By now, almost everyone knows the initials GPS stand for Global Positioning System and that GPS is a satellite-based radio navigation system that can greatly help us in a variety of ways, such as for guidance on streets or during outdoors events such as fishing or hiking. It also is well known that private companies use GPS to optimize delivery routes and determine location of vehicles, becoming more efficient at their jobs. Even cell phones now have a GPS chip that can be used for guidance or for help. What about in agriculture? In what forms can we use a GPS receiver to become more efficient in what we do? This article will try to explore some ways that farmers can make use of a GPS receiver to increase their efficiency and cut costs.

GPS Basics

The U.S. Department of Defense began developing the world’s first GPS system in 1973. In 1978 the first 1,900-lb. satellite was launched into a twice-daily orbit at a 55 degree angle to the equator. These satellites are 17 feet across with solar panels extended. Each satellite contains a computer, four atomic clocks accurate to a billionth of a second (0.000000001 second), and a 50-watt radio. Twenty-four satellites orbit at an altitude of 10,800 miles. Six different “spreads” of four satellites each orbit 60 degrees apart where they pass over the equator. With this configuration, at least five satellites are in view at nearly all times from any point on Earth.

These satellites continuously broadcast data, including time (to 1 billionth of a second) and their precise location relative to Earth at that instant. On the ground, all GPS receivers contain a computer and an ephemeris (or almanac) that provides information about each satellite’s location. A GPS receiver determines its position by continuously calculating its distance from three or more GPS satellites based on their time and location data—a process known as “triangulating.” Some GPS systems can guide massive agricultural machines as accurately as +/- 1 inch (2.5cm) or closer from an established row while moving at speeds of 12 mph (19 km/h) or faster.

GPS accuracy

The accuracy of a GPS receiver, or how close the position the receiver is reporting is to the real position, is a function of several factors, including but not limited to the type and cost of the receiver, the type of differential correction being used and the weather. In general, more expensive receivers have better computers and clocks, which allow them to perform calculations faster, translating into higher accuracy. Higher accuracy can also be obtained by using differential correction, a method where stationary receivers are used to figure out an error rate and transmit it to roving receivers, which in turn apply this correction to their signal, increasing their accuracy significantly.

There are several sources of differential correction available, some free of charge, some not. Perhaps the most used are WAAS (Wide Area Augmentation System) provided free of charge by the FAA (Federal Aviation Administration, www.faa.gov) and the beacon system provided by the US Coast Guard (www.navcen.uscg.gov). Most receivers today are programmed to receive both signals. OmniSTAR (www.omnistar.com) is a company that is present in several countries offering subscription-based, wide-area differential GPS service. Why use a subscription service while you can use a free WAAS differential GPS? Sometimes only a paid service can offer you the accuracy and reliability your business needs.

GPS applications in agriculture

Everything that we do on the farm, such as planting, scouting, spraying or harvesting, can be referenced to using coordinates (latitude and longitude). Latitude and longitude are the unique address of every inch of ground on the face of the earth, and that’s what the GPS receiver reports. We can use this unique address to store information and plan activities for each location. Let’s see some examples:

·Soil sampling: farmers have to pull soil samples and send them to the laboratory to verify nutrient levels (P, K, micro-nutrients) before planting. Fertilizer programs are developed based on the results obtained from those samples. If GPS coordinates (latitude and longitude) are known for each sample, a nutrient map can be developed (using specialized software). Nutrient maps are very useful to show nutrient variation throughout the farming area. Fertilization programs are developed based on nutrient maps and crop needs. Specialized machinery can distribute fertilizer or lime to the fields based on nutrient maps and current coordinates.

·Dynamic soil mapping: an alternative to traditional soil sampling, soil mapping uses a GPS receiver and sensors, such as pH meter or soil electrical conductivity, to map an area and guide liming operations or to build soil texture maps that can be used to create different management zones. Veris technologies (www.veristech.com) from Salina, Kan., is a popular manufacturer of soil-mapping tools.

·Variable-rate fertilization: once soil samples are transformed in nutrient maps and a fertilization program is on the way, a farmer may opt to use specialized machinery to deliver variable quantities of nutrients based on field requirements and GPS location. The ability to vary the quantity of fertilizer being delivered to different places is the heart of a precision farming plan. The logic is simple -- to adjust fertilizer delivery based on field requirements instead of delivering a blanket rate for the entire field.

·Machine guidance: from simple lightbars that can aid drivers to keep straight lines to expensive GPS-based equipment that can automatically steer farm machinery and implements, machine guidance is becoming the fastest-growing GPS-based technology on use at the farm. Guidance systems help farmers avoid waste of chemicals and fuel, reduce operator fatigue and increase hours of operation. Several companies offer GPS-based guidance equipment to farmers -- Trimble Navigation (www.trimble.com), Topcon (www.topconpa.com), Ag Leader (www.agleader.com) and most of the machinery manufactures such as John Deere (www.deere.com) and Case IH (www.caseih.com) offer solutions for machine guidance.

·Planting: the same concept of variable-rate fertilizing can be applied to planting. Machinery can actually vary the number of seeds per linear foot according to a map and GPS coordinates. Research is trying to provide answers for variable-rate planting. Another use of GPS-based technology during planting is the ability to shut off individual rows of the planter at the end of the field or in “no plant” zones.

·Spraying: in its simplest form, a GPS can be used to relay sprayer speed to a controller so it can adjust system flow to match the required volume. Variable-rate spray maps can be developed, and GPS receivers can be used to relay machine location and speed. GPS-based shut-off valves can shut off sections of the boom to avoid overlapping and waste of chemicals. Agricultural aircraft use GPS receivers to guide them throughout the field when spraying and to adjust flow rate to match aircraft speed.

·Harvesting: a yield monitor and a GPS receiver can aid a producer in mapping yield variation throughout the field. Yield maps are used in a variety of ways, such as provide information for future long-term fertilization programs, variety trials and contract negotiation.

Popular Web sites with information about GPS systems:

• Global Positioning System. Serving the World. Official U.S. Government GPS Web site. (http://www.gps.gov).
• Global Positioning System Home Page of the U.S. Forest Service (www.fs.fed.us/database/gps/). Lots of good information.
• Garmin’s “What is GPS” Web site. (www8.garmin.com/aboutGPS/)
• Global Positioning System, from Wikipedia. http://en.wikipedia.org/wiki/gps.
• The Global Positioning Systems Resource Library. http://www.gpsy.com/gpsinfo/
• GPS tutorial offered by Trimble. Access at www.trimble.com/gps/index.shtml