[aprssig] SSID for RF only Station with backup power

[Robert Bruninga]

>> * ... APRS must expand its symbol set by using OVERLAYS
>> * We need concurrence from authors on receipt of an overlay:
>>   - Receipt on existing "overlyable characters works"
>>   - Receipt on other symbols will not break anything
>>   - Eventually receipt of the overlay may be displayed
somehow
>>   - An overlay on a base symbol can trigger a different
symbol
> 
> ... I'd favor the major.minor.more-minor scheme from the 
> military spec:  It gives us incredible capabilities as well 
> as great compatibility for apps that don't implement the 
> 2nd/3rd level of detail or don't happen to have
> the latest definitions.  

Actually, the APRS Overlay concept is pretty much a 3 tier
system.  We have the primary table of about 90 symbols.  Then we
have the Alternate set of about 90 symbols, then the OVERLAYS
give a third tier of detail on those 90 symbols.  I think all
existing APRS apps do the first two tiers already, and if they
don't implement overlays, then they stop at the 2nd tier, but
still the basic symbol at that second level is displayed.

Bob, WB4APR