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The use of GPS and mobile mapping for decision-based precision agriculture
Information Technology Behind Precision Agriculture
As indicated earlier, much of the ability to implement precision agriculture is based on information technologies; in particular, global positioning and navigation and geospatial mapping and analysis. From being to end, the cornerstone for precision agriculture is based on precise locations and time. Fortunately the Global Positioning System (GPS) provides positioning, velocity, and timing capability, and Geographic Information Systems (GIS) provide mapping and analysis capability.
Global Positioning System
The United States of America developed and initiated the NAVSTAR Global Positioning System (GPS) for the purpose of providing navigation and positioning capability anywhere on earth, anytime, under any and all conditions. GPS provides a Standard Positioning Service (SPS) which is available to anyone to use at no cost. SPS is governed by the Federal Radionavigation Plan, which specifies the level of accuracy and integrity that the service must provide.
The initial design of SPS included an introduced error know as Selective Availability (SA). With SA, the GPS-SPS provided horizontal positional accuracies on the order of 100 meters. This level of accuracy was useful for general navigation and positioning but not for precision agriculture.
On May 1, 2000, then President Clinton issued and Executive Order to eliminate SA. At midnight May 1, 2000, SA was permantely turned off.
The elimination of SA immediately improved the performance of GPS by almost 10 fold. Horizontal accuracies have been reported as good as 4-6 meters but generally closer to 10 meters. Currently, the 1999 Federal Radionavigation plan is being updated to reflect the changes in the GPS-Standard Positioning Service after the elimination of Selective Availability.
Even with the elimination of SA, some precision agriculture techniques require even better accuracy, generally on the order of 1- 2 meters. Fortunately, there is a method of improving GPS accuracy called Differential Global Positioning System (DGPS).
Differential Global Positioning System
DGPS is a technique that corrects for some of the natural and introduced errors common to normal GPS observations, thus improving GPS positions. DGPS corrections can be applied in two ways:
- Post Processing - a technique that requires the GPS user to collect GPS data and then, using specialized software, process the GPS data with DGPS data, collected at the same time, from a known location like a base station or permanent reference station.
- Real Time DGPS - Real time DGPS (R/T DGPS) allows the GPS user to immediately take advantage of differential corrections that are broadcast in real time from DGPS services. The obvious advantage is the immediate improved accuracy that allows for:
- Single visit; Time is valuable and costly return trips can be avoided.
- Increased Efficiency: R/T DGPS eliminates post processing and saves money, time and energy.
- Better Results: R/T DGPS provides the capability for accurate mobile mapping.
R/T DGPS Services
R/T DGPS services are provided by Commercial DGPS services and
by Government DGPS services. Both Commercial and Government
provided DGPS services to the user by either satellite-based systems
or land-based systems. These services are augmentations to GPS.
U.S. Government DGPS Augmentations
Until recently, GPS users who wanted to get differential corrections
had to pay a subscription fee to a private company or maintain
their own base station in order to acquire differential corrections.
Today, there are U.S. Government R/T DGPS services that are
available to anyone at no cost:
Wide Area Augmentation System (WAAS)
The Wide Area Augmentation System, known as WAAS, is a
satellite-based system that is being developed to meet Federal
Aviation Administration (FAA) requirements for a safety-critical
navigation system. It is designed for aviation, providing improved
accuracy, integrity, and availability of the basic GPS signals at 300
feet above ground. This system will allow GPS to be used as a
primary means of navigation for enroute travel and non-precision
approaches in the U.S., as well as for Category I approaches to
selected airports throughout the nation.
WAAS is a relatively new system and is currently under
development and testing prior to FAA certification for safety-of-flight
applications. Initial testing began in August 2000. Currently,
while testing, there are planned outages. Even though WAAS is
available throughout the U.S. it is still difficult to rely on. As WAAS
becomes more capable, GPS receivers that also have WAAS capability
will be very useful.
Nationwide Differential GPS Service (NDGPS)
In 1990 the US Coast Guard (USCG) established a ground-based
differential GPS, covering the coastal areas and navigable waterways.
In 1994 a GPS Augmentation study was conducted and recommended
that the Department of Transportation (DOT) expand the USCG's
maritime system to provide continues marine and land coverage for
surface users. In 1997, Federal agencies began planning for a DGPS
system to provide Nationwide coverage; the Nationwide Differential
GPS (NDGPS).
Using the USCG's existing DGPS radiobeacon network as a
template, covering both coastlines, the Gulf of Mexico, the Great
Lakes, and major inland waterways, the NDGPS program intends to
densify the existing network with dual redundant terrestrial and
waterway coverage, providing service to the remaining fifty-five
percent of the continental US and Alaska. Under an exemplary
cooperative effort, many government agencies are contributing to
this program. A total of 53 Ground Wave Emergency Network
(GWEN) sites located around the country are being converted from
their old communications configurations to real-time GPS reference
station installations. Differential corrections will be broadcast, at
around 300 kHz, from 90-meter-tall GWEN transmission towers.
Once fully operational, the NDGPS will cover the nation
with the most accurate and reliable navigation system that the
country has ever had. GPS users, both civilian and government, will
have free access to the NDGPS.
The NDGPS will augment the existing satellite system with
ground-based radio transmitters, known as reference stations. The
reference stations will broadcast a signal from a transmitter located
at a known fixed location on the ground. Users who receive the
ground-based signal in addition to the normal GPS satellite signals
will be able to determine their position with greater accuracy.
Reasons for using NDGPS
Real Time Horizontal Accuracies of 1-5 meters.
The positional accuracy that is supported by the NDGPS system
varies with the user's distance from the reference station. Generally
the horizontal accuracy is about 1 meter for each 100 meters from
the reference tower.
Availability - 99.7%
Availability for a given broadcast is defined as the percentage of
time in a one-month period during which a DGPS broadcast transmits
healthy correction signals at the specified output level. The current
NDGPS is designed for, and is operated to maintain a broadcast
availability level, which exceeds 99.7%.
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