We depend on the small screen, several years older technology as an invaluable source of "more help" while making passages across the Sea of Cortez as well as along the California coastline. Imagine receiving a readout detailing other vessels underway in your "possible collision" area clearly indicating courses, speed, closest point of approach....Who really cares how far away a neighbor boat is located in a marina slip?
I urge all cruising sailors (especially for night passages) to buy and learn how to use AIS.. This technology is fantastic! We also believe in radar and chartplotters, as well as deptsounders, radios and eyesight as on watch gear. Best wishes and be careful!

Hi, its not about being in a marina.... its just that the marina is a good location to observe these issues.

The issue is that if their AIS is transmitting false postion info in the marina there is ever reason to believe that it is doing the same when they are at sea...which is a worry.

I have no idea why they are behaving like this and have no chance to ask about the installations... the owners being 'out of towners'.

[DotDun]

Originally Posted by cal20dennis

My AIS receive only is found inside a Standard Horizon VHF radio. We depend on the small screen, several years older technology as an invaluable source of "more help" while making passages across the Sea of Cortez as well as along the California coastline. Imagine receiving a readout detailing other vessels underway in your "possible collision" area clearly indicating courses, speed, closest point of approach....

Imagine affording the same to that other vessel by allowing them to see your course, speed, and CPA via a transmitting AIS....??

Originally Posted by cal20dennis

Who really cares how far away a neighbor boat is located in a marina slip?
I urge all cruising sailors (especially for night passages) to buy and learn how to use AIS.. This technology is fantastic! We also believe in radar and chartplotters, as well as deptsounders, radios and eyesight as on watch gear. Best wishes and be careful!

I urge all cruising sailors considering AIS to buy a transceiver. A receive only unit does not contribute to the AIS system.

[dridas]

Originally Posted by Paul L

If the boats are on the hard you can have some multipath error, where Sat signals get bounced off of buildings/landmasses and take a different path to the receiver than directly from the Sat.

I would add that if boats are near hard surfaces or structures this can be a major contributing factor. In fact, this is the one of the best reasons I've seen posted in this thread.

Not likely caused by a bad signal, digital signals carry checksums, so if it isn't received correctly, it's discarded.

The other thing that I might add is that if A signal is lost, then the accuracy could be affected, but GPS will usually only calculate with 3 signals.

The multipath makes sense. For every 1 nanosecond that the signal takes to be received, the accuracy could be off by 12 inches per nanosecond. That can add up quickly. GPS's calculate the amount of time a signal takes to be received in order to determine distance from the satellite, which is then triangleated from other satellites to determine location and altitude.

[Snowpetrel]

Originally Posted by Paul L

If the boats are on the hard you can have some multipath error, where Sat signals get bounced off of buildings/landmasses and take a different path to the receiver than directly from the Sat.

I've seen this multipathing (at least thats the best explaination) a few times on a few boats around cliffs or ice, particularly with poorly sited antenna's. Position jumps up to half a mile or so for up to 30 seconds, then jumps back.

I have also heard from reliable sources of ships steaming around with a constant substantial position error showing on AIS, maybe datum or some other user setting like antena offset ? Not much beats the mk1 eyeball, or the mk 2 upgrade (a good pair of binoculars!)

[Paul Elliott]

Originally Posted by Snowpetrel

I have also heard from reliable sources of ships steaming around with a constant substantial position error showing on AIS, maybe datum or some other user setting like antena offset ? Not much beats the mk1 eyeball, or the mk 2 upgrade (a good pair of binoculars!)

I saw this problem in person, back in 2006 I think, when a large ship passed me one mile to the north. It's AIS showed it about a mile to the south of me. I was down at the navstation at the time and called up to the crew to keep an eye on that ship to the south. I was quite surprised when I came on deck to sort out the confusion.

I discussed this with a friend at the Coast Guard and he told me that in some older nav systems the GPS position is modified by the plotter to compensate for differing chart datums. Unfortunately this modified position was also being sent to the AIS transponder. Warnings were sent out to shipowners, etc, about this problem, and probably most have corrected it. I've not seen this problem since then, but then again I only saw it once so it must not have been very common.

[CaptTom]

Another common error I've seen is inaccurate data entry by recreational boaters with Class B transceivers. They might show up on AIS as a 42-meter commercial vessel, but turn out to be a 42-foot recreational vessel. For some reason that 40-50 foot range seem to be the worst offenders.

[El Pinguino]

Its not just a local issue.... here is another one in Melbourne, Orstralia.

I'm not getting bent out of shape over this issue, I'm putting most of my time into researching the quality of various rums, however I did find three in two different marinas ( edit.. that was out of a sample of about 10 boats) while waiting for the kettle to boil this morning.

To narrow down the cause someone needs to take out an offending boat and monitor it while it sits at anchor for a week.

[Paul L]

Another interesting error occurs when the SBAS information (position augmentation) is received by an AIS box from beyond the area where it is valid. In NZ/Aus you can receive Japanese SBAS constellation, MSAS. The info is not valid for the area and can cause the fixes to jump significantly and the fix to be occasionally lost. Disabling SBAS fixes this issue. We picked up this problem when we got south of the equator and mid-way through the Pacific. It was a problem along the NZ coast.