Compressed Geolocation Frames

A compressed geolocation frame has a payload of either exactly 17 or 19 bytes.

Callsign	SSID, Path Code & Data Type Code	Compressed Data
4 bytes	1 byte	12 (or 14) bytes
CCCC	D	/XXXXYYYY$cs(aa)

where:

• CCCC: 4 bytes for the compressed 6 character Callsign

• D: compresses into 1 byte:

  • the SSID (between SSID 0 [none] and 15; included),

  • the Path Code (between path 0 [none] and 3; included), and

  • the Data Type Code = 0

• /: the Symbol Table Identifier

• XXXX: the Base91 compressed longitude

• YYYY: the Base91 compressed latitude

• $: the Symbol Code

• cs: the compressed course (in degrees) and speed (in knots)

• (aa): optionally, the compressed altitude (in feet)

Note:

• Terrestrial objects do not require sending altitude data. Anyhow, GPS height readings are notorious for being significantly inaccurate.

• APRS-IS already understands Base91 XXXXYYYY compression when altitude aa is not used.

• In absence of altitude aa, the i‑gate adds the Compression Type Byte T right behind cs.

• When aa is present, the i‑gate will instead need to decompress the whole frame and forward the uncompressed frame to APRS‑IS.

• The parenthesis are not sent; these merely indicate optionality.