Key operational parameters of GPS
receivers
Important operational parameters of GPS receivers
include accuracy, correction service, and hardware selection.
These technical aspects are explained in detail by our earlier
publications (Andrade-Sanchez and Heun (2010 and 2011))
AZ1558 January 2012
Pedro Andrade-Sanchez and John T. Heun
with particular emphasis in application in Arizona. The user
must carefully select between multiple options in order to
obtain satisfactory performance when implementing this
technology in mechanized operations.
Accuracy. GPS receivers are built to achieve certain accuracy
levels depending on their internal components, enabled
communication protocols, and unlocked capabilities in
firmware. Accuracy is perhaps the single most important
factor; therefore it is worth checking the manufacturer’s
claims on pass-to-pass and year-to-year accuracy. Remember
that GPS costs are related to their accuracy levels, so it pays
to do a careful analysis of accuracy needs in your farm.
Differential Correction. This is an essential function of
modern GPS receivers that is needed to obtain adequate
levels of accuracy for machine applications. Most agricultural
applications require sub-meter (less than 3 ft) pass-to-pass
accuracy, but very often accuracy is needed as close as a few
inches and even at sub-inch level. Correction services in the
U.S. such as Wide Area Augmentation System (WAAS) are
available free of charge. Private firms such as OMNI-Star
and John Deere provide excellent services to subscribing
clients by charging fees according to the accuracy levels.
When selecting a particular GPS receiver, it is important to
consider whether it comes loaded with WAAS or OMNIStar
capabilities, and to confirm that the receiver can be
upgraded to perform at higher levels of accuracy (RTK).
Real-time kinematics (RTK) is the most accurate GPS
correction system used in agricultural applications. Growers
using RTK GPS need access to a ground base station through
radio link. Users can choose to buy their own base station
or subscribe to a local network of RTK signal correction
stations. Subscribing to an existing station makes good
economic sense if it is available in the area. Both options
are very common in Arizona and in the near future we
expect that more will become available. The Department of
Transportation (DOT) is currently working in some states to
provide wide-range coverage of RTK-level correction signals
through their Continuously Operating Reference Stations
2 The University of Arizona Cooperative Extension
(CORS) system. There is much potential of widespread use
of RTK correction under the CORS infrastructure. A model
of application of CORS to agriculture has been tested in the
state of Alabama (Winstead et al., 2009). Private providers
are testing the delivery of correction signals through cellphone
modems.
Hardware Selection. One aspect often ignored is the
selection of the external antenna that has to match the
capabilities of the receiver. Newly released integrated GPS
systems require dual-frequency antennas to track L1 and
L2C satellite signals.