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[aprssig] AIS and DSC - lots of questions

WB4GQK at aol.com WB4GQK at aol.com
Wed Apr 6 16:22:05 UTC 2005


Hi All,

First of all let me discuss DSC Digital Selective Calling. This is a 10-bit 
code where the first 7 bits are information bits. Bits 8,9,10 indicate the 
number of B elements that occur in the first 7 bits. That is a B element is the 
number 0 while a Y element is a number 1.  

This scheme is an error detecting code, since accuracy is of primary 
importance. On VHF, CH 70, 156.525 MHz the Time Diversity Interval is 33 1/3 ms using 
600 bits/sec  F1B 170 Hz tones. On HF there is one (1) DSC frequency on each 
of the 6 marine bands. HF uses J3E emission, Audio tones of 1700Hz with +/- 85 
Hz shift with a modulation rate of 100 Baud. The TDI for HF is 400 ms.

There are 27 "symbols" (actually they are service commands) used in the DSC 
coding structure. 12 of these "symbols" deal with distress/mayday situations, 
the other 15 are for general telecommands. Theoretically all vessels are 
supposed to apply for a 10 digit ship's identification number. This is called 
Maritime Mobile Service Identification. So if you have the MMSI of a friends' boat 
you can store it in your VHF radio. Then by selecting the vessel you want to 
call you can key the DSC function and that vessel and no other will be alerted 
by an alarm. That vessel then sends an ACK back to the caller and IF THE OWNER 
CONNECTED A GPS DATA SIGNAL TO HIS VHF the caller will also receive a LAT/LON 
position of the vessel he called. It is estimated that less than 10% of the 
private vessels that have DSC capable VHF radios do not have them connected to a 
GPS. What a shame.

The HF DSC system has some real problems, the main one of which is having 
both radios on the same calling channel. You can readily see if one radio is 
setting on the 4 Meg frequency and the calling vessel is on the 22 Meg freq never 
the twain shall meet.

The AIS or Automatic Identification System is quite different than the DSC 
system. Obviously because it serves a different purpose. The data rate for AIS 
is 9600 bits/sec and it operates on marine VHF channels 87A and 87B only! 
161.975 MHz and 162.025 MHz. The AIS VHF transceivers come equipped with a built in 
GPS receiver. It's primary purpose is to provide traffic control shore 
stations complete information about the speed/location/course/type of vessel/ etc. 

AIS with it's Self Organizing Time Division Multiple Access, the system can 
handle 2250 time slots of 26.67 ms each or TDMA of 4500 messages a minute! 
Compared to DSC which can only handle 4 to 9 polled messages a minute. The AIS 
data signal is a standard 8 bit ASCII code. The code structure contains 99 
service messages.

The technical differences in the code notwithstanding, the simplistic 
statement is:
DSC = "I am calling you " and maybe I get your position information
AIS=  "to whom it may concern" here is where/what I am, how fast I'm going 
and where I'm headed! Best advised you stay out of my way!

De Jim




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