Re: Difference between GPS and measured distace.

On Mon, 26 Aug 2024 21:23:11 GMT, Roger Merriman <roger@sarlet.com>
wrote:

cyclintom <cyclintom@yahoo.com> wrote:

I've already said that it is probably a radio interference problem since
it only occurs on GPS near the Airport. Once I installed the speed sensot
on the wheels it disappeared. Do you suppose that's a "Tom thing"?

It is likely to be a "Tom Thing" when only Tom has the problem.

Since I ride though almost certainly closer as Heathrow was built close to
the various then villages and the roads many haven’t changed alignment, let
alone it’s size and I use the same unit no problem nor does it seem to be a
reported problem to the best of my knowledge.

Is your cycling computer setup to use the same Satellites as Tom's
computer?  Tom is using a Garmin 830, which offers the option of using
GPS and *EITHER* Galileo or GLONAS satellite constellations.  Each
constellation uses a different frequency range (or band).  The Garmin
830 doesn't have enough horsepower to use more than 2 constellations
at one time.

Meanwhile, you are in the UK where Galileo is the most commonly used
for navigation.  I don't know (or remember) which cycling computer you
have.  If it only does Galileo then interference from a US located GPS
pseudolite (ground based satellite) will act very different compared
to a UK based pseudolite operating on Galileo frequencies.  The
difference in pseudolite frequencies will make the two cycling
computers respond differently to ground based interference.
<https://en.wikipedia.org/wiki/Pseudolite>
The interference could be coming various communications equipment
located at the airport.  These will also likely be somewhat different
in the US and UK.


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