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[aprssig] Re: AIS and DSC - lots of questions

WB4GQK at aol.com WB4GQK at aol.com
Fri Apr 8 03:25:49 UTC 2005


Hi

Obviously KC2MMi  has made several good points, but the fact remains a large 
number of boaters who have purchased at least marine VHF radios have not 
applied for a MMSI, and most likely the reason is the cost of $200 to obtain a 
ships license. As long as the sailor is sailing close inshore there is not too 
much to worry about. 

But since this door has been opened let me expose what's on the other side. 
For the average boater they don't even need a VHF radio much less an HF rig! 
However there is a small group of we boaters that get into really going 
off-shore. In my case I actually have participated in Open Ocean Racing in 2001 and 
2002.  I have signed my boat and crew again for the St Pete-Isla Mujeres MX 
,which is 549 miles of open blue water racing. The race starts on the 29 th of 
April. You can track me again on HF APRS every hour.

My case is special but let me stipulate right up front ANY boat that desires 
to enter this race must be inspected by the Southern Ocean Racing Committee, 
SORC, and meet their requirements which include a LICENSED Marine VHF AND a 
Marine SSB HF rig. The rules are they must BOTH be capable of DSC and have GPS 
data fed into them so that if a MAYDAY is sounded BOTH rigs will transmit the 
alarm and provide the GPS coordinates of the site. And yes I am also required to 
carry a 409 EPIRB. My boat will be docked at the race Headquarters the day 
before just so the committee can examine and test all the safety and electronic 
equipment before they will allow me and crew to participate.  

But you know something, I feel a strong responsibility for the  guys who are 
willing to risk their neck in the pursuit of a challenge. Besides I don't 
think I could face their first mates if anything went wrong. So every safety 
device is up-to-date, including the inspection of the emergency liferaft.

Now this is the extreme right hand side of boating so anything goes in 
between until you get to the other side where many cruisers don't even have any kind 
of a 2 way radio. That's their choice.

Now let me cover AIS. You are absolutely correct it is designed to work 
primarily  in crowded harbors. But does it surprise anyone that the AIS units are 
working offshore? They ABSOLUTELY DO! For one reason, all big shipping vessels 
ply charted routes. If you examine a Gulf of Mexico navigation chart you will 
see the big shipping lanes very well delineated. In the race this month we 
will be crossing three (3) heavily traveled shipping lanes. The purpose of AIS is 
to not only transfer information to controllers on shore but to alert other 
vessels of their presence and INTENDED COURSE. These big ships pass within just 
a few miles of each other while still 150 miles off shore in the Gulf. One 
thing you must realize is the closing velocities on these big carriers approach 
50 knots! They want to be sure the other vessel sees them and is aware what 
their intentions are, the AIS data is all displayed on their navigation screens. 
Radar just pictures where they are located, and you don't know if they are 
watching their screen! At least AIS will produce an alarm when it sees a 
possible track crossing.

To summerize when we made the race to Isla Mujeres in 2002 I was able to copy 
AIS data from a number of these big freighters and being able to pick up a 
position, course and speed at least 10 minutes before the small boat radar could 
even see a blip much less plot it's course was indeed a godsend.

Very obviously this business about DSC, AIS depends entirely on where you 
live, where you sail, how often and especially what kind of sailing you perform. 
My case is different. Right now there are 38 boats entered in the race, three 
(3) of which are not only good friends but also HAMS! N4MKT, KG4IXZ and KM4YB 
have entered their boats. And as long as we are within 35 miles of each other 
we will be using DSC to contact each other. After that we will switch to HF. I 
believe I will be the only one using HF APRS.

I want to finish with a comment about "intelligent" radios. I believe what 
you are referring to here is ALE or Automatic Link Establishment.  I have the 
ALE software installed on the computer on the boat. My Icom M802 SSB is fully 
under the control of the computer and could respond to the commands from the ALE 
software. However I am missing one piece of gear and that is a must for this 
ALE system to operate. I need a second completely separate antenna and 
receiver that is broad band enough to recognize DSC signals calling my MMSI from ANY 
of the marine bands from 22 mHz all the way down to 4 mHz frequencies.  
Someway this data has got to interface with the ALE program and it then switches the 
SSB  to say channel 12401, commands the SSB to auto tune and then send an ACK 
along with my position coordinates. I have done one heck of a lot of reading 
on this subject and from what I have seen is that these add-ons are  very very 
iffy even if they work at all.  And this secondary system is not cheap you 
are looking at over a grand installed and working. The two systems I have seen 
offered are basically a computer type of receiver that performs high speed 
scans of the DSC calling channels on each of the bands. The problem is my boat's 
computer load is becoming horrendous already. The navigation program is first, 
WeatherFax is second, WinAPRS is third, the SSB controller is fourth. I have 
scrubbed Winlink so it will not be running at all during the race. I just don't 
feel comfortable with additional operational programs.

Now you have the view from the extreme right.

73 de Jim




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