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Important GPS Accuracy Definitions:
PASS TO PASS accuracy measures
the relative accuracy over a 15 minute interval - usually thought of as
guess row error when driving rows, or skip / overlap from one pass to the
next when driving swaths. A Trimble GPS receiver with pass-to-pass
accuracy of +/- 4 inches skip or overlap, 95% of the time.
YEAR TO YEAR accuracy is the
measure of repeatable accuracy that you can drive the same rows a day, week,
month, or year later. So, a +/- 1 inch year-to-year accuracy means you
can drive the same rows next year within 1 inch of this years rows, 95% of
the time.
GPS EXPLAINED
Differential GPS (DGPS) WITH BEACON CORRECTION
The vehicle with a GPS antenna receives GPS signals from the
GPS satellite constellation. The Beacon receiver, at a known location, receives
GPS signals. The beacon generates an equation that changes the location of where
the GPS satellites say it is, to where it knows it is, and then sends the
equation known as the 'correction message' to the GPS antenna on the
vehicle—which then applies the correction.
DIffERENTIAL GPS (DGPS) WITH WAAS AND
OMNISTAR CORRECTION:
The vehicle with a GPS antenna receives GPS signals from the GPS satellite
constellation. The WAAS and OmniSTAR services have many GPS receivers at known
reference locations that send the correction messages to control
stations which then uplink the message to a geostationary satellite (WAAS or
OmniSTAR). The geostationary satellite (WAAS or OmniSTAR) then sends the
correction message to the GPS antenna on the vehicle, which applies the
correction.
RTK (REAL TIME KINEMATIC)
This is a highly precise technique that results in one inch year-to-year
accuracy. RTK GPS requires two specialized GPS receivers and two radios.
One GPS receiver is set up as a base station within radio range of the field you
are working so it can send the correction message to the roving receiver. Both
receivers collect extra data from the GPS satellites, known as L2 Band, that
enables better precision.
*Information from
Trimble Precision Ag Product Brochure.
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