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iG5 User Manual 76
Wikipedia has an excellent article on the Fresnel effect:
https://en.wikipedia.org/wiki/Fresnel_zone ; be sure to check
out the section on ‘Fresnel Zone Clearance’ mid -article.
The Fresnel effect
explains for why
your GPS receiver
will track a
satellite which is
fully behind a
building or
ridgetop. The
beam width is
wide enough that
a portion of the
signal reaches the GPS receiver, even thoug
h the beam’s
center is fully blocked by the building.
Tracking a satellite means that the satellite is ‘visible’ to your
receiver, however just tracking is not sufficient to accurately
evaluate a carrier-phase position.
To compute an accurate position, your receiver needs a very
clean signal with few reflections, obstructions or delays. Any
object blocking a part of the beam can be a source of
reflection, attenuation or delay.
Clear path means that you don’t just need a small opening in
the trees for a laser beam to shoot through. You need an
opening in the trees large enough that
most of the energy
which is spread out over the Fresnel beam width reaches the
receiver with no obstructions.
How wide is the Fresnel beam along the path? Much wider
than you think!
Here is a beam-width chart for GPS L1 (1.575 GHz):