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From: Catrike Ryder <Soloman@old.bikers.org>
Newsgroups: rec.bicycles.tech
Subject: Re: Garmin altitude problems
Date: Sun, 11 Aug 2024 07:14:17 -0400
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On Sun, 11 Aug 2024 10:24:10 GMT, Roger Merriman <roger@sarlet.com>
wrote:

>Catrike Ryder <Soloman@old.bikers.org> wrote:
>> 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.
>> 
>All of the big three ie Garmin-Wahoo-Hammerhead on there upper end devices
>all have barometric sensors for improved accuracy, I’m assuming that while
>GPS Altitude is indeed more accurate flying or sailing, this isn’t the same
>cycling with sky cover and undulating land.
>
>Ie clearly must be some engineering reason to do so, most likely the above.
>
>Roger Merriman
>

I don't pay any attention to altitude on my rides, although it is
recorded on my Garmin computer. There are 30/40 foot variations in
maximum altitude when I compare identical rides, so whatever method
the computer uses is worthless.