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

AIS Position Error?
 

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.

Same thing onboard of Motion².
I have setup a Raymarine E80 together with Raymarine S3G course
computer (build in rate gyro!) together with AIS (via Easyais receiver)
and all NMEA data of the Seatalk bus via a graphic ST60 display. All
NMEA data flows go via Multiplexer to Bluetooth (and laptop behind it)
as well as to the E80 (at baud rate 38k).
The baud rate is very important due to the amount of NMEA data of the
RateGyro, other NMEA and AIS (more the 200 hits in the Rotterdam area).
The normal 4k NMEA speed is simply to slow to transport all the data.
With the mulitplexer receiving multiple data streams (from S3g and
EasyAis and ST60) at 4k and sending it out at 38k to E80 (and
Bluetooth), it all works without any collisions or loss of data.

Dirk

AIS Position Error?
 

"Paul" wrote in
:


"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

I really don't know much about AIS data and how often ships report, but do
other reports from that ship show it moving in a reasonable manner?

-- Geoff

AIS Position Error?
 

"Meindert Sprang" wrote in
:

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

Meindert


SALESMAN! SALESMAN ALERT!!.....(c;

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

AIS Position Error?
 

"Paul" wrote in
:

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.



Oh, oh....WARNING WARNING....HE'S BYPASSING NMEA ALTOGETHER!

Next thing you know he'll insist all instruments in the whole boat be
Bluetooth....negating needing multiplexers or even wires to make it all
talk to each other!

What a pleasure THAT will be to install! I'm all for it!

I can hear it in the Bluetooth headset telling the helmsman, "Come left to
course 085. May I energize the autopilot for you? Say 'yes' or 'no'."

"Radar, how far is that ship just off the port bow?"...."Eight point four
nautical miles. I'm tracking it for collision avoidance, sir. Please
slack the sheet on the Genoa. I'm detecting it stalling at this relative
wind setting, slowing us down."

No knobs at all at the helm. You just tell it what you want through your
Bluetooth headset.....(c;

--
"Thank you, HAL."..."You're quite welcome, Dave."
There's amazing intelligence in the Universe.
You can tell because none of them ever called Earth.

AIS Position Error?
 

Well Larry,
I just put almost that on an owners new boat.
http://www.tacktick.com/
It's all BT and solarpowered (except the depth sounder needs a battery).
It doesn't quite have the functionality you looking for, but may with
the next firmware upgrade.
Matt Colie

Larry wrote:
"Paul" wrote in
:


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.




Oh, oh....WARNING WARNING....HE'S BYPASSING NMEA ALTOGETHER!

Next thing you know he'll insist all instruments in the whole boat be
Bluetooth....negating needing multiplexers or even wires to make it all
talk to each other!

What a pleasure THAT will be to install! I'm all for it!

I can hear it in the Bluetooth headset telling the helmsman, "Come left to
course 085. May I energize the autopilot for you? Say 'yes' or 'no'."

"Radar, how far is that ship just off the port bow?"...."Eight point four
nautical miles. I'm tracking it for collision avoidance, sir. Please
slack the sheet on the Genoa. I'm detecting it stalling at this relative
wind setting, slowing us down."

No knobs at all at the helm. You just tell it what you want through your
Bluetooth headset.....(c;

AIS Position Error?
 

"Matt Colie" wrote in message
...
Well Larry,
I just put almost that on an owners new boat.
http://www.tacktick.com/
It's all BT and solarpowered (except the depth sounder needs a battery).

Not quite. They do not use BT, that would use too much energy. Tacktick
developed their own very low power radio and protocol for this. It's a shame
they still need a wire to your computer. We made a customized BT multiplexer
for them, but when the prototypes were ready, they bailed out....

Meindert

AIS Position Error?
 

Matt Colie wrote in news:A8%Ig.1676$Wf3.379
@newsfe02.lga:

I just put almost that on an owners new boat.
http://www.tacktick.com/
It's all BT and solarpowered (except the depth sounder needs a
battery).
It doesn't quite have the functionality you looking for, but may with
the next firmware upgrade.
Matt Colie



Lionheart is wifi-powered, herself. The serial port on a Noland
multiplexer goes to a Webfoot serial-to-ethernet interface:
http://www.i****chdogs.com/DataSheets/WF111803.pdf
It's fully DHCP addressable and gets its IP from a Netgear wireless
router's Ethernet port. Up to 256 wifi devices can connect to it, like
The Cap'n running in the nav laptop through the "virtual serial port"
software that came with the Webfoot. This handles the problem of serial
interface only on The Cap'n's archaic interface. With the Wifi laptop,
you can get data off the boat even up in the parking lot of the marina,
half a mile away.

One day I happened to have the laptop in my haulings and stopped by the
marina office to discuss an electrical problem due to salt rot in the
pedestal box. "Which slip is your boat in?", they asked me. I couldn't
remember the number, so whipped out the laptop and brought it out of
hibernation as I knew the system on the boat was running when I left.
Booted the Cap'n and clicked FIND SHIP zoomed in tight and said, "Right
HERE on J-dock, as you can see by the live display from our network.", as
the marina people noticed the boat moving slightly on her GPS string and
the depth changing slightly because the tide happened to be ripping out.

I think we could sail it from the beach or dingy quite a ways away, sorta
"remote control". With this in mind, I WEP protected the router so some
smartassed competitor in a race couldn't take control...not good...(c;

With the wifi link, it all integrates quite nicely to the old NMEA
network, just like it were on a wired serial cable. It's totally
transparent to The Cap'n and the users, who do nothing but turn it on.
Great fun laying on a beanbag with the computer on your lap, beer in one
hand, clicking a new waypoint and calling back to the slaves in the
cockpit, "COMING ABOUT!" as the big wheel spins over by the B&G Pilot
tugging on her rudder post bellcrank.

Look on their faces when it worked the first time?......PRICELESS.

Larry
Third Mate - Bilge Pumps and Electronics

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

AIS Position Error?
 

"Meindert Sprang" wrote in
:

Not quite. They do not use BT, that would use too much energy.
Tacktick developed their own very low power radio and protocol for
this. It's a shame they still need a wire to your computer. We made a
customized BT multiplexer for them, but when the prototypes were
ready, they bailed out....



What a waste when the wifi I installed works so good. It comes with the
virtual serial port for the old nav software to install into the laptop(s).
The Webfoot:
http://www.i****chdogs.com/DataSheets/WF111803.pdf
interfaces your serial port data to the wifi link, automatically, on
bootup. All you have to tell the virtual serial port is what IP the
Webfoot is using, which is always 192.168.0.2 on the Netgear wifi.

Shame they're still draggin' the wire around or some awful-priced custom
radio system. Wifi doesn't use any power and it all runs great off the 12V
battery system.

I'm afraid BT might not have the range through the fiberglass and coring
materials to get through the whole boat. Wifi easily does and is CHEAPER!

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

AIS Position Error?
 

Larry wrote:
Matt Colie wrote in news:A8%Ig.1676$Wf3.379
@newsfe02.lga:


I just put almost that on an owners new boat.
http://www.tacktick.com/
It's all BT and solarpowered (except the depth sounder needs a

battery).

It doesn't quite have the functionality you looking for, but may with
the next firmware upgrade.
Matt Colie




Lionheart is wifi-powered, herself. The serial port on a Noland
multiplexer goes to a Webfoot serial-to-ethernet interface:
http://www.i****chdogs.com/DataSheets/WF111803.pdf
It's fully DHCP addressable and gets its IP from a Netgear wireless
router's Ethernet port. Up to 256 wifi devices can connect to it, like
The Cap'n running in the nav laptop through the "virtual serial port"
software that came with the Webfoot. This handles the problem of serial
interface only on The Cap'n's archaic interface. With the Wifi laptop,
you can get data off the boat even up in the parking lot of the marina,
half a mile away.

One day I happened to have the laptop in my haulings and stopped by the
marina office to discuss an electrical problem due to salt rot in the
pedestal box. "Which slip is your boat in?", they asked me. I couldn't
remember the number, so whipped out the laptop and brought it out of
hibernation as I knew the system on the boat was running when I left.
Booted the Cap'n and clicked FIND SHIP zoomed in tight and said, "Right
HERE on J-dock, as you can see by the live display from our network.", as
the marina people noticed the boat moving slightly on her GPS string and
the depth changing slightly because the tide happened to be ripping out.

I think we could sail it from the beach or dingy quite a ways away, sorta
"remote control". With this in mind, I WEP protected the router so some
smartassed competitor in a race couldn't take control...not good...(c;

With the wifi link, it all integrates quite nicely to the old NMEA
network, just like it were on a wired serial cable. It's totally
transparent to The Cap'n and the users, who do nothing but turn it on.
Great fun laying on a beanbag with the computer on your lap, beer in one
hand, clicking a new waypoint and calling back to the slaves in the
cockpit, "COMING ABOUT!" as the big wheel spins over by the B&G Pilot
tugging on her rudder post bellcrank.

Look on their faces when it worked the first time?......PRICELESS.

Larry
Third Mate - Bilge Pumps and Electronics


I think you may want to go a little farther than depending on WEP for
protecting your network... Netstumbler requires about 1 minute's worth
of traffic to break a WEP key.

bob

AIS Position Error?
 

"Larry" wrote in message
...
Shame they're still draggin' the wire around or some awful-priced custom
radio system. Wifi doesn't use any power and it all runs great off the
12V
battery system.

To say that Wifi does not use any power is a bit steep. For instance, the
Wifo module I'm planning to use in my Wifi mux uses 450mA at 3.3V. Compare
that to a BT module consuming 20mA.
And the RF circuits in the Tacktick units are even more low power, being
solar powered and able to run some 50-60 hours without charge. Their power
budged is so low that they had massive problems when they developed the big
displays with an LCD area compared to the large B&G instruments. Go figu
not enough power to drive the LCD!

I'm afraid BT might not have the range through the fiberglass and coring
materials to get through the whole boat. Wifi easily does and is CHEAPER!

Dunno, the BT we use is a class 1, with a range up to 200 meters. If you
stick a class 1 BT dongle in your laptop too, you'll have about the same
range as Wifi but on a much lower power budget.

A nice detail: the well known sailor Pete Goss used Tacktick on his last
Round Britain race and hooked it up to our BT mux to get the data on hus
ruggedized tablet PC while sitting on the outriggers of his trimaran. Lucky
for us, Tacktick's sales manager told him to use our thingy :-)

Meindert

AIS Position Error?
 

"Geoff Schultz" wrote in message
6...
"Paul" wrote in
:


"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

I really don't know much about AIS data and how often ships report, but do
other reports from that ship show it moving in a reasonable manner?

-- Geoff

Since this was a "ships passing in the afternoon" situation, I only had the
the one encounter with them. It looked like their reported longitude was
more or less correct -- their position was moving from west to east in a way
that matched what I was seeing with my eyes. It was just that the latitude
was consistently off by perhaps a minute.

Someone has mentioned (on the Panbo blog) that they saw a ship in San
Francisco reporting her position as being in the middle of Treasure Island
(a small island in the bay), so perhaps this problem isn't unprecedented. I
still have no idea what could be causing this error, other than an
un-reported GPS failure causing the position reporting to be running in
dead-reckoning mode. That would be two unrelated failures, or a failure and
a design flaw. In any case, I'm definitely going to keep using my eyeball
to verify what the AIS reports.

-Paul

AIS Position Error?
 

"Larry" wrote in message
...
"Paul" wrote in
:

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.



Oh, oh....WARNING WARNING....HE'S BYPASSING NMEA ALTOGETHER!

Next thing you know he'll insist all instruments in the whole boat be
Bluetooth....negating needing multiplexers or even wires to make it all
talk to each other!

What a pleasure THAT will be to install! I'm all for it!

I can hear it in the Bluetooth headset telling the helmsman, "Come left to
course 085. May I energize the autopilot for you? Say 'yes' or 'no'."

"Radar, how far is that ship just off the port bow?"...."Eight point four
nautical miles. I'm tracking it for collision avoidance, sir. Please
slack the sheet on the Genoa. I'm detecting it stalling at this relative
wind setting, slowing us down."

No knobs at all at the helm. You just tell it what you want through your
Bluetooth headset.....(c;


Well, technically I was still using NMEA -- the BT connection was just
substituting for the serial cable.

I agree with the sentiment though, NMEA 0183 is old and slow. I would
prefer WiFi. BT networking is pretty primitive and the port-count
limitations on most devices make a it difficult to create a flexible
multiport system. BT still has the power advantage though, and I hate
wasting Amp-Hours (thus the Pocket PC for my logging application instead of
an always-on laptop).

-Paul

AIS Position Error?
 

RW Salnick wrote in news:ed2bcl$aiu$1
@gnus01.u.washington.edu:

I think you may want to go a little farther than depending on WEP for
protecting your network... Netstumbler requires about 1 minute's worth
of traffic to break a WEP key.

bob



Not a problem. We're dealing with sailor-bankers, sailor-lawyers, sailor-
just-plain-rich-people.....not intensive hackers with banks of computers
and millions of lines of code experience. From my experiences helping them
getting their VHF antenna to radiate, reliably, and some of the other
situations they've asked me to help with, there's no danger at all.

If they can't figure out how to hook up the new starting battery to a
Yanmar, there's little danger they're going to break the WEP code in the
wireless lan any time soon....(c;

Some of the lawyers can't load a flashlight. I've never figured out why
society allows lawyers to make so MUCH money with so little brains. How
stupid.

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

AIS Position Error?
 

"Meindert Sprang" wrote in
:

450mA at 3.3V

450ma x 3.3V = 1.485w at 13V = .114A x 24 = 2.74AH which isn't much a
significant part of a small 330AH house battery, even smaller on the two
banks of beasts we drag around in the stern. 2.7AH a day isn't much of
an issue, is it? The stern running light uses 10-20 times that much
overnight.

PLEASE tell me you're getting the 3.3V from a highly efficient tiny
switching supply....not some 7800 series analog heat sink heater
regulator....please? 450ma x 13V off an analog regulator = 5.85w or
10.8AH/day....not so good....not a catastrophy, but not so good. My
friend Dan's Hatteras 56 got 12VDC to run the toys from the 32VDC train
engine power that ran the 8V92TA beasts-in-the-bilge. Most of the power
went up in smoke because they had 3 HUGE heatsinked analog regulators
wasting the 20 volt difference between the systems, heat created in
hundreds of watts. Of course, with power to burn on this oil burner, it
wasn't much of an issue, but seemed an awful waste as the main salon's
air conditioner had to pull this heat out of the cabin, wasting more
power from the two gensets. Me? I used to tell the pump boys at the
diesel dock to "fill 'er up and don't forgit ta wash the windshields"
(5). It wasn't my money, after all...(c;

We quibble about the tiniest things.....

When I used to work for the Federal government, we had a saying that went
something like:

"In any meeting, the amount of time spent on any line item was
proportional to the inverse square of its cost."

A $2 whazzit took hours to fight over. The $480,000 diggit took 10
minutes, mostly in motions to pass it. I never saw 'em buy something
over $1M, but that might have happened during one of my yawns and I
missed it.

If 2.74 AH/day is too much of a strain on the system, should we REALLY
sail it out of the harbor in the first place??



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

AIS Position Error?
 

"Paul" wrote in
:

I agree with the sentiment though, NMEA 0183 is old and slow. I would
prefer WiFi. BT networking is pretty primitive and the port-count
limitations on most devices make a it difficult to create a flexible
multiport system. BT still has the power advantage though, and I hate
wasting Amp-Hours (thus the Pocket PC for my logging application
instead of an always-on laptop).



Wifi Ethernet transceivers are so cheap there's 4 of them coming out built
right into tiny telephone handsets to hook Skype-in-EEPROM directly to the
router, eliminating running Skype on desktop or laptop completely. I bet a
fully Ethernet-compliant chipset and radio transceiver now costs OEMs less
than $20, maybe even less than $10. Amazing what Asians can do with slave
labor.

The Chinese company making Ipods for Apple was investigated for child labor
abuses. The investigators didn't find anything wrong. They were paying
the help nearly $100/month for six, twelve-hour days a week.

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

AIS Position Error?
 

Larry wrote:
"Meindert Sprang" wrote in
:

450mA at 3.3V

450ma x 3.3V = 1.485w at 13V = .114A x 24 = 2.74AH which isn't much a
significant part of a small 330AH house battery, even smaller on the two
banks of beasts we drag around in the stern. 2.7AH a day isn't much of
an issue, is it?


It wouldn't if it were the only WiFi network hw on the boat;
unfortunately one is not very much use :-) by definition you need two.

The power draw starts to add up if you are going to replace all
NMEA/Seatalk/whatever-bus drivers with WiFi. 3 sensors, 6 displays, 1 PC
adds up to 27 Ah... that's no longer neglible.

--
Kees

AIS Position Error?
 

"Larry" wrote in message
...
"Meindert Sprang" wrote in
:

450mA at 3.3V

450ma x 3.3V = 1.485w at 13V = .114A x 24 = 2.74AH which isn't much a
significant part of a small 330AH house battery, even smaller on the two
banks of beasts we drag around in the stern. 2.7AH a day isn't much of
an issue, is it? The stern running light uses 10-20 times that much
overnight.

Ahem, does a PDA or laptop have a 330Ah battery?
The average battery life of a PDA (Paul said he uses one) halves if you
enable its Wifi. Not so for BT.

PLEASE tell me you're getting the 3.3V from a highly efficient tiny
switching supply

I am, don't worry :-)

Meindert

AIS Position Error?
 

"Larry" wrote in message
...
Wifi Ethernet transceivers are so cheap there's 4 of them coming out built
right into tiny telephone handsets to hook Skype-in-EEPROM directly to the
router, eliminating running Skype on desktop or laptop completely. I bet
a
fully Ethernet-compliant chipset and radio transceiver now costs OEMs less
than $20, maybe even less than $10. Amazing what Asians can do with slave
labor.

The only problem is that Wifi is only cheap if you buy 100k chipsets or
more. Most, if not all Wifi chip vendors won't talk to you or send you a
datasheet if you don't sign a contract first for at least 100k units. So for
low volume applications, Wifi is very expensive. The Wifi modules I use cost
around $100 each, even in 100's.

Meindert

AIS Position Error?
 

Meindert Sprang wrote:
"Larry" wrote in message
...
Wifi Ethernet transceivers are so cheap there's 4 of them coming out built
right into tiny telephone handsets to hook Skype-in-EEPROM directly to the
router, eliminating running Skype on desktop or laptop completely. I bet
a
fully Ethernet-compliant chipset and radio transceiver now costs OEMs less
than $20, maybe even less than $10. Amazing what Asians can do with slave
labor.

The only problem is that Wifi is only cheap if you buy 100k chipsets or
more. Most, if not all Wifi chip vendors won't talk to you or send you a
datasheet if you don't sign a contract first for at least 100k units. So for
low volume applications, Wifi is very expensive. The Wifi modules I use cost
around $100 each, even in 100's.

Meindert



Meindert,

I assume you use these modules in your non-marine products right?

To me having a WiFi multiplexer doesn't make sense, as there is no
"serial profile" defined on top of Ethernet, by my understanding. Even
if there were in my opinion it would more sense to have an (wired)
Ethernet version of your multiplexer instead? A lot of people
considering this type installation might already have a PC/access point
installed, and it would mean higher reliability for users willing to run
wires. People that really want wireless you can sell a $50 access point
to... Ethernet modules are surely a lot less expensive?

The new integrated systems (Furuno, Raymarine, Garmin) also use wired
ethernet, so the infrastructure is getting installed already.

What we need now is a standard for transmitting NMEA and NMEA-2000
(like) data over Ethernet/UDP. Guess that won't happen for a while...

--
Kees

AIS Position Error?
 

If they can't figure out how to hook up the new starting battery to a
Yanmar, there's little danger they're going to break the WEP code in the
wireless lan any time soon....(c;

No, but someone onshore near their boat certainly can. Use WPA and you're
done.

Some of the lawyers can't load a flashlight. I've never figured out why
society allows lawyers to make so MUCH money with so little brains. How
stupid.

You've obviously never needed effective legal service. But hey, play dumb
until you do, then $350/hour to keep you out of prison will seem CHEAP.

AIS Position Error?
 

Miendert,
You are obviously in the position to know.
I just installed all the stuff and it sure looked like BT, but come to
think of it - it did not ever claim to be.

That is all good to know. I'm sure that the owner will be back wanting
something added that will be an issue.

Matt Colie

Meindert Sprang wrote:
"Matt Colie" wrote in message
...

Well Larry,
I just put almost that on an owners new boat.
http://www.tacktick.com/
It's all BT and solarpowered (except the depth sounder needs a battery).


Not quite. They do not use BT, that would use too much energy. Tacktick
developed their own very low power radio and protocol for this. It's a shame
they still need a wire to your computer. We made a customized BT multiplexer
for them, but when the prototypes were ready, they bailed out....

Meindert

AIS Position Error?
 

Paul wrote:
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



I beleive I have read somewhere that professional systems are able to
utilize AIS targets as input for target tracking on a conventional radar
system/screen. Has anyone been able to test anything like this on
consumer equipment? Would it even be technically possible? As Paul
mentiones in his post above it can be somewhat dangerous to rely only on
AIS data, even if one is not using Pauls own software ;)
Being able to push the AIS data into the radar screen would be a nice
way to get both on one single screen (why not chart as well).

I understand that on a platform like Garmin 3010, able to provide chart
+ radar and AIS, the above is somewhat possible since I can get a good
visual check if the radar and AIS target are inline or not. But the
tracking I dont know about. Given the poor AIS support on the Garmin
units at this stage I would guess not.

Cheers
david

AIS Position Error?
 

Kees Verruijt wrote in news:44f57d46$0$4530
:

To me having a WiFi multiplexer doesn't make sense, as there is no
"serial profile" defined on top of Ethernet, by my understanding.

It's terrible, but please don't tell my captain. He thinks it's wonderful,
like I do....(c;



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

AIS Position Error?
 

I beleive I have read somewhere that professional systems are able to
utilize AIS targets as input for target tracking on a conventional radar
system/screen. Has anyone been able to test anything like this on
consumer equipment? Would it even be technically possible? As Paul
mentiones in his post above it can be somewhat dangerous to rely only on
AIS data, even if one is not using Pauls own software ;)

Cheers
david

David,
do you mean something like this? see the pictures he
http://uk.pg.photos.yahoo.com/ph/dba...449re2&.src=ph
As you can see, AIS targets are displayed on the radarscreen of the
raymarine E80. You can activate an AIS sourced CPA alarm together with
a marpa alarm.

Experiences:
To be honest, I most of the time use radar overlay (on a Navionics map)
during our crossing from the Netherlands to Norway. I still use rader,
because:

First: Keep in mind that fishingships don't use AIS yet. So if you only
look for AIS targets, you will miss some importent fishing ships. These
ships always seem to change course suddenly or at the wrong moment.

Second, AIS data is not always correct. For example, If a ship is not
moving, compass data can be wrong when its derived from a GPS. In the
harbour of Rotterdam, there are several ships which are not positioned
right. (bow to the water instead of lying parallel to the kay). What
also happened during our crossing is that AIS targets where lost while
others stayed. Happened in 2 occasions. I guess somebody did a
reboot(?) We also met a ship which was according to the ais data "under
sail" but it was a huge tanker at 20 knt speed. (BTW, I have a 2
channel receiver)

Dirk

AIS Position Error?
 

On Mon, 28 Aug 2006 00:19:10 -0700, "Paul" wrote:

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

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 discovered a problem over a year ago with Furuno GPS units interfaced into
AIS systems. I reported it to the US Coast Guard & they published some info
he

http://www.uscg.mil/hq/gm/moa/docs/1-05.pdf

More info on the fix he

http://www.uscg.mil/hq/gm/moa/docs/furunosafety.pdf

So, next time you chat to them, ask them if they have a Furuno GP80 or GP90
GPS, and if they do, let them know there is a firmware upgrade. I found the
problem initially when installing an AIS base station & seeing about 10% of
the vessels transmitting positions on land when I could see them out the
window. The bust was about 250 metres in my case, and the offset to WGS-84 on
the paper charts that everyone cheated off was 250 metres. I went onboard a
couple of the vessels to check out their systems & needless to say, the
Captains were pretty shocked when I plugged my notebook into their AIS pilot
port & showed them their boat 200m up the shoreline.

Dave

AIS Position Error?
 

Dave Baker wrote in
:

Captains were pretty shocked when I plugged my notebook into their AIS
pilot port & showed them their boat 200m up the shoreline.

Dave



You lawyers reading this....Stop drooling on your keyboards....

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

AIS Position Error?
 

"Dave Baker" wrote in message
...
On Mon, 28 Aug 2006 00:19:10 -0700, "Paul" wrote:

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

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 discovered a problem over a year ago with Furuno GPS units interfaced
into
AIS systems. I reported it to the US Coast Guard & they published some
info
he

http://www.uscg.mil/hq/gm/moa/docs/1-05.pdf

More info on the fix he

http://www.uscg.mil/hq/gm/moa/docs/furunosafety.pdf

So, next time you chat to them, ask them if they have a Furuno GP80 or
GP90
GPS, and if they do, let them know there is a firmware upgrade. I found
the
problem initially when installing an AIS base station & seeing about 10%
of
the vessels transmitting positions on land when I could see them out the
window. The bust was about 250 metres in my case, and the offset to WGS-84
on
the paper charts that everyone cheated off was 250 metres. I went onboard
a
couple of the vessels to check out their systems & needless to say, the
Captains were pretty shocked when I plugged my notebook into their AIS
pilot
port & showed them their boat 200m up the shoreline.

Dave,

Thank you for this information. I don't know what equipment was used on the
ship I encountered, but this is a completely plausible explanation for what
I was seeing. Perhaps I will try to locate the ship owner and send them a
message -- it couldn't hurt!

What port were you in when you discovered this problem?

Regards,
Paul

AIS Position Error?
 

"Kees Verruijt" wrote in message
...
I assume you use these modules in your non-marine products right?

No, I am going to use one of these in a MiniPlex-42Wi.

To me having a WiFi multiplexer doesn't make sense, as there is no
"serial profile" defined on top of Ethernet, by my understanding.

There is, sort of. Telnet is a standard protocol to replace serial
terminals. And I found out that the modules from Lantronix, which come with
a serial port redirector, talk almost standard telnet. If I open telnet on
their IP and port, I see the serial data on my screen.

Even
if there were in my opinion it would more sense to have an (wired)
Ethernet version of your multiplexer instead?

Coming too: the MiniPlex-42E.

The only "problem"so far is that the port redirector software which creates
a virtual com port opens a point-to-point TCP/IP session with the modules
while I like them to talk UDP. That way, any computer on the net can read
the NMEA data and talk to it, instead of just one.

A lot of people
considering this type installation might already have a PC/access point
installed, and it would mean higher reliability for users willing to run
wires. People that really want wireless you can sell a $50 access point
to... Ethernet modules are surely a lot less expensive?

Well, al lot of professional race-sailors like to use Wifi instead of BT
because most tablet PC have Wifo built in standard while BT if often an
option. And Wifi is slightly higher power than class I BT devices.

The new integrated systems (Furuno, Raymarine, Garmin) also use wired
ethernet, so the infrastructure is getting installed already.

What we need now is a standard for transmitting NMEA and NMEA-2000
(like) data over Ethernet/UDP. Guess that won't happen for a while...

Well, let's define one!
It would be interesting though to see how the major brands transfer NMEA
over ethernet. But indeen, UDP is the best way since it is also a
connectionless/broadcast protocol just like NMEA0183 and NMEA2000.

Meindert

AIS Position Error?
 

On Fri, 1 Sep 2006 19:07:49 +0200, "Meindert Sprang"
wrote:

Well, let's define one!
It would be interesting though to see how the major brands transfer NMEA
over ethernet. But indeen, UDP is the best way since it is also a
connectionless/broadcast protocol just like NMEA0183 and NMEA2000.

Agreed. I use UDP in my stuff. It makes it easy to keep the network
"open" in that any device on the subnet can grab the data and use it.
For that reason, I just send the NMEA sentences one at a time, with no
mods or higher level constructs that a listener would have to be aware
of. The only "gotcha" (and it's irrelevant to the vast majority of
uses) is that all the routers I've looked at block UDP broadcasts from
going off the subnet, for good and sufficient reasons. If you want to
broadcast to another subnet you have to target the UDP to a specific
address on that subnet and let it repeat the message as a broadcast on
that subnet. At least, that's the way I solved the problem in my
stuff.

Unfortunately, I've found that most of the potential listeners to the
NMEA data I'm making available are so busy making everything
proprietary that this is less of a benefit than I hoped.


-----------------------------------------------------------------------------------------------------------
Glen "Wiley" Wilson usenet1 SPAMNIX at world wide wiley dot com
To reply, lose the capitals and do the obvious.

Take a look at cpRepeater, my NMEA data integrator, repeater, and
logger at http://www.worldwidewiley.com/

NMEA - WiFi - BT (was AIS Position Error?)
 

wrote in message
...
On Fri, 1 Sep 2006 19:07:49 +0200, "Meindert Sprang"
wrote:

Well, let's define one!
It would be interesting though to see how the major brands transfer NMEA
over ethernet. But indeen, UDP is the best way since it is also a
connectionless/broadcast protocol just like NMEA0183 and NMEA2000.

Agreed. I use UDP in my stuff. It makes it easy to keep the network
"open" in that any device on the subnet can grab the data and use it.
For that reason, I just send the NMEA sentences one at a time, with no
mods or higher level constructs that a listener would have to be aware
of. The only "gotcha" (and it's irrelevant to the vast majority of
uses) is that all the routers I've looked at block UDP broadcasts from
going off the subnet, for good and sufficient reasons. If you want to
broadcast to another subnet you have to target the UDP to a specific
address on that subnet and let it repeat the message as a broadcast on
that subnet. At least, that's the way I solved the problem in my
stuff.

Unfortunately, I've found that most of the potential listeners to the
NMEA data I'm making available are so busy making everything
proprietary that this is less of a benefit than I hoped.


This thread has drifted in a direction that I like a lot. Since my original
AIS-specific question has likely been answered, I believe that the NMEA /
WiFi / BT topic deserves its own thread so it doesn't get lost. Of course I
still look forward to further AIS discussions.

I agree that WiFi and wired ethernet are the direction to go if you want to
build an open nav-electronics system (and I certainly do). BT is probably
better for the extremely power-sensitive applications, such as handhelds and
of course cellphone earphones. Meindert, I would definitely be a customer
for a wired ethernet mux. Of course, I've just got one boat, so I would
only buy one or two muxes.

I remain vigilant when it comes to power drain on my boat. I have three
100W solar panels on board, but depending on the cloud cover, and my point
of sail relative to the sun, I need to run the engine to charge batteries
for about an hour a day -- more if I am using the B&G hydraulic autopilot.
At anchor, the panels essentially keep up with the lower power drain. This
is why I am avoiding running a laptop full-time, and worry about the power
requirements of the electronics.

Of course if I shut off all the junk and just sail, I only have to power the
tricolor and compass light. But I do like my toys (and cold drinks).

-Paul