The technology that enables auto-steering is rooted in the U.S. Government's Global Positioning System [GPS]. Developed initially for military applications, the first satellite was launched in 1978. The GPS became fully operational in 1995 and was quickly adopted for various applications, including agriculture.
The total GPS operating system is comprised of 24 satellites in 6 orbits around the globe to ensure that almost every point on earth has 9-12 satellites above its horizon. (To see the coverage in your area, check out the www.newholland.com/na Web site for a free, downloadable GPS mission planning software tool.* When you select your location and day, this software will show you the number of satellites visible for every hour of the day. See Figure 1.)
GPS vs. DGPS
The GPS signal alone is not accurate enough for various precision agricultural tasks such as spraying, planting and harvesting because errors are introduced in the measurement system in a number of ways:
- Inaccuracies in GPS receiver clocks
- Satellite orbit variations
- The configuration of the satellites in view
- The atmospheric impact of bending the radio signals from the satellites
- A signal may be received directly from a satellite or via a reflection from another object causing a "multi-path" which can
confuse the position calculation.
These errors can introduce an inaccuracy of a few meters, which is problematic for most agricultural operations. The Differential GPS or DGPS corrects for these errors.
DGPS relies on a reference station whose position is measured very accurately. The station receives the GPS signal and at each instant of time it can calculate its position from the GPS (akin to how your GPS receiver does it). Since its exact location is known, the difference between the calculation and the accurate knowledge of the station's position can be made available to the correction signal provider. The correction signal provider "packages" all the differences from all reference stations and provides it to the user's receiver. Now, your receiver knows about errors in similar receivers around it and can insert a correction in its calculation to yield a better accurate position.
Corrections may come into your receiver via either stationary satellite transmission (for correction signals yielding results** of 4" – 10") or via direct transmission via a local reference station for a more accurate signal of 1". The main reason for the improved accuracy with the local reference station is that the correction is derived from conditions very close to where your equipment operates. This methodology is called Real Time Kinematic (RTK).
In order to receive the correction signal, you must possess the proper receiver. The NH252 receiver is capable of receiving the free WASS signal provided by the U.S. Government or the paid subscription signals provided by OmniStar. An RTK base station is required for the most accurate
correction for your GPS measurements.
Mission Planning Software
Free, downloadable GPS mission planning software is a powerful tool that determines the visibility of GPS satellites. Input your location to the software so you can find out satellite availability and better plan your fieldwork.*
Easy-to-read bar graphs give you an hourly summary of the number of visible satellites, while sky plots show you where those satellites are located. You can anticipate potentially degraded GPS performance and plan your work to avoid costly downtime.
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* Go to www.newholland.com/na. On the home page, click on: >Products>Agricultural Guidance Systems>IntelliSteer™ Auto Steering Systems>IntelliSteer™ Tools
** The accuracy of any auto-steering system is dependent on satellite communication, field location, terrain, tractor and implement condition and setup, system installation,
and calibration. A field demonstration and trial is the best way to determine actual guidance system accuracy in each situation.
How does GPS work?
GPS works in five logical steps:
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Triangulation. By very accurately measuring the distance of a location from three satellites, it is possible to "triangulate" a position anywhere on earth.
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An accurate measure of distance. To measure the distance to satellites, the GPS receiver uses the travel time of radio signals.
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Perfect timing. To measure the travel time, GPS requires extremely accurate timing.
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Exact satellite location. To achieve the highest accuracy, the exact location of the GPS satellites in space must be known.
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Corrects errors. Finally, corrections must be made for any delays the signal experiences as it travels through the atmosphere.
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The Different Kinds of Differential Signals
1. WAAS
The Wide Area Augmentation System (WAAS) was created by the Federal Aviation Administration (FAA) as a free-to-air differential correction service. The WAAS network consists of 25 ground reference stations located in the United States. This network receives GPS signals from all GPS satellites in view. The GPS data is sent to a master control site, where GPS differential corrections and GPS satellite health data are transmitted to geostationary satellites. These satellites broadcast the information to all WAAS-capable GPS receivers, which then decode the WAAS signal to provide real-time differential corrections. WAAS signals are broadcast free to all WAAS-enabled receivers. Currently, WAAS coverage is only available in the United States and parts of Canada.
2. OmniSTAR VBS or HP
OmniSTAR is a commercial subscription service that provides real-time correction for GPS measurements. OmniSTAR continuously monitors the signals of all GPS satellites at over 90 known locations around the world, a number of which are in North America. The reference stations measure the errors from all GPS satellites in view at their locations and compute corrections. OmniSTAR utilizes several commercial geostationary satellites around the world to deliver these corrections to customers.
OmniSTAR VBS (Virtual Base Station) subscriptions are available worldwide within the OmniSTAR coverage area. It provides similar accuracy to WAAS DGPS or to within ten inches.
An OmniSTAR HP (High Performance) subscription provides accuracy within four inches for subscribers worldwide within the OmniSTAR coverage area.
3. A Real Time Kinematics (RTK) base station
An investment in an RTK base station provides the most accurate correction for your GPS measurements. You can locate the reference station in a permanent location, or you can use a mobile unit that is tripod or post-mounted. The reference station transmits real-time positional carrier phase data over a radio datalink to the receiver on your tractor or equipment. The receiver is then able to compute its exact position in your field to within one-inch accuracy.
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