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From: Catrike Ryder <Soloman@old.bikers.org>
Newsgroups: rec.bicycles.tech
Subject: Re: Garmin altitude problems
Date: Sat, 10 Aug 2024 05:35:41 -0400
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On Sat, 10 Aug 2024 08:54:20 GMT, Roger Merriman <roger@sarlet.com>
wrote:

>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. 
>> 
>It is how GPS without a barometric sensor do but it’s less accurate though
>by how much and how much you care.
>
>I’m more likely to note the temperature with all the caveat’s though more
>likely in winter.
>
>Roger Merriman

GPS altitude calculation should be more accurate if it can read four
satellites.

********************************
Is GPS Accurate for Altitude?
Private Pilot
Made Easy

There is no such thing as a ‘perfect’ system, and GPS is no exception.
When it works, it is highly accurate. In fact, there are currently
plans to eventually replace conventional aviation navigation aids with
GPS (GBAS) landing systems.

However, GPS altitude can be affected by the following: –

    Ionospheric errors – The Earth is surrounded by layers of a dense
atmosphere that change in size and thickness depending on solar
activity and the time of day. This can cause minor altitude errors.
    Clock errors – GPS clocks are phenomenally However, even a
minuscule disruption to their timing can cause errors. This doesn’t
happen often, and GPS receivers normally discard a timing signal they
recognize as erroneous.
    Rounding errors – Generally, these errors are small, but in an
aircraft, precision matters.
    Satellite signal loss – Like any piece of electrical equipment,
Satellites do sometimes stop working or are rebooted. For a GPS to
provide accurate altitude readings, it needs multiple satellites,
normally four. If the fourth drops out at a critical time, this could
cause issues.

https://pilotinstitute.com/altimeter-vs-gps-altitude/

*********************************

I understand that using four satellites, time errors can be
compensated, and four satellites are required for altitude
calculations anyway.  It seems to me that the biggest problem with GPS
altitude is possible loss of satellite reception, which is a serious
issue for airplanes, but not such a big deal for land and sea
operations. Why Garmin and other land based GPS systems don't use GPS
altitude is a mystery to me.