[SPAM][aprssig] Garmin emulation

Scott Miller scott at opentrac.org
Wed Jan 11 14:57:54 CST 2006

> What is the difference between NMEA and GARMIN ?

Just about everything.  =]

> Can you "poll" for a position in GARMIN's protocol ?

Basically, you've first got to send a 'start pvt' packet (for position,
velocity, and time) to get it to start sending periodic PVT packets.  The
position comes back as 64-bit floating point in radians.  Velocity is in
meters/second north and east, also floating point.  To convert to course and
speed, you've got to do some trig.  Course you can get from the arctangent
of the north velocity divided by the east velocity, and speed from the
square root of the sum of the squares of both components.

You can also poll for individual position/velocity/time components, but
they're just as ugly if you get them that way.  For a PC it's not a big
deal, but for an 8-bit processor, it's really not fun.

Waypoints are a pain because Garmin uses several different data formats,
depending on the model.  Newer models support a capabilities format that
tells you which one they're using.  Otherwise you have to figure it out from
the packet length.  Waypoint positions are 32-bit integers.


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