iGage iG9 User Manual
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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 though 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):
1 foot above your GNSS antenna, the beam width is 1.6’ in diameter. 20 feet above the
Rover
antenna (perhaps the midpoint of tree canopy), the 1
st
Fresnel beam diameter is 7 feet! A clearing in
the treetops 100’ above your antenna needs to be 16’ in diamet
er.
At the midpoint between your receiver and the satellite, the Fresnel beam is over 6,000 feet in
diameter! And that is for the signal for a single satellite, multiply this by the number of tracked
satellites and there is signal energy everywhere.
OPUS Best Practices Conclusion
There are lots of things that can go wrong with OPUS occupations. Some you can control, some you
can’t.
If you stack multiple problems:
Bad Constel Short Occu Moderate Bad HI => FAILURE
Your OPUS solutions will fail or have high RMS estimates and the time you spent collecting the
observation will be wasted.
The OPUS family of online tools: OPUS-Static, OPUS-RS, OPUS-Projects are amazing. They allow users
to generate reliable X, Y and Height coordinates for GPS suitable locations, anywhere in the world.
Hopefully by utilizing the simple rules presented in this chapter, all your jobs will be
OPUS-Successful!
