iGage iG4, Руководство пользователя

Страница: 49 / 57

49
Compare these two sky plots (same time, same location):
GPS Only GPS + G G BeiDou
More satellites is better. More signals is better. Even though the iG4 tracks GPS, GLONASS, Galileo
and BeiDou satellites, OPUS currently only uses the GPS observations. So a great constellation like
the one on the right is reduced to the minimal constellation on the left.
A modern GNSS RTK receiver has and uses more signals at all times than the OPUS processing tools..
Baseline Distance
OPUS processes GPS baselines from your receiver all the way back to each individual CORS station.
Typically these will be 45 KM (28 miles) to 150 KM (93 miles) baselines. In some areas the nearest
CORS station might be 250 KM distant!
RTK processes the baseline from your RTK Base to your RTK Rover which typically will be less than 10
KM (6 miles.)
Short baselines ‘Fix’ more easily and have substantially less noise.
#13 Fresnel Zone Considerations
Most GPS users think of the radio path from their receiver to each of the satellites is like a small
laser beam. This is incorrect.
The GPS beam width is spread out in a cigar shaped area known as the ‘Fresnel Zone’.
Fresnel is pronounced with a silent-s: Frenel), named after French physicist Monsieur Fresnel.
Wikipedia has an excellent article on the Fresnel effect: https://en.wikipedia.org/wiki/Fresnel_zone ;
be sure to checkout 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 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):