[aprssig] station capability flags

[Jason Winningham]

There was some discussion in the past few days about not knowing  
which stations were acting as Igates.  One thing that has always  
seemed missing in the APRS protocol is a way to indicate station  
attributes.  Selecting an icon is quite restrictive.  What if it's a  
digi with a weather station, or emergency power, or an Igate, or all  
of the above?  How you you indicate more than one, without doing  
network traffic analysis?

In the absence of a dedicated field added to the protocol, it seems  
to me we could get a very limited set of flags by using the TOCALL  
SSID bits.  A quick scan of my tnc log file didn't show any TOCALL  
addresses using an SSID other than 0.

One approach would be using the 4 bits to flag the 4 most useful  
attributes that may not be indicated by the station's icon.

Another method, more complex, would be to define 15 common sets of  
attributes indicated by the non-zero SSID.

-Jason
kg4wsv