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From: Jeff Liebermann <jeffl@cruzio.com>
Newsgroups: rec.bicycles.tech
Subject: Re: Difference between GPS and measured distace.
Date: Tue, 27 Aug 2024 16:51:11 -0700
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On Tue, 27 Aug 2024 22:56:42 GMT, Roger Merriman <roger@sarlet.com>
wrote:

>I have the 830 like Tom does.
(...)
>I still think this is a Tom issue the devices are same and easy enough to
>change the preferences and see if it makes any difference.

If I suspect interference, I sometimes use two or more different GPS
receivers and watch the numbers change on the status display.  There
are many apps for displaying GPS performance on a smartphone.  I don't
know what's available on the Garmin 830.  Probably fairly minimal.  If
there's interference, one or more of the displays are going to show
erratic changes in C/No (carrier to noise ratio) and the various DPO
(dilution of position) accuracy values.
<https://gisgeography.com/gps-accuracy-hdop-pdop-gdop-multipath/>
Unfortunately, such a test is not perfect.  Multipath and reflections
can produce the same erratic numbers as interference.  Therefore,
running such a comparison while moving or next to moving traffic is
not going to work.  

Admittedly, I've never tried this with a cycling computer because they
usually don't display C/No and DoP for individual satellites.  I've
always done it with smartphones and GPS diagnostics apps.  

Don't expect to easily find the interference source.  For that, I use
a spectrum analyzer and a very directional antenna tuned to the
applicable GPS constellation frequency.  It's very difficult to obtain
sufficient gain with a very broadband GPS antenna that covers all the
constellations.  I have one horn antenna cut for the GPS band, but I
have nothing for Glonass or Galileo.  So far, I've found a few GPS
interference sources more by inspection than with direction finding.
Most of the time, I can't find anything suspicious.  Most of the
direction finding and transmitter hunting I've done in the last 3
decades has been chasing down sources of Wi-Fi interference and the
usual intermodulation mixes at crowded radio sites.

For the non-believers in GPS interference, here are some papers
written about a local GPS interference (or jamming) incident:
<http://www.learnbydestroying.com/jeffl/GPS%20Jamming%20Moss%20Landing/>
Spoiler:  The interference was caused by a broadcast TV antenna
mounted amplifier and I had nothing to do with finding the source.


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