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This is the typical protocol used by UKHAS members to transmit GPS data balloon to ground on the unlicensed bands, although any custom data may be tacked onto the end of the string.
$$<CALL SIGN>,<INCREMENTAL COUNTER ID>,<TIME HH:MM:SS>,<LATITUDE-NORTH DD.DDDDDD>,<LONGITUDE-EAST DDD.DDDDDD>,<ALTITUDE METERS MMMMM>,<CUSTOM DATA><NEWLINE>
<CALL SIGN> simply identifies your balloon - doesn't have to be a real, licensed callsign.
<INCREMENTAL COUNTER ID> also helps people receiving many transmissions at once. This should go up for each new transmission. It can go up 1, 2, 3, or be the milliseconds since poweron, or whatever you want.
After “<ALTITUDE>,” the distributed tracker or anyone else not prepared for your custom data will not plot your data however the DL system will log the data for review. If you want to send anything else down, you can do so in that space.
The newline at the end is not required, however it does make the protocol much more easier to human-read.