oakleyb
Well-Known Member
Finally got my NASA 3 wired up and talking to chart plotter, Standard Horizon CP180. I get plenty of targets with COG and speed but no vessel name. Is this correct ?
AIS Targets have no name
- Thread starter oakleyb
- Start date
SimonP85
Well-Known Member
I've seen the same on our SH GX2100E. I believe the COG and speed information is transmitted more regularly than name, destination etc so sometimes the targets lack these details to begin with.
pvb
Well-Known Member
That's correct. Position, course, speed, etc, is transmitted every few seconds, but the vessel name and other static info only gets transmitted every 6 minutes, so will take a while to show up.
oakleyb
Well-Known Member
super, thanks for the replies
pvb
Well-Known Member
As your Nasa AIS 3 isn't a dual channel receiver, it might take longer than 6 minutes for the vessel name to show.
Giblets
Well-Known Member
Ahh, that would explain why we get the MMSI number up well before the name is displayed.
BrianH
Well-Known Member
All receivers have a display delay between target data and Name data for the reason pvb gives earlier, that the name is held in a separate, static data sentence transmitted only every 6 minutes while the target displayed is from the dynamic data sentence transmitted every few seconds. Both sentences contain the relevant MMSI and when the static data eventually arrives, it is matched to the target by MMSI and only then displayed.
The Nasa receivers are not dual, simultaneous channel receivers, which can exacerbate the delay. All AIS transponders automatically alternate transmissions between the two AIS frequencies (VHF 161.975 Mhz and 162.025 Mhz to double the system bandwidth). An alternating frequency receiver such as the Nasa may not be listening on the frequency carrying a transmission and will consequently miss it. For the dynamic data sentence this does not matter as the frequency (time) of transmission is only a matter of seconds and the next transmission will be received when the sender changes to the alternate channel frequency - provided the receiver has not as well changed the listening frequency, in which case he must wait for the next ... However, in the case of the static data sentence sent every six minutes it can often take up to 12 minutes or even longer (by the 6 minute multiple) to be included in the target data.
prv
Well-Known Member
It's pretty poor that NASA are still selling a single-channel AIS receiver nowadays - they must be about the only ones still doing so!
Pete
Pete
pvb
Well-Known Member
Strolls
Well-Known Member
️ That was a wonderful explanation, Barnac1e. ��
BrianH
Well-Known Member
Thanks, [blush]. I like to keep reminding potential AIS receiver buyers to be fully informed and research the market thoroughly. Too many are lured by a slightly cheaper price and a name that has dominated the market too long, when so many other excellent products offer an improved functionality. The technology has truly moved on.
VicMallows
Well-Known Member
This comment is NOT relevant to the OP who is taking the RS232 data directly to a chart-plotter ..... but may be of interest to anyone experiencing lack of the static data (ship name etc) when connected to a PC via a serial-to-usb converter:
My NASA AIS2 was connected to a PC via a serial-usb converter. Dynamic data was displayed correctly by the PC chart plotter software, but no static data was EVER received, no matter how long you waited. Eventually established that the static data sentences being received over the USB were truncated. A new serial-to-usb converter solved the problem. For some reason the old converter could not handle the static sentences. (longer?; insufficient buffer in converter?;timing error?).
My NASA AIS2 was connected to a PC via a serial-usb converter. Dynamic data was displayed correctly by the PC chart plotter software, but no static data was EVER received, no matter how long you waited. Eventually established that the static data sentences being received over the USB were truncated. A new serial-to-usb converter solved the problem. For some reason the old converter could not handle the static sentences. (longer?; insufficient buffer in converter?;timing error?).
pvb
Well-Known Member
Too true! For less than £30 extra, you can get a proper dual channel parallel receiver.
Boo2
Well-Known Member
Yes, I've seen this on my RO4800, vessel name seem to take longer to show up than other info.
<Edit: thanks to Pete and Barnac1e for the explanations of the reason>
Boo2
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BrianH
Well-Known Member
For those using PC plotting and with such difficult-to-trace problems it can be helpful to use an NMEA message display utility and one of the many on-line message protocol description sites.
One solution is the free OpenCPN application - an excellent navigation program in its own right that supports AIS reporting. Under options>connections check the "Show NMEA debug window" box, whereupon a window will open containing the streaming messages that can be compared to the correct format protocol. Invaluable for those frustrating installation times when nothing seems to be working when you can check if the AIS receiver is indeed feeding the messages into the application.
