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From: Jeff Liebermann <jeffl@cruzio.com>
Newsgroups: rec.bicycles.tech
Subject: Re: Garmin altitude problems
Date: Mon, 12 Aug 2024 10:32:57 -0700
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On Sat, 10 Aug 2024 09:21:49 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

>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.

Argh!  Much of what I wrote above is wrong.  I hate it when that
happens.

GPS position and altitude are not derived from satellite Doppler
shifts.  GPS uses TDOA (time difference of arrival) before the clock
in the GPS receiver is synchronized to the satellite clock, and then
switches to TOF (time of flight) after synchronization.  The TOF
delays from 3 satellites are used to calculate a 2D solution (no
altitude) or 4 satellites for a 3D (including altitude) solution using
multilateration.  No Doppler shift is involved in the calculation.

However, Doppler shift is used for calculating speed:
<https://www.e-education.psu.edu/geog862/node/1786>
<https://www.vboxautomotive.co.uk/index.php/en/how-does-it-work-gps-accuracy>

More later, hopefully with fewer mistakes.

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