AIS..how accurate is yours???

[rognvald]

"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." El Pinguino


Simple and correct logic by El Pinguino. Far too many sailors place unreasonable faith in their electronic aids. This I believe is a problem. Electronic ATON's are only brush strokes that are used to complement the painting. Commonsense and visual aids are needed to render it complete. Good luck and safe sailing.

[dridas]

Originally Posted by rognvald

"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." El Pinguino


Simple and correct logic by El Pinguino. Far too many sailors place unreasonable faith in their electronic aids. This I believe is a problem. Electronic ATON's are only brush strokes that are used to complement the painting. Commonsense and visual aids are needed to render it complete. Good luck and safe sailing.

The problem with this thought is that if you believe that the errors are caused by multipathing, then at sea you should not have that problem. Defining "at sea" as being away from any building or other obstruction that causes the multipathing.

Keep in mind.... Multipathing occurs when a GPS antenna is inside of a structure which causes the signal to be bounced, and in turn delayed.

The GPS sends an atomic time from x location, that is received by X receiver. The amount of time that it takes to travel is calculated as the distance from that satellite. Multiple signals must be received in order to determine location on Earth.

[Paul L]

Originally Posted by rognvald

"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." El Pinguino


Simple and correct logic by El Pinguino. Far too many sailors place unreasonable faith in their electronic aids. This I believe is a problem. Electronic ATON's are only brush strokes that are used to complement the painting. Commonsense and visual aids are needed to render it complete. Good luck and safe sailing.

While I agree with you, it is not much different than saying visual aids are only brush strokes to render it complete. There are plenty of visual aids which are off station. It all comes down to the standard saying " prudent mariners will not rely solely on any single aid to navigation"

[rgleason]

http://www.cruisersforum.com/forums/....php?p=2242522

[El Pinguino]

Originally Posted by dridas

The problem with this thought is that if you believe that the errors are caused by multipathing, then at sea you should not have that problem. Defining "at sea" as being away from any building or other obstruction that causes the multipathing.

Keep in mind.... Multipathing occurs when a GPS antenna is inside of a structure which causes the signal to be bounced, and in turn delayed.

The GPS sends an atomic time from x location, that is received by X receiver. The amount of time that it takes to travel is calculated as the distance from that satellite. Multiple signals must be received in order to determine location on Earth.

Well, some people believe in sky fairies but if my position as transmitted by my AIS was jumping around like a one legged tap-dancer when tied up I would not simply be assuming that all was well at sea.... I'd be anchoring somewhere for a day or two and making quite sure it wasn't doing it when away from any external strucures likely to cause multipathing.

A random marina this morning (Aucklamd).... looked at the track of four yachts...this was the fourth one. The other three were steady as a rock despite being in the same environment.

[Paul L]

Originally Posted by El Pinguino

Well, some people believe in sky fairies but if my position as transmitted by my AIS was jumping around like a one legged tap-dancer when tied up I would not simply be assuming that all was well at sea.... I'd be anchoring somewhere for a day or two and making quite sure it wasn't doing it when away from any external strucures likely to cause multipathing.

A random marina this morning (Aucklamd).... looked at the track of four yachts...this was the fourth one. The other three were steady as a rock despite being in the same environment.

It's possible this caused by the issue I pointed out in post #45

[El Pinguino]

Originally Posted by Paul L

It's possible this caused by the issue I pointed out in post #45

So that would be happening both alongside and at sea?

Not so common on the Hamble but they are there.......

[kmacdonald]

I don't have AIS and don't see the need in it.

[Paul L]

Originally Posted by El Pinguino

So that would be happening both alongside and at sea?

Not so common on the Hamble but they are there.......

Yes, the problem occurs based on geographic area and unit. Most of the Class B electronics are designed by SRT and then re-labeled by OEMs. So th einners of many diferent vendors class B AIS units are the same. I first found this issue because my AIS would occasionally report a lost GPS signal, even when there were many satellites in view. I also would see an occasional position jump when using the anchor watch that was driven off the AIS GPS' signal. After working with the manufacturer to sort it out they came up with the explanation I gave above. Turning off the SBAS support made the GPS positions rock solid again.

[Jim Cate]

Ping, after reading the earlier posts, I did a little experiment whilst tied to the dock and being essentially motionless:

Set up the Vesper Vision in anchor watch mode, and recorded several hours of positions (it places a dot on the screen at regular intervals... not sure how often, but many times a minute as far as i could tell). They moved about somewhat, with a fairly random distribution over a distance of around 20 feet. There was one set of data looking like a breadcrumb trail, moving off in an arc of some 30 feet and then a brief period of random motion around that center, and then a pretty rapid return to the previous centroid.

I also looked at the Marine traffic position report for the same period. It showed a perfect string of positions in a N/S line, moving back and forth slowly over an estimated 40 feet, with one excursion off the the west. This was a single data point, and was not repeated.

I'm not too sure what this proves, but the difference between the two data sets is interesting in that in my case the Marine Traffic display does not seem to reflect reality. The internal GPS in the Vesper unit seems pretty stable, the wi-fi connected display on OCPN agrees with the Vesper display and I don't see how my broadcast position could be different from that data. Conclusion in this case is that Marine traffic does some massaging of the broadcast data... perhaps it is a matter of the resolution with which they display the historical data.

None of this looks like the giant excursions you have presented!

Puzzling, eh?

Jim

[Snowpetrel]

Originally Posted by 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.

A very interesting bit of info, thanks Paul. Is this a common problem for many GPS recievers or just the specific ones inside the common AIS units?

Sent from my SM-G930F using Cruisers Sailing Forum mobile app

[El Pinguino]

Originally Posted by Jim Cate

None of this looks like the giant excursions you have presented!

Puzzling, eh?

Jim

Puzzling indeed.
I'm not suggesting that all boats are doing this...maybe 5%..but some are doing it.

If they were all doing it it would make more sense.

Nothing extreme to be found at Manly... just several like this one... maybe you could pop round and ask what their setup is......

[Jim Cate]

Originally Posted by El Pinguino

Puzzling indeed.
I'm not suggesting that all boats are doing this...maybe 5%..but some are doing it.

If they were all doing it it would make more sense.

Nothing extreme to be found at Manly... just several like this one... maybe you could pop round and ask what their setup is......

Ok, I'll see if I can find them on board. It is the only other overseas yacht here at present... we heard them negotiating clearance with Customs as they approached a while back,but have not seen them on board (a longish walk to where their berth is, so we don't check too often).

cheers,

Jim

[dridas]

Hey Jim. You might ask if there GPS receiver is WAAS (wide area augmentation system) compatible. If they are foreigners, from outside the US, it's possible their GPS is not set up for WAAS. Normal GPS, actually old GPS systems, had an accuracy of 100m.

If I understand WAAS correctly, GPS signals (not all of them) are updated with corrections that account for changes in the ionisphere, which brings accuracy down to a few meters. If the AIS is broadcasting location data that is not predicted, corrected, or accurate, the location will not be stable.

I'm just throwing out guesses here. True accuracy of a GPS is dependent upon many factors, and signals that bounce like that one are either receiving inaccurate time from the satellites, systems are lagged during the calculations, or there simply isn't enough accuracy in the signal being received.

There may be another cause, but without knowing the offending signals setup, it's hard to narrow down why.

[Snowpetrel]

A slight error here, real target is at the red arrow, Ais shows it slightly out of position, for some reason this unit had a bunch of alarms that knowbody (including the techs that installed it) seems to know how to fix. Prehaps its due to this?