[aprssig] Freq Info for NMEA and other Trackers

[Robert Bruninga]

> Will GHz be supported or an additional digit for the MHz?

Yes, if you look at the spec, it goes to letters.
AXX.XXXMHz = 12XX.XXX MHz,
BXX.XXXMHz = 24XX.XXX MHz> 

> Also, if it's fixed format like this, the MHz 
> would only be "cosmetic" and not really needed.

Since this is an optional field in an otherwise free text field,
it needs the "MHz" as confrimation that in fact, the digits in
that location actually refer to a frequency.

Hope that helps.
Bob