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[aprssig] garmin gps and compass heading

Stephen H. Smith wa8lmf2 at aol.com
Sat Apr 29 01:21:14 UTC 2006

ramirault at verizon.net wrote:
> ----- Original Message ----- From: "Wes Johnston"
> Subject: [aprssig] garmin gps and compass heading
> Does anyone know if the garmin units with compasses output a seperate 
> compass heading?  Note that the compass heading may be different than 
> the direction of travel (for example if the unit is held horizontal 
> and moved backwards or left/right.)  My intent is to have a serial 
> compass that shows which way the gps unit is oriented.
> ----------------------------------------------
> What all this has to do with compass output (with those GPS units with 
> an electronic compass.. the only ones in consideration here I hope) 
> ??  I have no idea.

Are you referring to compass-like displays computed from normal GPS 
data,   or to a GPS that has a separate hardware (perhaps an electronic 
flux-gate compass) device built into it,   that generates additional 
words in the data stream output through the same serial port as the GPS.

In a normal GPS, the compass-like display ("heading") is computed by 
comparing the previous position fix with the current one,  and 
determining the change.  The result is that :

1) You must move between two fixes to get any kind of heading reading

2)  It doesn't matter which way you hold the unit since the change in 
position on the earth's surface will be the same.

3)  Since the calculated position on most GPS units "wobbles" or "slops" 
back and forth a bit when you are standing still,  you will receive an 
erratic and constantly changing meaningless "heading" while you are 
standing still.  (You must KEEP MOVING to keep the heading meaningful.)

4)  Some GPS units have a kind of firmware "squelch" that stops updates 
(and keeps repeating the last fix) when you are moving less than  .5 MPH 
or so,  to suppress the random jitter in the lat/long readout when you 
are stationary.  In this case, the heading will either be the last one 
while you were still moving,  or the heading will disappear when you 
fall below the speed threshold considered to be stationary.

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com

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