AIS..how accurate is yours???

[JPA Cate]

Average, about 8 mi for sailboats, though one at 21, which was a surprise, sometimes as close as 4 mi., so it's a range and may depend on multiple factors, installation and atmospherics being two.

Ships, routinely 50 n. mi. or more.

Vespermarine, with an internal GPS, Class B, masthead antenna, with splitter.

Our plots of other vessels do not jump around like El Ping's. Long ago, we had a GPS that occasionally gave bad fixes: gave an sog of 651 knots --for a sailboat with a 29 ft. waterline, for instance. Maybe GPS anomalies from their transmitter to your receiver?

Ann

[SaltyMetals]

Anne,
Interesting. When interrogating the AIS info on another boat I often see the same thing, giving crazy speeds or length info. Then after a while it stabilises and gives what looks like reasonable data. I wonder if it is the weakness of the transmitted signal of the vessel I am looking at or it is caused by apparent defects in my own AIS system. (Raymarine splitter and tx/rx ais . ( think that makes it a Class B ).

[Dockhead]

Originally Posted by SaltyMetals

Anne,
Interesting. When interrogating the AIS info on another boat I often see the same thing, giving crazy speeds or length info. Then after a while it stabilises and gives what looks like reasonable data. I wonder if it is the weakness of the transmitted signal of the vessel I am looking at or it is caused by apparent defects in my own AIS system. (Raymarine splitter and tx/rx ais . ( think that makes it a Class B ).

Just to be technically correct -- you don't do any "interrogating" of other vessels' AIS data. It is broadcast. You merely receive it.

Interrogation does take place in DSC position requests. But that has nothing to do with AIS.

[DotDun]

Originally Posted by SaltyMetals

Anne,
Interesting. When interrogating the AIS info on another boat I often see the same thing, giving crazy speeds or length info. Then after a while it stabilises and gives what looks like reasonable data. I wonder if it is the weakness of the transmitted signal of the vessel I am looking at or it is caused by apparent defects in my own AIS system. (Raymarine splitter and tx/rx ais . ( think that makes it a Class B ).

AIS signals are transmitted as digital data, it would be hard to imagine that if the signal was weak you would decipher anything at all. The speed data comes from the GPS SOG whereas the vessel length data is part of the static data stored in the AIS transceiver.

I would guess the erratic location data comes from a poorly placed GPS antenna the AIS transceiver is using. I've seen several installations with the antenna mounted inside the vessel buried behind a nav station with lots of other electrical connections/gear close by. I'm sure anytime another piece of equipment is powered up, the noise near the GPS antenna is pretty high. It makes one wonder what the AIS GPS has for a HDOP.

[DeepFrz]

Marine traffic .com uses land based receivers that decode the signals that are transmitted from the ships.

I was told by a marine electronics expert that marine traffic.Com is relying on land based signals to place your position on a map on an Internet site.

You should get much better performance from your AIS system. However, a class B system only transmits at 2 watts so any inefficiency in the antenna system will cut down your radius of reception.

[dridas]

Question.....

Is transmitted AIS data sent as a digital or analog signal?

[AedanC]

Originally Posted by dridas

Question.....

Is transmitted AIS data sent as a digital or analog signal?

Digital.

[El Pinguino]

Originally Posted by Ann T. Cate

Average, about 8 mi for sailboats, though one at 21, which was a surprise, sometimes as close as 4 mi., so it's a range and may depend on multiple factors, installation and atmospherics being two.

Ships, routinely 50 n. mi. or more.

Vespermarine, with an internal GPS, Class B, masthead antenna, with splitter.

Our plots of other vessels do not jump around like El Ping's. Long ago, we had a GPS that occasionally gave bad fixes: gave an sog of 651 knots --for a sailboat with a 29 ft. waterline, for instance. Maybe GPS anomalies from their transmitter to your receiver?

Ann

They aren't mine, Ann, they are from Marine Traffic.... I don't have the facility to track for those sort of periods

You say your Vesper has 'internal GPS'? Does that mean no external GPS ant at all? I can see that leading to problems.

SaltyMetals doesn't say where he is... if in a marina with high buildings around etc that will cruel his range.

[AedanC]

Originally Posted by El Pinguino

You say your Vesper has 'internal GPS'? Does that mean no external GPS ant at all? I can see that leading to problems.

We have a Vesper XB-6000 and it comes with an internal GPS antenna that works very well in our case. There is an SMA connector on the rear where you can connect an external antenna if you want, but we havn't felt it to be necessary yet.

[DotDun]

Originally Posted by AedanC

We have a Vesper XB-6000 and it comes with an internal GPS antenna that works very well in our case. There is an SMA connector on the rear where you can connect an external antenna if you want, but we havn't felt it to be necessary yet.

How are you judging the quality of the GPS fix on your AIS?

[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.

[AedanC]

Originally Posted by DotDun

How are you judging the quality of the GPS fix on your AIS?

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.

[El Pinguino]

Originally Posted by AedanC

We have a Vesper XB-6000 and it comes with an internal GPS antenna that works very well in our case. There is an SMA connector on the rear where you can connect an external antenna if you want, but we havn't felt it to be necessary yet.

Hmmm, I can see a bit of light here.
On a thread the other day someone was wondering where the builders had hidden his AIS... turned out it was behind a settee in the saloon...... I can see that sort of thing leading to GPS reception issues.

Archimedes, 2 or 300 metres away on the hard, keeps ringing bells on my display.... seems she is going to run me down......

[El Pinguino]

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.

One on the hard, two in the water, no high ground or high buildings nearby.....

[Paul Elliott]

Originally Posted by DotDun

AIS signals are transmitted as digital data, it would be hard to imagine that if the signal was weak you would decipher anything at all.

Yes. The data is sent via FSK (Frequency Shift Keying) and the message includes a 16-bit error-checking field. With this level of error-checking the likelihood of having an undetected error is not zero, but is very low. A received message that fails the error-check is generally discarded. In some advanced systems (such as satellite reception of AIS) this error code is used to try to repair the errors, but I don't think this technique is being used in any of our receivers or transponders.

With my spreader-mounted AIS antenna, and a class-B transponder, I usually am received by other vessels out six to ten miles. I can usually pick up Class-A signals at a 12-mile range, and sometimes out 100 miles. On rare occasions I have received transmissions at a 1000-mile range. However, range much beyond the visual horizon depends on specific propagation conditions and are pretty unusual. I receive Class-B signals out to 6-10 miles reliably.