Re: Garmin altitude problems

On Sat, 10 Aug 2024 14:12:12 -0400,
  Frank Krygowski <frkrygow@sbcglobal.net> wrote:

 On 8/10/2024 12:21 PM, Jeff Liebermann wrote:

 
GPS is far more accurate measuring 2D surface distances than
3D measuring altitude. "Grade, Elevation, and GPS Accuracy
FAQ"
<https://support.ridewithgps.com/hc/en-us/articles/4419010957467-Grade-Elevation-and-GPS-Accuracy-FAQ>
 
Garmin rates their GPS elevation accuracy at +/-400 ft and
their barometric elevation accuracy at +/-50 ft.
 
If anyone wants me to explain why in excruciating detail if
anyone is interested.  The short version is that GPS works by
measuring the Doppler shifts produced by multiple satellites
across the sky.  The motion of the satellite has to be mostly
toward or away from the GPS receiver.  For altitude, that
happens best when the satellites are near the horizon.  A
satellite directly overhead does not move toward or away from
the GPS receiver and is therefore useless for computing
altitude.  Since the horizon tends to be cluttered with
mountains, trees, buildings, reflections etc, the signals from
the best positioned satellites can't be used.  Just to make
things difficult, the better GPS antennas are designed to
suppress ground reflections which makes using low elevation
satellites even more difficult to use.

Yep, and nice explanation.

 IIRC, Jobst explained that with some trigonometric detail many
 years ago.
 
 And it's all irrelevant for Florida bike paths.

LOL  Reminds me of a tandem rally in Sebring a few years ago.  At
one point we rode with a couple from Ft. Lauderdale and had a hard
time keeping straight faces when they complained about the hills.

--
Ted Heise      <theise@panix.com>       West Lafayette, IN, USA