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[aprssig] Protocols

Andrew Rich vk4tec at people.net.au
Thu Apr 3 20:50:15 UTC 2008


Hi

I have been doing some work with RADAR protocols, that by nature have to be small in payload and contain as much info as possible.

I am seeing some principles coming through that might apply to a future spec for APRS

Asterix is the name of the protcol and is widely used in commercial position reporting systems.

What I found is that even in my own dabblings here, I can pack data much more efficiently.

You would loose the ability to read in plain text, but the on air bandwidth would be greatly improved.

Astrerix works something like this

40 00 16 30 27 34 56 43 45 45 67

40 = This a a category 40 data burst, its rules and data items are laid out in the category 40 spec
00,16 = there is 16 octets of data in this data burst
30 = FSPEC or field spec.

"27 34 56 43 45 45 67" is the data that follows

The FSPEC is a bit indicator of what data is following

0010 0101 might be an example to say that a station id, followed by position report, followed by a weather report is coming.

In one byte you have specified the messages that follow, and you can BCD pack location info into the data that follows.

I am going to be re-visting my glider tracker in an effort to get the payload right down.

Cheers

Andrew VK4TEC
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