Precision farming in the northern grains region
Application of GPS in farming systems

Key points

• Global positioning systems (or GPS) offer growers a practical, affordable, and simple means of locating sites within a paddock.
• A GPS uses coded signals from a number of satellites to calculate the position on the earth's surface.
• Generally, precision is matched by receiver cost, but even cheap GPS units can provide accuracy to within 10-20 m.
• Application of GPS to farming systems includes self-steerage (or guidance) systems, data monitoring systems, and mapping facilities.

What is GPS and how does it work?

The Global Positioning System (GPS) is a satellite navigation system providing worldwide coverage. A group of 24 satellites, circling twice-daily 20,000 km above the earth's surface, transmit coded signals that are picked up by GPS receivers. The constellation of navigation satellites around the earth enables position to be determined anywhere at any time, and in any weather condition - for free!

By recognising the codes for each satellite, the receiver can determine the time taken for the signal to be transmitted. The GPS uses this information to then calculate the distance to each satellite. Once four or more satellites are located, the GPS "triangulates" the distances to provide a location on the earth's surface, i.e. longitude, latitude, and elevation.

However, the signal is still prone to a number of errors that can reduce the positional accuracy. These include atmospheric errors, multi-path errors, satellite and receiver errors, and intentional errors.

Atmospheric errors are introduced as the signal passes through the atmospheric layers. Charged particles and moisture droplets delay the signal, leading to timing inaccuracies. Atmospheric errors may range from 3 to 50 m, depending on the time of day and the arrangement of satellites in the sky. A "dual-frequency" GPS minimises these errors through computer modelling or by comparing the relative speeds of two different signals - but these receivers are costly.

Multi-path errors occur when the signal bounces off obstructions, such as buildings or sheds, before reaching the receiver. Such errors may exceed 100 m in certain situations. Complex signal rejection procedures - or simply using the GPS in wide-open spaces - should minimise these errors.

Satellite (or "ephemeris") errors result when the broadcast orbit differs from the actual orbit. The US Department of Defence uses radar to determine these errors, and any updated positional information can be added to the satellite code to reduce this error. Receiver errors result largely from noise or the use of inaccurate clocks inside the GPS unit - but can be minimised with more expensive clocks.

To prevent hostile parties using GPS, an intentional error was added to the satellite signal. This code degradation, known as "selective availability" (SA), introduced a random clock error into each satellite. When SA is operative, range measurements may be biased by as much as 30-50 m. Applying a "differential correction" minimised this intentional error (see "dGPS receivers" below) - however, SA was removed in May 2000, vastly improving the accuracy of all GPS units.

Tips for improving the accuracy of GPS

For any GPS receiver, there are several tips that can improve the positional accuracy.

1. Ensure at least four or more satellites have been sighted - it may take 10-30 seconds for the receiver to locate their codes. Avoid buildings, sheds, gullies or mountains.
2. The quality of the GPS signal is indicated by the PDOP (position dilution of precision): a low PDOP, such as less than 5, indicates the satellites are nicely spaced to give your position. A high PDOP may mean you need to wait several minutes before trying again, or that nearby obstructions are preventing the satellite signals being received. Sometimes, even facing towards the equator where more satellites are located can improve the PDOP.
3. For a highly accurate stationary position, average the GPS reading over a period of time. For example, collect signals over 2 min and then average the latitude, longitude and elevation.
4. Ensure the correction signal (for beacon-corrected GPS units) is less than 30 seconds old.

Application of GPS in farm management

Guidance systems

A number of visual or mechanical steerage-assist guidance systems for tractors or spray-rigs are commercially available (e.g. AgGuide, Trimble, AgSystems, Rinex). A major advantage with these systems is the reduced cost of overlap for each spray/fertiliser application, estimated conservatively at 10-12% for every operation.

The system also removes the need to use physical markers to identify strips within the paddock, however start-up costs can be high, and problems such as driver over-compensation or stress on the steering system are still being documented.

Visual guidance systems provide a visual tool, such as a light-bar or a revolving compass, to assist the operator in the direction of travel. Generally, these systems are less invasive to the steering assembly, can be re-installed to new machinery, and ensure the driver is still fully in control of the operation. However, the reaction time for the operator to sense a new direction can introduce a low degree of overlap. Although the traffic pattern cannot be repeated, a traffic map can be reproduced from the GPS. Light-bar systems cost around $7,000 (prices as at December 2003).

Automated guidance systems must be mounted within the steering apparatus of the rig, and are more difficult to install/remove. A traffic map can be created prior to the operation and loaded into the on-board computer, which then applies the GPS as the navigation tool. Within-paddock obstacles, such as contour banks, trees, gullies or fences, need to be entered into the system, and the driver is always able to over-ride the steering system.

The next generation of GPS-guided systems will be able to steer remotely and autonomously (see www.uga.edu/aboutUGA/compete-rowbot.html as an example).

Monitoring systems

Crop scouting, using the GPS and a mobile logging device (e.g. palmtop or laptop computer), enables the user to locate, identify and tag sites within a paddock. A number of portable monitors with high-capability software are available. These might include:

• Weed monitoring - both populations and weed species could be entered.
• Soil depth - moisture probes could be used at regular intervals to find soil rooting depth as an index of plant-available water capacity.
• Soil erosion events - regular monitoring of soil losses after a potentially erosive rainfall event can indicate sites of high risk, and the usefulness of any remedial action.
• Salinity - similarly, points of seepage can be noted regularly to indicate the spread or containment of water tables.
• Insect monitoring - current insect populations and ongoing yield-loss estimates could be logged during the scout.

Mapping facilities

The GPS is the engine that enables site-variable factors to be mapped including:

• Yield mapping - using a yield monitor and suitable software, the GPS enables one to log yield during the harvest operation. Some harvester manufacturers, including Case, John Deere, provide yield-monitoring capability with their latest combines - but require add-ons (i.e. GPS, data cards, and software) to upgrade to the yield-mapping function.
• Terrain mapping - some yield-mapping packages enable elevation to also be collected. While the vertical accuracy is typically a third that of horizontal accuracy, errors can be removed by averaging elevation from a number of maps. Elevation maps can be produced in many yield-mapping software packages.
• Soil mapping - using instruments, such as electromagnetic induction sensors or soil colour/texture sensors, the GPS enables a map to be constructed of soil changes within a paddock.
• Field size and boundaries - the GPS enables the size of paddocks to be determined, and provides a map of paddock boundaries and crop rotations.

Choosing a GPS

A range of GPS receivers is available for an equally broad range of prices (as at August 2001). The key is to work out the purpose for using the GPS.

Basic GPS receivers (GPS)

A basic GPS receiver is able to provide a position with some degree of error. These devices are generally cheap (less than $900), provide on-screen tracking and are usually hand-sized. Accuracy was relatively poor until SA was turned off, however a basic GPS is now capable of sub-20 m accuracy. There are a range of manufacturers including Garmin, CSI, Trimble and Magellan.

Differential GPS receivers (dGPS)

There are a number of systems developed to minimise the SA, and improve positional accuracy to between 2-5 m. Although these devices are more expensive than basic GPS devices (mostly $1,000-$7,000) and may require a well-placed antenna to receive the differential signal, they provide real-time qualitative information and can be programmable. The differential signal is calculated from a GPS located on a known site capable of correcting any SA or atmospheric errors. The correction factor is transmitted to the dGPS via a number of methods:

• Maritime radio beacons have been established by the Australian Maritime Safety Authority  along the coast, particularly adjacent to the Great Barrier Reef, to broadcast the correction signal for ships. The accuracy of the transmitted signal declines with distance from the station (usually about 1 m error per 150 km).
• A correction service is available, through subscription, from AUSNAV via FM radio (e.g. Triple J). See www.amps.com.au/gps.htm for more information. Coverage is limited by the ability to pick up the FM broadcasts, which can decline in quality with weather or topography. Their coverage includes from the Sunshine Coast to Adelaide via Melbourne.
• An annual subscription to a specialised GPS operator, such as OmniSTAR, enables the correction signal to be broadcast via satellite. Subscription fees range from $2,00 per annum. GPS receiver manufacturers include CSI, Leica, RDS, OmniSTAR/Fugro, Novatel, Ashtech and Trimble.
• A system known as WAAS (wide-area augmentation system) has been well used in the USA to improve accuracy of GPS devices. It appears unlikely that Australia will receive this system due to the installation price, however the aviation industry is moving into a LAAS (local area augmentation system) for enhanced accuracy

Emerging technologies are integrating hand-held computers with GPS devices for mobile mapping applications. For example, the Leadtek Bluetooth GPS receiver allows you to receive GPS data on mobile handhelds wirelessly. The Heicom wireless GPS-handheld uses a compact flash card to provide the GPS hardware upgrading the handheld to operate as a GPS. See the Aus Navigation website for some examples of these technologies. NAVMAN is another card-based plug-in enabling seamless GPS data capture.

Table 1. Summary of advantages and disadvantages of correction factor methods

	Advantages	Disadvantages
Marine beacon	Free signal Cheap receivers ($1,500)	Less accurate away from coast No signal over 400 km from nearest beacon signal.
FM broadcast	Cheap set-up costs (GPS receiver $700-3,500; FM receiver <$1,000)	Annual subscription fees Limited coverage
Satellite	Total coverage across Australia and across locations Highly accurate (sub-metre)	Receivers can be expensive ($6-10,000) Annual subscription fee

Web sites on GPS technology

The best up-to-date information on GPS technology can be found on the web; much of this is updated fairly regularly.

GPS providers/manufacturers

• Omnistar
• Thales Navigation
• CSI Wireless
• Magellan  
• Garmin  
• Novatel
• Trimble 

GPS signals are most useful when a map can be created or a sample point can be relocated. The easiest way to achieve this is to purchase a GPS unit with mapping capability. Alternatively, one might consider purchasing a palmtop computer with a GPS data-capture software package. Some that are currently available include:

• OziExplorer
• CatchmanPlus
• Fieldworker
• AgGPS EZ-Map

Further information

• Global Positioning System Consortium (GPSCO) 1998. 'Exploring GPS - a GPS users guide.' (GPSCO: Bathurst)  ISBN 0 9585281 0 1.
• GIS User: bimonthly magazine focusing on Australasian geographic information systems applications.
• Measure & Map (M&M); bimonthly magazine on surveying and mapping in Australia.
• Asian Surveying and Mapping Magazine: a monthly online magazine with articles on regional mapping and satellite/GPS issues.

See also the other notes in the precision farming in the northern grains region series:

• What is "precision farming"?
• Soil compaction and controlled traffic farming.
• Using variable rate technology (VRT) in cropping land.
• Identifying subsoil constraints.

Key contacts

The DPI&F Business Information Centre (phone 13 25 23) provides generalist information and specialist referral services for the cost of a local call from anywhere in Queensland. The Business Information Centre is open Monday to Friday 8 a.m. to 6 p.m. (excluding public holidays).

FS0419. Last updated 14 January 2004