OK, the APRS Frequency spec has been updated. In addition to the existing Tnnn, Dnnn, these were added: 1750 and NoPL. http://www.ew.usna.edu/~bruninga/localinfo.html http://www.ew.usna.edu/~bruninga/aprs/freqspec.txt And if you have no PL and no tone, then those 4 bytes may be used for other purposes. Bob, WB4APR