Re: Garmin altitude problems

On Sat, 10 Aug 2024 04:39:21 -0400, Catrike Ryder
<Soloman@old.bikers.org> wrote:

On Fri, 09 Aug 2024 17:36:48 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:
>

On Sat, 10 Aug 2024 07:02:48 +0700, John B. <slocombjb@gmail.com>
wrote:
>

The basic system sounds much like my old airplane altimeter. But how
is atmospheric pressure due to temperature handled?

>
The MEMS barometric sensor, which I guessed was being used by Garmin,
has a built in temperature sensor and compensates internally.
<https://www.st.com/resource/en/datasheet/lps28dfw.pdf>
The data sheet claims "embedded temperature compensation" and +/-1.5C
temperature accuracy.  However, I can't tell if the pressure numbers
the sensor produces are pre-compensated internally in the sensor, or
if the pressure is calculated externally with a connected micro
controller.  In other words, I don't know how Garmin does temperature
compensation.
>
The supplied code from Sparkfun shows that both pressure and
temperature are output by the sensor.  However, it does not show any
external processing which would be needed to provide compensation.
<https://learn.sparkfun.com/tutorials/sparkfun-absolute-digital-barometer---lps28dfw-qwiic-hookup-guide#lps28dfw-arduino-library>
Once again, I don't know how Garmin does temperature compensation.
>
I blundered across this explanation of how temperature compensations
works in an aircraft altimeter:
"What does setting an altimeter actually do to the altitude?"
<https://aviation.stackexchange.com/questions/87848/what-does-setting-an-altimeter-actually-do-to-the-altitude>
I don't really know if that helps answer your question but it is
interesting reading.

Calculating altitude with GPS seems to me to make more sense.

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.

If you want accurate elevations, the antenna needs to be larger and
more complex than what's found in Garmin products.  Something like
these:
<https://www.google.com/search?q=gps+choke+ring+antenna&tbm=isch>
<https://en.wikipedia.org/wiki/Choke_ring_antenna>




--
Jeff Liebermann                 jeffl@cruzio.com
PO Box 272      http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann      AE6KS    831-336-2558