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Path: ...!eternal-september.org!feeder3.eternal-september.org!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail From: Jeff Liebermann <jeffl@cruzio.com> Newsgroups: rec.bicycles.tech Subject: Re: Garmin altitude problems Date: Sat, 10 Aug 2024 09:21:49 -0700 Lines: 72 Message-ID: <ur3fbj9aqa6ocp24t4u2cprtotbvjm2ju3@4ax.com> References: <HAOsO.12050$MJMb.1498@fx43.iad> <6he7bjp0qtkrif3t3n72q46tk0peot758q@4ax.com> <3Y0tO.41962$nLcd.12608@fx09.ams4> <gbn9bj9g6lg7bmfjcnr0pheah6b8nsjog5@4ax.com> <3aqabj58cc9e2fkintbin8c2v4kqqo0fhp@4ax.com> <d5icbjpe7men8nrimh37lu9cd3hnsvh7d8@4ax.com> <abbdbjhcae4uf1fqgd7df8ioof8rdd0ssu@4ax.com> <bubdbjdlj11teosdun77r4pj5qn3kdhe71@4ax.com> <li9ebjh2k7kij85184o819lahdqeo4qjst@4ax.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Trace: individual.net XMLqqEGSLWzrpUXny5k8rgOuCQx9MRCWmW7lKrGYbj8ZpQ/Pq9 Cancel-Lock: sha1:43NXafEvZxuzEo8ZLR1i3dmKfiY= sha256:UygC8BG+qC/ccBdXyMLctpDlnVe9ugZIxD1alvCG9XM= User-Agent: ForteAgent/8.00.32.1272 Bytes: 4541 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