[aprssig] AIS and DSC - lots of questions
WB4GQK at aol.com
WB4GQK at aol.com
Wed Apr 6 12:22:05 EDT 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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