Hoolie
Well-Known Member
I have a short length of coax that will link the AIS antenna to the radio. Simple to disconnect the AIS box and connect to the VHF. No complicated switching needed.
Masthead VHF antennas
- Thread starter Hoolie
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JumbleDuck
Well-Known Member
Hmm. I have to disconnect two leads and reconnect one of them. You have to do the same, adding a coax extension as well. In what way is flicking a switch more complicated than either of these?
PaulRainbow
Well-Known Member
AIS is, like VHF (because it IS VHF) line of sight, plus a bit for atmospheric refraction.
Any further is caused by various "anomalies". Those distances are not reliable, whereas line of sight is.
bbg
Well-Known Member
Flicking a switch is obviously less complicated, but adds the potential error of putting (or leaving) the switch in the wrong position. I'm not saying it is a bad idea, just one that comes with risks. Seriously, undoing two co-ax cables and re-connecting one is not that complicated. You just need to make sure the cable(s) are long enough. And that you don't try to use the VHF when it is not connected to an antenna.
JumbleDuck
Well-Known Member
Oh I know, I know, and the idea is in abeyance, What I'd really like, one day, is a big red button marked "emergency" which switches the radio and GPS onto a backup battery and then automatically switches the VHF to the highest available antenna. On the other hand, KISS has its attractions too.
Arida
Well-Known Member
Are you sure about it? The AIS transmit capable splitters I have seen don't seem to have enough connections to allow control of the AIS or the VHF voice radio. They are advertised as giving voice priority which is correct of course.
My assumption (and I admit it is only that) is that they are RF sensed on both the Voice and AIS ports but that the logic is such that if the voice radio is transmitting it will always win - that is it will be connected directly to the antenna. At this point the AIS will not work (in either TX or RX) but will hopefully be connected to a dummy load to avoid the (fairly remote) possibilty of damage to its power amplifier.
RichardS
N/A
The active splitter will ensure that the VHF and AIS cannot both transmit power to the antenna at the same time. The switching is built into the splitter.
Richard
RichardS
N/A
I wonder if what we are missing here is digital error correction?
We all know that an analogue vinyl record with a scratch cannot be corrected as the information is gone/modified for ever, whereas a CD with a scratch can sound exactly like the CD without the scratch. In fact, the signal is identical as the digital error correction reconstructs the missing data exactly. And CDs are "transmitting" vastly more complex constantly-changing than AIS, where much of the data is unchanging, and CDs are managing this trick with a relatively primitive correction algorithm compared to today's digital media.
If I'm right, we need to be circumspect about any comparison with analogue VHF and "line of sight" as error correction could easily re-create a digital blob on a screen with relatively little data provided each flash of incomplete data gives enough for the error correction to fill in the rest. OK, that digital blob may not have 100% authentic fresh data .... but who cares. As soon as it comes into "view", which is when it becomes dangerous, all the data becomes live which is when you really need it.
No-one would suggest that a scratched CD which still plays is any less authentic than the unscratched original, so the error-corrected "anomaly-derived" AIS data might well be as authentic as the line-of-sight data depending upon how good the error correction is ....... and we can finally put "line-of-sight" in its proper context.
Richard
We all know that an analogue vinyl record with a scratch cannot be corrected as the information is gone/modified for ever, whereas a CD with a scratch can sound exactly like the CD without the scratch. In fact, the signal is identical as the digital error correction reconstructs the missing data exactly. And CDs are "transmitting" vastly more complex constantly-changing than AIS, where much of the data is unchanging, and CDs are managing this trick with a relatively primitive correction algorithm compared to today's digital media.
If I'm right, we need to be circumspect about any comparison with analogue VHF and "line of sight" as error correction could easily re-create a digital blob on a screen with relatively little data provided each flash of incomplete data gives enough for the error correction to fill in the rest. OK, that digital blob may not have 100% authentic fresh data .... but who cares. As soon as it comes into "view", which is when it becomes dangerous, all the data becomes live which is when you really need it.
No-one would suggest that a scratched CD which still plays is any less authentic than the unscratched original, so the error-corrected "anomaly-derived" AIS data might well be as authentic as the line-of-sight data depending upon how good the error correction is ....... and we can finally put "line-of-sight" in its proper context.
Richard
PaulRainbow
Well-Known Member
Not sure where you went with that Richard 
We're not making comparisons with "analogue VHF", it IS analogue VHF.
It's all pretty simple really, it transmits line of sight (plus a little for atmospheric refraction). If it gets bounced around by mountains, large container ships, atmospheric conditions or UFO's it goes further, faster, around corners etc.
Given your location you're most likely getting regular, almost "reliable" transmissions from non-line of sight vessels due to local geography.
We're not making comparisons with "analogue VHF", it IS analogue VHF.
It's all pretty simple really, it transmits line of sight (plus a little for atmospheric refraction). If it gets bounced around by mountains, large container ships, atmospheric conditions or UFO's it goes further, faster, around corners etc.
Given your location you're most likely getting regular, almost "reliable" transmissions from non-line of sight vessels due to local geography.
RichardS
N/A
Not sure where you went with that Richard
We're not making comparisons with "analogue VHF", it IS analogue VHF.
Not my suggestion ...... MikeCC in post #24. I thought he was an expert looking at his sig.
Richard
PaulRainbow
Well-Known Member
Not my suggestion ...... MikeCC in post #24. I thought he was an expert looking at his sig.
Richard
Not sure what Mike was referring to there Richard.
RichardS
N/A
Wkipedia entry:
AIS uses the globally allocated Marine Band Channels 87 & 88.
FM modulation is not used.
AIS uses the high side of the duplex from two VHF radio "channels" (87B) and (88B)
Channel A 161.975 MHz (87B)
Channel B 162.025 MHz (88B)
The simplex channels 87A and 88A use a lower frequency so they are not affected by this allocation and can still be used as designated for the maritime mobile frequency plan.
AIS uses GMSK modulation. In digital communication, Gaussian minimum shift keying or GMSK is a continuous-phase frequency-shift keying modulation scheme.
Based on the Wiki, it looks like I was right Paul, and digital error correction is probably the missing factor after all!
Richard
PaulRainbow
Well-Known Member
Wkipedia entry:
AIS uses the globally allocated Marine Band Channels 87 & 88.
FM modulation is not used.
AIS uses the high side of the duplex from two VHF radio "channels" (87B) and (88B)
Channel A 161.975 MHz (87B)
Channel B 162.025 MHz (88B)
The simplex channels 87A and 88A use a lower frequency so they are not affected by this allocation and can still be used as designated for the maritime mobile frequency plan.
AIS uses GMSK modulation. In digital communication, Gaussian minimum shift keying or GMSK is a continuous-phase frequency-shift keying modulation scheme.
Based on the Wiki, it looks like I was right Paul, and digital error correction is probably the missing factor after all!
Richard
But that's only digital modulation, same as mobile phones use. Still subject to the same rules of "line of sight".
Not sure they'd use error correction on a system like AIS Richard. It's a mandatory system for commercial vessels and i'm not sure it would be considered good practice to be filling in missing data. But who knows.
Alpha22
Well-Known Member
Digital signal modulation will always out perform FM voice modulation.
Keeping it simple.....
From my house in UK with big antennas and 100W of power I can get:-
50-100 miles on FM...
Most of Europe, Middle East and East Coast North America on SSB
The rest of the world on morse (primitive digital signal modulation.) I can even hear my own echo coming right back round over the North Pole when beaming towards Australia........ that's long range.
This is on HF, not VHF but it serves to show how much further digital modes will travel compared to voice.
Again trying to keep it simple .... GMSK is kind of like SSB in that there is no carrier to get destroyed by noise. Thus preserving the signal longer.... and further. In poor reception conditions your DSC mayday is FAR more likely to be received and understood than a voice transmission.
AIS has a kind of built in error correction in that the receiving station 'builds up a picture' of the transmitting station over several transmissions...... if some data is not received in the first transmission it is "filled in" by data from subsequent transmissions. "Correct" data is not removed if not received in the subsequent transmissions.
Keeping it simple.....
From my house in UK with big antennas and 100W of power I can get:-
50-100 miles on FM...
Most of Europe, Middle East and East Coast North America on SSB
The rest of the world on morse (primitive digital signal modulation.) I can even hear my own echo coming right back round over the North Pole when beaming towards Australia........ that's long range.
This is on HF, not VHF but it serves to show how much further digital modes will travel compared to voice.
Again trying to keep it simple .... GMSK is kind of like SSB in that there is no carrier to get destroyed by noise. Thus preserving the signal longer.... and further. In poor reception conditions your DSC mayday is FAR more likely to be received and understood than a voice transmission.
AIS has a kind of built in error correction in that the receiving station 'builds up a picture' of the transmitting station over several transmissions...... if some data is not received in the first transmission it is "filled in" by data from subsequent transmissions. "Correct" data is not removed if not received in the subsequent transmissions.
JumbleDuck
Well-Known Member
Wouldn't receiving wrong information be seen as worse?
PaulRainbow
Well-Known Member
Yes, that's what "could" happen with CD style error correction. No error correction doesn't equate to wrong information, it equates to no information, which IS better than wrong information.
RichardS
N/A
Reading the post from Alpha22 we seem to have cracked it. VHF voice and VHF AIS do not have the same transmission characteristics and error correction is implemented.
However, as I said in my "error correction hypothesis" post, digital error correction does not generate the wrong signal. It either re-creates the original data exactly or it fails. It does not "guess" what the missing data might be. Using the analogy with CD, and assuming there is a something similar with AIS, there is no chance that wrong data would be displayed. It would be corrected or flagged up as being missing.
Richard
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JumbleDuck
Well-Known Member
Error correction doesn't make a wild guess at missing data; it reconstructs it exactly using redundant information in the original message and if it can't do that it says so. There is no conceivable situation in which error correction leads to less reliable information transfer.
As it happens, though, AIS does not use error correction. It uses standard NMEA0183 format sentences with a two-byte checksum. Eric S. Raymond (yes, that Eric S. Raymond) has done sterling work in working out details of the format, which is otherwise hidden behind proprietary-ness.
http://catb.org/gpsd/AIVDM.html
BrianH
Well-Known Member
Your link is indeed an excellent reference to AIS message formats (and as such it resides as a prime AIS reference in my bookmarks). However, it does not cover the signal transmission/receive decoding layers, which I had always assumed contained some level of cyclical redundancy check (CRC) code. Difficult to access due to (as you mention) propriety restrictions (that damned Swede again) but I distinctly recollect that in the AIS specifications are references to such in one of the data link layers.
Back in my comms programming days, the normal application of CRC for error detection was to have the receiver request the sender to repeat the transmission but, of course, with AIS the transmission is one way and it can only recognise a data error and discard the message. In practical terms this is no big deal, the next one will be along shortly. I am aware of CRC algorithms that can reconstruct the data but would not expect such an overhead in AIS message packet processing layers due to that frequency of repetition.
PaulRainbow
Well-Known Member
The error correction being discussed here, from post 48, does just that.
" as the digital error correction reconstructs the missing data "
Indeed.
My points were simple.
a) AIS is, like VHF, line of sight (plus the usual addition for atmospheric refraction). This is a well documented scientific fact.
b) AIS isn't using any form of error correction to fill in missing data and display vessels that are not within that line of sight. In Richards case, he is surrounded by mountainous islands, the mountains being the reason he sees vessels "around corners".
