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Author Topic: AIS versus APRS  (Read 14699 times)
N8QH
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Posts: 19




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« on: March 14, 2013, 03:00:48 PM »

It's amazing how much better the Maritime Automatic Identification System (AIS) works - at nearly the same frequency (160 MHz) and with only 25 watts of power.

Here's the range of my AIS receiver: Reception distance in nautical miles: Average: 54.64 / Max: 264.36 (those are ephemeral stats based on the current positions of vessels - but they are typical).

You can see my stats here: http://www.marinetraffic.com/ais/stationdetails.aspx?station_id=595

And my coverage area here: http://www.marinetraffic.com/ais/default.aspx?zoom=7&centerx=-122.4805&centery=37.50246&oldmmsi=595

I also find HF APRS on 30 meters is about twice as reliable, based on packet loss rations, than VHF APRS.

My conclusion: VHF APRS doesn't work anywhere near as well as it could. And it isn't due to packet transmission frequencies: large ("Class A" AIS) vessels transmit position reports every 2 seconds. The problem with APRS is it's near complete lack of control over transmission contentioning.

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N8BOA
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Posts: 101




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« Reply #1 on: March 15, 2013, 04:28:59 AM »

Apples or Oranges
   AIS is not using the same protocol as Packet ax.25. There is no two way communication no CRC error checking. I honestly I don't what protocol AIS uses but it is one way thus they likely use a FEC  (Forward Error Check) that does not require two comms like CRC does. Also all of AIS is over water not land like APRS so propagation is defendant as well.
Sean   
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N8BOA
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Posts: 101




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« Reply #2 on: March 15, 2013, 04:36:59 AM »

http://www.wavecom.ch/PDF/Advanced_Protocols/AIS.pdf

"
the core of AIS is the concept of Self Organizing Time Division Multiple Access (SOTDMA) which is used for Class A de-vices. Each frame (one minute long) is divided into 2250 slots
which gives 4500 slots as AIS signals are transmitted on two frequencies, AIS1 (161.975 MHz) and AIS2 (162.025 MHz). Based on previous traffic history, each station when transmitting re-
serves a future slot for its next message. Messages may be longer than one slot. To avoid overlap stations continuously synchronize to each other (see fig. 4).
Base stations use another ac-cess method, FTDMA (Fixed TDMA) with fixed slots. Class B equipment use CSTDMA (CarrierSense), an adaptation of the Ethernet collision sense access
method."
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N8QH
Member

Posts: 19




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« Reply #3 on: March 15, 2013, 03:04:01 PM »

Apples or Oranges
   AIS is not using the same protocol as Packet ax.25. There is no two way communication no CRC error checking. I honestly I don't what protocol AIS uses but it is one way thus they likely use a FEC  (Forward Error Check) that does not require two comms like CRC does. Also all of AIS is over water not land like APRS so propagation is defendant as well.
Sean   

>  AIS is not using the same protocol as Packet ax.25.
Correct. AX.25 is primitive and long in the tooth. In computing terms, it's a relic out of the Cro Magnon era.

> Also all of AIS is over water not land like APRS so propagation is defendant as well.
I'm plotting vessels with AIS that are on the other side of the Santa Cruz mountains, while I'm in a mountainous valley where even FM broadcast is barely receivable.  I have an APRS receiver (Kenwood D710) with an identical antenna, at exactly the same height on the same mast that hasn't even comparatively 10% the range to APRS targets - some of which are blasting 50 watts. AIS is 25 watts for "Class A" and 2.5 watts for "Class B".

It's the protocol that's bad - not the propagation. We Hams should be able to do better.
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K0JEG
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Posts: 672




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« Reply #4 on: March 16, 2013, 06:42:51 AM »

AIS utilizes GMSK (Gaussian Minimum Shift Keying) modulation at 9600 bit/s. Adjacent channels can be spaced either 12.5 kHz or 25 kHz. Data is encapsulated in HDLC like packets, which are NRZI encoded, see fig 5 and fig. 6.

D-Star uses GMSK and has position reporting built in, although it doesn't have much in the way of packetization. It would be nice if the community viewed the protocol as more than a voice mode, so maybe we could retire our Bell 212 modems.
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W0DLM
Member

Posts: 76




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« Reply #5 on: March 20, 2013, 07:56:11 AM »

I'm plotting vessels with AIS that are on the other side of the Santa Cruz mountains, while I'm in a mountainous valley where even FM broadcast is barely receivable.
There are AIS repeating stations in operation in some areas.  Might well be one on one of those mountains, which would, of course, explain your reception.
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W5LZ
Member

Posts: 477




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« Reply #6 on: March 21, 2013, 07:55:59 AM »

There are a number of differences between AIS and APRS.  One is professional the other is amateur.  They have different reasons for 'being' too.  And maybe the biggest difference is cost.  Could APRS be 'improved'?  That depends on what the purpose of that improvement is and how it's used.  That improvement also means it's not so 'modifiable', it's more 'restrictive'.  But it could certainly be configured to include more information of some kind.  Then again, that would mean restructuring the packet 'string' which would mean a change in protocol.  That would be sort of self-defeating.  The two systems are not competitive.
 - 'Doc
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N1ZZZ
Member

Posts: 161




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« Reply #7 on: October 03, 2013, 01:03:30 AM »

We replaced the AIS on my ship earlier this year. Cost was $5,500.  That's a Class A unit.

If anyone is going to cross the ocean in a sailing yacht, I would hope that they have an AIS unit so that the big ships can see them at a decent range.

I find AIS to be of great help in traffic management and collision avoidance.  In the open sea, with my antenna height of about 100 feet off of the water and a 5/8 wave vertical antenna, I regularly pick up other ships in the 20-30 nm range and sometimes out to 60 nm.  Frankly, anything over 24 miles is a waste of my time as I am not thinking that far ahead.

I don't use VHF APRS out here, so can't comment on a direct comparison.

 I do have an HF APRS station on the ship.  Using Robust Packet Radio and my 30' wire at the same height but with 100W out, I am getting single hop paths of 3,000-4,500 miles on 30 meters, and 24/7 coverage up to 2,500 miles.

73
Jeremy N1ZZZ
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