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[aprssig] Protocols

Scott Miller scott at opentrac.org
Thu Apr 3 20:58:08 UTC 2008


You can cram a lot of data in there if you sacrifice human readability. 
  That's what I was going for with the OpenTRAC spec:

http://www.opentrac.org/spec.pdf

It makes some compromises for extensibility, though - in particular, 
using length fields and identifiers where you could get by without them 
if you needed less flexibility.

8 bytes is enough to represent any 2D position on the Earth with about 2 
centimeters resolution.  6 bytes is enough for typical APRS use.

Scott
N1VG

Andrew Rich wrote:
> 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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