RTL-SDR + OpenCPN on a mac

[rgleason]

I mean Jeff Robins. Sorry .

[NahanniV]

Originally Posted by bdbcat

Rick...

Just ordered one. Will let you know how it goes.

I have been studying the interaction between simple one-channel AIS receivers and most Class B transponders, and the results are a bit concerning, if my data and observations are true.

I have been using an SDR for spectrum analysis, and a dAISY programmed for single-channel-only mode to look at some real life data.

1. I find that most (all?) single channel receivers only switch channels when they detect an error on receive, usually due to transient noise.

2. I also find that many Class B transponders transmit on only one of the authorized TX channels, most of the time. They are supposed to alternate channels for "periodic repeated" messages, but they don't all seem to do this.

Result: If I am listening on Channel 1, and your transponder is stuck on Channel 2 for a long time, I may never see you. Or it may take a very long time (multiple minutes). This problem is exacerbated by the long reporting period and subordinate network access methods of Class B transponders.

So, in short, I'm beginning to conclude that the utility of Class B for inshore work to be compromised and sometimes dangerously misleading for collision avoidance. This does not mean that the utility for offshore long-range collision avoidance is null. It just means that one should not rely exclusively on AIS-B for close-in navigation.

But we knew that already....


Food for thought...
Dave

I think statement: "the utility of Class B for inshore work to be compromised and sometimes dangerously misleading for collision avoidance" is quite misleading.

It seems to me that the problem you outline is really the fault of substandard one channel receivers.

In my opinion, SDR AIS receivers are a fun experiment, but nothing more, never to be trusted for navigation.

Also, there is no reason for a single channel receiver, hopefully they will disappear from the market.

If buying a receiver be sure to get a dual channel, not multiplexed. They are the same price or less as inferior ones. More importantly, consider a transponder, you will probably buy one eventually.

Cheers,
JM.

[astuder]

I don't want to hijack this unrelated thread, so feel free to respond over here:
http://www.cruisersforum.com/forums/...er-154962.html

Originally Posted by bdbcat

1. I find that most (all?) single channel receivers only switch channels when they detect an error on receive, usually due to transient noise.

dAISy hops after a successful message, message with error, or ~100ms without activity. It was channel hopping faster in the original design, but there is some overhead per hop which reduces the number of messages received. You may have noticed that the number of messages received per minute increases when channel hopping is turned off.

While theoretically hopping could increase the number of messages received if you're smart and really fast, which some competitors claim. I haven't seen that working in practice yet.

Originally Posted by bdbcat

2. I also find that many Class B transponders transmit on only one of the authorized TX channels, most of the time. They are supposed to alternate channels for "periodic repeated" messages, but they don't all seem to do this.

Interesting observation and a strong argument against turning off hopping. I will have to add a note to my documentation.

[rgleason]

PerhapsI should not have posted the dAISy HAT in this thread, but that's where dAISy was posted originally as a very good contender and in my opinion, a better solution than rtldsr, with channel jumping single reciever. The HAT is two channel!
Thank you Adrian, for your thoughts. Its clear that the quality of the gps matters, the programming and speed of the processor also aftect 2 channel switching response.
I think bdbcat's observations are useful considerations in purchase and use.

[jeffrobbins]

Originally Posted by bdbcat

2. I also find that many Class B transponders transmit on only one of the authorized TX channels, most of the time. They are supposed to alternate channels for "periodic repeated" messages, but they don't all seem to do this.

I'm not sure which Class B's you've tested, but for Class B's that are certified by CE, USCG, FCC, etc they will alternate transmission channels for both dynamic and static reports. A Class B might not transmit if there is very high RSSI or high Class A traffic on the channel (among a few other reasons but not likely to be your situation) but in that case the reporting period will be skipped.

[bdbcat]

Jeff...

Thanks for the comments.

I agree with your comments regarding SDR receivers, and the shortcomings of single-channel receivers in general. But integrated single channel receivers are real, and widely installed in the field. So that makes them worthy of at least a clear understanding of their performance and behavior.

I remain interested in the fact that Class-B transponders may skip report periods randomly (due to noise), and single channel receivers choose their reception channel according to non-deterministic logic and/or conditions specific to the individual receiver's firmware. To me, this means that Class B position/velocity reports from single channel receivers should be viewed with just a little bit of suspicion.

As do you, I hope more 2 channel receivers come to market at competitive prices, especially for those not contemplating installing a transponder for whatever reason.

So Adrian, thanks for bringing us the 2 channel dAISy HAT.

Thanks again
Dave

[jeffrobbins]

Originally Posted by bdbcat

I agree with your comments regarding SDR receivers, and the shortcomings of single-channel receivers in general. But integrated single channel receivers are real, and widely installed in the field. So that makes them worthy of at least a clear understanding of their performance and behavior.

I was referring to transmit from Class B transponders in my previous post. But single channel receivers, regardless of their switching time, will miss transmissions. Transponders receive simultaneously on both channels and must be able to receive without error at least 90% full slot utilization on both channels which exceeds real-world scenarios.

Originally Posted by bdbcat

I remain interested in the fact that Class-B transponders may skip report periods randomly (due to noise), and single channel receivers choose their reception channel according to non-deterministic logic and/or conditions specific to the individual receiver's firmware. To me, this means that Class B position/velocity reports from single channel receivers should be viewed with just a little bit of suspicion.

Class B selects its transmission slots based on listening to the signals on each channel and selecting candidate slots that haven't been previously reserved. It then listens to determine if other traffic may be using the same candidate slot. To determine this it must establish the background noise signal level but when the noise level is quite high it can't determine if the slot is available. However, this rarely occurs unless there is something wrong on-board. Class B's report when the noise level is too high so the owner can isolate and correct the problem.

So, it has nothing really to do with whether a single channel receiver can receive Class B packets. That has to do with the receiver's capabilities and of course whether it's actually receiving on the channel at the time of transmission. It doesn't matter whether the transponder is a Class A or Class B as they operate using the same alternating channel scheme. In the end a single channel receiver will always miss at least 50% of potential transmissions.

[bdbcat]

Jeff...

Thanks for the technical color. It helps us all to occasionally dig into the black box.

I stand by my real-world experience on East coast US. Class-B targets moving at 10+ knots are rarely anywhere near their last reported position as received, and the performance is significantly worsened with single channel receivers in high signal-density environments.

I recommend upgrading to 2 channel receivers if AIS is an important part of your cruising environment.

Dave

[jeffrobbins]

Dave, your observation makes sense. A 30 second transmission interval and moving at 10 knots results in 150 meters between transmissions.

And a single channel receiver gives you only 50% of the messages so the vessel moves 300 meters between received positions.

[barnakiel]

No manual but have you tried googlin up and reading thru how others did this?

I was considering building a wifi router for rtl-sdr ais and at that time I remember finding a good number of websites and blogs explaining clearly what to connect/compile and set up.

Dig into the web. Send mails to bloggers. You will get your answers.

b.

[rgleason]

Thanks Dave, Jeff and Adrian.

Block Island Fast Ferry comes to mind. I hope Vesper XB-8000 is in our near future, but for the time being, Adrian, is the dAISy HAT dual channel receiver also capable of USB connection to a Win PC?

[AedanC]

Some time ago bdbcat wrote:

Originally Posted by bdbcat

Rick...

2. I also find that many Class B transponders transmit on only one of the authorized TX channels, most of the time. They are supposed to alternate channels for "periodic repeated" messages, but they don't all seem to do this.

Dave

In a fit of boredom a few nights ago I wrote a quick programme to analyse the AIS receive logs I had from last season. After removing all stations where I had less than 50 samples I ended up with 79 separate MMSI numbers, 78 of which transmitted with a more even balance than 65% channel A or B. I received the remaining one 213 times, of which 212 were on channel A and 1 on channel B. I was using a Vesper XB-6000 which I believe is a two channel receiver.

Interestingly the one offender was a lighthouse....

Aedan

[jeffrobbins]

Good data and that's about what I would expect, except for the lighthouse. Yes, the XB-6000 is a two-channel simultaneous receive.

For the lighthouse... if it is transmitting Aid to Navigation messages (not vessel position reports) then it could be configured to transmit on only one channel, although that would be an atypical choice.

[AedanC]

Originally Posted by jeffrobbins

For the lighthouse... if it is transmitting Aid to Navigation messages (not vessel position reports) then it could be configured to transmit on only one channel, although that would be an atypical choice.

I did capture one transmission on the other channel, so I suspect it's not configured like that. I won't be back that way until next summer I'm afraid...

Aedan