AIS Position Error?
 

Has anyone seen a ship position as reported by AIS being off by over one
nautical mile?

I was recently sailing from Hawaii to San Francisco, and encountered a
freighter that eventually passed about one mile north of us. We were both
heading east. The strange thing is that their position as reported by their
AIS transmitter showed them passing about one mile to the south of us! I am
using my own AIS program, so I assumed at the time that there was a bug in
my code. I captured the raw NMEA data (an option in my program), and after
looking at it, and running it through some commercial programs, it seems
that my code was OK and the ship was reporting its position incorrectly.

I've seen bad data show up in the "static and voyage related data" messages,
but until now haven't seen bad latitude / longitude data. The reference
position offset fields only allow for up to 63 meters of beam offset, so
that couldn't account for the position error I was seeing. Since the
"position report" message should get it's position directly from a GPS unit,
I can't understand how an error of this magnitude could occur. I have not
seen any similar errors with other ships. I will be digging deeper into the
captured data to look at the "position accuracy" and "time stamp" fields to
see if perhaps the ship was in some sort of dead reckoning mode or was
reporting other position-fixing errors.

I guess that this underscores the need for a visual or radar confirmation
during a close AIS encounter! Obviously the calculated CPA was quite
different from the actual.

FYI, here is the ship data my program saved:

Time: 8/4/06 3:01:52 PM
Name: LADY MADONNA
Callsign: 3EKW8
Latitude: 40.728833 deg
Longitude: -152.034833 deg
SOG: 12.2 kt
COG: 86.0 deg
Destination: CEDROS_MEXICO
Ship Type: Cargo ship
Ship Status: Under Way using engine
MMSI: 352730000

Any thoughts?

Thanks,
Paul

AIS Position Error?
 

"Paul" wrote in message
...
Has anyone seen a ship position as reported by AIS being off by over one
nautical mile?

I was recently sailing from Hawaii to San Francisco, and encountered a
freighter that eventually passed about one mile north of us. We were both
heading east. The strange thing is that their position as reported by
their AIS transmitter showed them passing about one mile to the south of
us! I am using my own AIS program, so I assumed at the time that there
was a bug in my code. I captured the raw NMEA data (an option in my
program), and after looking at it, and running it through some commercial
programs, it seems that my code was OK and the ship was reporting its
position incorrectly.

I've seen bad data show up in the "static and voyage related data"
messages, but until now haven't seen bad latitude / longitude data. The
reference position offset fields only allow for up to 63 meters of beam
offset, so that couldn't account for the position error I was seeing.
Since the "position report" message should get it's position directly from
a GPS unit, I can't understand how an error of this magnitude could occur.
I have not seen any similar errors with other ships. I will be digging
deeper into the captured data to look at the "position accuracy" and "time
stamp" fields to see if perhaps the ship was in some sort of dead
reckoning mode or was reporting other position-fixing errors.

I guess that this underscores the need for a visual or radar confirmation
during a close AIS encounter! Obviously the calculated CPA was quite
different from the actual.

FYI, here is the ship data my program saved:

Time: 8/4/06 3:01:52 PM
Name: LADY MADONNA
Callsign: 3EKW8
Latitude: 40.728833 deg
Longitude: -152.034833 deg
SOG: 12.2 kt
COG: 86.0 deg
Destination: CEDROS_MEXICO
Ship Type: Cargo ship
Ship Status: Under Way using engine
MMSI: 352730000

Any thoughts?

Thanks,
Paul

OK, I looked at the "position accuracy" and "time stamp" fields, and these
indicate that everything is in high-accuracy mode. What could be going on
here?

Thanks,
Paul

AIS Position Error?
 

"Paul" wrote in
:

Since the
"position report" message should get it's position directly from a GPS
unit, I can't understand how an error of this magnitude could occur.
I have not seen any similar errors with other ships.

If his GPS continues to report to the AIS transponder, even though it has
gotten confused from lack of signal or some idiot in engineering blinking
the power on and off causing it to reset, I can see that happening. Ever
notice how the GPS stores the last position it could get before it lost the
signal? That position, I'm sure, is what it's sending on NMEA out so the
GMDSS has the last valid data for the HF/VHF distress signal as she rolls
over.

I think his GPS was unlocking on loss of signal or had a power glitch from
the generator room.

--
There's amazing intelligence in the Universe.
You can tell because none of them ever called Earth.

AIS Position Error?
 

"Paul" wrote in news:44f298f3$0$34535
:

Name: LADY MADONNA
Callsign: 3EKW8


Of course, in our usual mode of "strict radio silence", which I will never
understand, we could have CALLED 3EKW8 on Channel 16 or 13 and had a chat
with the mate on duty to ask him why his AIS was transmitting the wrong
position, right??

Did he not answer your call?? If not, did you try his MMSI on Ch 70? That
usually wakes them up, these days.

I'm sure the Lady Madonna doesn't bite. One first mate tried to trade
Lionheart for his 950' containership, but I told him my VISA card couldn't
feed it at the fuel barge...(c; I always call them late at night on 13.
It keeps both of us awake. They're usually bored to tears.

--
There's amazing intelligence in the Universe.
You can tell because none of them ever called Earth.

AIS Position Error?
 

"Larry" wrote in message
...
"Paul" wrote in news:44f298f3$0$34535
:

Name: LADY MADONNA
Callsign: 3EKW8


Of course, in our usual mode of "strict radio silence", which I will never
understand, we could have CALLED 3EKW8 on Channel 16 or 13 and had a chat
with the mate on duty to ask him why his AIS was transmitting the wrong
position, right??

Did he not answer your call?? If not, did you try his MMSI on Ch 70? That
usually wakes them up, these days.

I'm sure the Lady Madonna doesn't bite. One first mate tried to trade
Lionheart for his 950' containership, but I told him my VISA card couldn't
feed it at the fuel barge...(c; I always call them late at night on 13.
It keeps both of us awake. They're usually bored to tears.


Larry,

Actually, we had a nice chat on channel 16. They said they were picking up
"soap" at Cedros Island, then heading back to Japan. Perhaps he meant
nitrates, rather than soap? Lady Madonna is a bulk carrier. Anyway, the
language barrier made it seem unlikely that I would have been able to carry
on a technical conversation. Also, by the time I noticed the position
difficulty we had signed off, and I didn't want to bug them for what I
thought was an error in my own software.

We usually try to hail passing ships when on the high seas, since we can go
for over a week between sightings. Sometimes they answer, other times I
think they don't want to wake up the English-speaking crewmember. Having
AIS give me the ship name does make it more likely that they will respond
when I call.

As for the possibility that their GPS had lost lock, after my original
posting I looked at the message details in my AIS NMEA capture file, and the
data fields that should be used to indicate "position fixing" problems show
a high-accuracy fix during the encounter. Also, my own GPS status was good.
Obviously something was wrong, though.

Regards,
Paul

AIS Position Error?
 

Paul wrote:
Has anyone seen a ship position as reported by AIS being off by over one
nautical mile?

snip
Any thoughts?

Thanks,
Paul



This is really off the wall but is there a possibility that the datum
being used was that far off? Isn't the LAT/LON calculated for the chart
datum?
Jim

AIS Position Error?
 

"Paul" wrote in news:44f3193c$0$34553
:

"soap" at Cedros Island,

Wonder if the bulk soap was a hold full of washing powder or liquid
detergent. Man, what a mess if THAT leaked!...(c;

400,000 litres of dishwashing liquid, 12' seas and a 50 knot gale. Would
that clean the beach?....(c; I bet there'd be no tar balls from the oil
tanker leak 3 days ago!



--
There's amazing intelligence in the Universe.
You can tell because none of them ever called Earth.

AIS Position Error?
 

"Jim" wrote in message
. ..
Paul wrote:
Has anyone seen a ship position as reported by AIS being off by over one
nautical mile?

snip
Any thoughts?

Thanks,
Paul

This is really off the wall but is there a possibility that the datum
being used was that far off? Isn't the LAT/LON calculated for the chart
datum?
Jim

I don't think so, since the chart datum isn't an issue unless the positions
are being shown on a chart (whereas my display is similar to a radar
screen). Even so, my boat's position relative to the ship's position should
be displayed correctly regardless of any datum discrepancies. What I've got
is a situation where a ship that is physically to the north of me is
transmitting a latitude that is to the south of me.

I'm not completely ruling out cockpit error or bad code on my part, but I
can't find it and the raw data seems to exonerate me. If anyone wants to
help figure it out, here is the minimum NMEA data capture that shows the
situation. The first line is my position, and the second is the AIS message
from the ship:

$M2RMC,225040,A,4038.518,N,15149.375,W,5.6,072.5,0 40806,014.5,E,D*6
!AIVDM,1,1,,A,15@HsT001wE8wopG@0K5:3=N0@L6,0*6

The "$M2" has been substituted to indicate which multiplexer port the data
came from.
My position is 40.642433 deg (N), -151.820917 deg (W)
The reported ship position is 40.630167 deg (N), -151.822000 deg (W)
The range and bearing to the ship are 0.7NM, 183.8 deg true

Trust me, the ship was actually to the north. I have photos!

-Paul

AIS Position Error?
 

"Paul" wrote in message
...
$M2RMC,225040,A,4038.518,N,15149.375,W,5.6,072.5,0 40806,014.5,E,D*6
!AIVDM,1,1,,A,15@HsT001wE8wopG@0K5:3=N0@L6,0*6

The "$M2" has been substituted to indicate which multiplexer port the data
came from.

Hey, that's a nice feature. Smells like a MiniPlex to me....

Meindert

AIS Position Error?
 

"Meindert Sprang" wrote in message
...
"Paul" wrote in message
...
$M2RMC,225040,A,4038.518,N,15149.375,W,5.6,072.5,0 40806,014.5,E,D*6
!AIVDM,1,1,,A,15@HsT001wE8wopG@0K5:3=N0@L6,0*6

The "$M2" has been substituted to indicate which multiplexer port the
data
came from.

Hey, that's a nice feature. Smells like a MiniPlex to me....

Meindert

Yup, a Bluetooth one at that! I still need to send it in for the AIS port
upgrade though. The MiniPlex has been really nice to have on board. I can
use my PocketPC (running my custom software) to wirelessly interface to my
nav gear. I use it to display and keep a running log of the interesting
NMEA data. At the moment I am using a second bluetooth channel to connect
the AIS receiver to the PocketPC, but I hope that I with the upgrade I can
run everything through the MiniPlex.

-Paul