Agriculture Reference
In-Depth Information
5.5.1 S
PACE
-B
ASED
P
OSITIONING
S
YSTEMS
Advancements in sensing, communication, and control technologies coupled with
GNSS and Geographical Information Systems (GIS) are aiding the progression of
agricultural machines from the simple, mechanical machines of yesterday to the
intelligent, autonomous vehicles of the future.
The U.S. Global Positioning System (GPS) is maintained by the U.S. government
and has been in operation since the late 1970s. The benefits of GPS, specifically in
the agricultural industry, have been well documented as they have progressed from
point location mapping (soil sampling or yield monitoring) to real-time equipment
control (auto-steer or map-based automatic section control) (USCGNC, 2010a). To
increase the accuracy of the existing GPS network, additional technologies have been
developed by both public and private institutions. The Nationwide Differential GPS
System was developed for use in the United States and included beacons maintained
by the U.S. Coast Guard and the Department of Transportation. The Wide Area
Augmentation System (WAAS) is operated by the Federal Aviation Administration.
The WAAS network has become available for a variety of other users desiring sub-
meter accuracy who have compatible receivers. A more recently developed system for
improving GPS accuracy is the Continuously Operating Reference Stations (CORS)
that was initially created by the National Oceanic and Atmospheric Administration.
Since its inception, additional organizations have joined the network and provided
correction data from their land-based GPS stations (US-CGNC, 2010b).
The Global Navigation Satellite System (GLONASS) is a Russian-operated satel-
lite network that was developed in the late 1970s and was extended to non-military
use in 2007. GLONASS is comparable to the U.S. GPS system and was created
to provide real-time positioning data to compatible receivers. The GLONASS sys-
tem is continually upgraded as existing satellites exceed their service life and new
series replace them. The GLONASS-M series is currently in operation, with the
GLONASS-K1 series expected to become operational in 2011 (FSA-IAC, 2010).
Galileo, a GNSS, is being developed by the European Union (EU) to provide a
network of satellites separate from the Russian and U.S. systems now in use. The
Galileo system has been developed by the European Space Agency primarily to
provide real-time positioning data for civilian use and was designed to be compat-
ible with the Russian and U.S. systems. Two experimental satellites have been suc-
cessfully launched, and four additional satellites were launched in 2011 to validate
system operation (ESA, 2010).
The accuracy of differential global position systems (DGPS) degrades with
increasing distance to the reference station. For DGPS systems, an inter-receiver
distance of a few hundred kilometers will yield a submeter level accuracy, whereas
for RTK systems a centimeter level accuracy is obtained for distances of less than
10 km. To service larger areas without compromising on the accuracy, several refer-
ence stations have to be deployed. Instead of increasing the number of real reference
stations, VRS are created from the observations of the closest reference stations. The
locations of the VRS can be selected freely but should not exceed a few kilometers
from the rover stations. Typically, one VRS is computed for a local area and working
day.
