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error differential can be sent to another
GPS unit, a “rover”, that is out in the
field. This rover GPS unit is calculating
a position with error, but can apply the
error differential to this position and
correct it. Thus it is known as differential
correction.
Differences between types of differential
correction are in how the base
unit transmits the error differential
to the rover unit. But what are the
differences that make RTK so much
more accurate than the other methods
of differential correction? What must
take place that accounts for the inch or
less of accuracy?
Two main things account for RTK’s
accuracy: the accuracy of the known
position for the base station and the
proximity of the base to the rover.
The accuracy of the known base position
is determined during the setup
of a RTK base station. Recording and
averaging GPS positions for a 24 hour
period provides a location coordinate
with sub-inch accuracy. In addition
receivers used to set up a base station
are high quality units that also receive
other GNSS (Global Navigation Satellite
Systems). This means that besides
using GPS satellites, they can also
use GLONASS (Russian equivalent to
GPS) and Galileo (European) satellites,
which increases the accuracy of the
base location. The higher accuracy of
the RTK base station location, results
in the higher accuracy of a RTK calculated
position.
The other aspect of RTK is the proximity
between the base and the rover.
Other types of differential correction
system cover 30 to 300 miles. Most
RTK systems are designed for operation
in a 10 mile or less distance. Some
manufacturers will base estimates of
accuracy on the distance between base
and rover.
West Hills College has two specific
methods of RTK differential corrections
for use by students and the Farm
of the Future. The Farm of the Future
is 256 acres that was donated to West
Hills College to serve as a model of advanced
technology in agriculture. It includes
a pistachio orchard, various field
crops, equipment and an irrigation
system. A solar field supplies all of the
power for the farm. As new technology
becomes available, it is researched
or industry partners help provide the
resources. For example, West Hills
has received donations of a fixed wing
and rotor UAS drones which are used
as a part of industry demonstrations
and for student lab exercises. Students
were able to experience flying the UAS
to capture imagery from several fields.
In the same way students gain valuable
experience using the RTK. Farm of the
Future includes a mobile RTK station
and a cellular network RTK. Though
both are not necessary, each adds to
community needs and student learning.
The components of the West Hills
mobile RTK station include a Trimble
GNSS antenna, a Trimble 750MSL
GNSS base receiver, a SiteNet 900 radio
transmitter with antenna, deep cycle 12
volt battery, Trimble FMX1000 display
monitor, Trimble portable tripod, and
cabling to connect it all. The mobile
part of this RTK is the tripod that can
be setup next to the field in which
the signal will be used. This keeps the
signal as close as possible to the rover
and makes this the most accurate GPS
correction.
The antenna, receiver, and transmitter
are mounted somewhere on the tripod
which is placed securely at a location
next to the field. Cabling attaches
the GNSS antenna to the receiver and
another cable attaches the receiver, battery
and transmitter. The battery needs
to be a large tractor battery to provide
power for at least 24 hours of power.
Connecting the cabling is actually the
easy part. The system must be setup
so that each component communicates
Photo Credit: Terry Brase
The electronics for a SmartNet RTK include the receiver, cellular gateway, and power supply. The cellular antenna is on
the outside of the waterproof box and can be seen on page 24.
Page 26 Progressive Crop Consultant March/April 2017