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Things to know before you buy a guidance system

Global Positioning System (GPS) guidance systems are one of the latest and most popular advancements in farm equipment.

DGPS guidance steering aides guide the operator through the field and can be used in place of visual marking systems such as row and foam markers. These systems need an operator steering the vehicle at all times and can be accurate up to 20 inches. These systems are prone to drift (short-term repeatability). A typical system will cost between $2600 to 5,000.

The auto-steering guidance systems have the capability to steer a machine across the field. At the end of the field, the driver must steer the machine to the next pass where the auto-steering system will steer itself as directed.

The task of choosing a guidance system can be challenging and overwhelming because different brands offer different features. Buyers need to decide what their needs are and then choose a guidance system that matches their farming situation. Knowing the answers will help to identify the appropriate guidance system.

There are three main sources of differential signal available for use with DGPS-based guidance system.

1. The United States Coast Guard Beacon Signal (USCGBS) is limited to major bodies of water and rivers. The USCGBS is free. As the distance increases (>100 miles) from a beacon tower to the mobile antenna, the signal may be degraded or interrupted by weather or structures.

2. The second differential source is fee-based through commercial differential signal service providers. This differential source is a satellite-based correction signal and is available worldwide.

3. The newest source of differential signal is Wide-Area Augmentation System (WAAS) from the Federal Aviation Administration. This differential source is also a satellite-based correction signal. WAAS has performed well in most applications and can provide a free differential signal for the agricultural user. For guidance steering aids, the accuracy of the different DGPS single frequency correction sources should be similar for field applications.

For guidance applications, the pass-to-pass accuracy (short-term repeatability) needs to be as close as possible. The guidance systems will usually have a pass-to-pass accuracy within 20 inches of the desired line. With an 20-inch accuracy, a guidance system can be used on a sprayer, fertilizer spreader, tillage equipment and depending on how accurate guess rows are needed, it may be used to plant soybeans. The use of guidance steering aids to plant row crops such as corn or wheat is not recommended at this time. For needed accuracies less than 20 inches, the guidance options to increase accuracy usually require an auto-steering system.

To select a guidance system for the farming operation a few issues need to be addressed:

What applications will be performed with the guidance system? The guidance steering aid will help in most agricultural operations. The exceptions may be planting. The operator will have to determine the level of error associtated with guidance steering aids and planter guess row width.

What equipment will the guidance steering aid be used in? Does the equipment have an enclosure or not? If there is no enclosure then there needs to have a guidance steering aid that is waterproof.

Where will the guidance steering aid be mounted? If it is inside a cab, then a suction cup mount will work or placing it on a flat surface (if available). If the guidance steering aid is to be mounted on the hood, then the hood should either be metal or have some type of magnetic mount available. Suction cups are not recommended for the hood. Fiberglass hoods can be difficult to mount guidance systems. The mounting of the guidance display may be a factor in which system suits the operation. Some display units are manufactured for inside the cab only, where others can be used inside or outside of the cab.

Features on the visual display will aid the driver in staying on the desired line of travel such as heading error (look ahead), swath number, or error from desired line of travel. These features help the driver determine the amount of correction needed for turning and staying on desired line of travel.

Make sure the GPS receiver has an update rate that will meet the needs of the guidance system. Most guidance systems have an update rate of 5-Hz to operate efficiently and correctly. A faster update rate does not increase GPS receiver accuracy, it provides more location information per second.

Guidance displays are either a light bar or visual screen. Light bars come in many versions, but the main premise is a row of lights that indicate where the vehicle is in respect to the desired line of travel. The visual screen may be a computer screen or LCD screen. The visual screen may help when driving curves. Both types of visual displays will aid the driver on the desired line of travel effectively. It is up the driver to determine which type of screen is suitable for the operation.

Many guidance systems have the option (serial port) for a datalogger such as PDA or yield monitor console where an application record or field information can be collected. This information can be part of the spray record or planting pattern to overlay with future yield data. Some guidance systems will calculate boundary or applied acres, this will depend on the model and specifications.

Prices differ when comparing guidance systems. Make sure that the price quoted has all of the necessary components and cables to complete the intended tasks. If an external device uses GPS such as a controller or computer, there may be special cabling needed.

Do you have an agronomy question? Email We'll send some of the most common questions to professionals in the industry and see what they say. Look for answers in upcoming Agro-Connect Ask the Experts columns.

Global Positioning System (GPS) guidance systems are one of the latest and most popular advancements in farm equipment.

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