Selecting A GPS Receiver

In addition to the article in the July 2007 issue, "Precision Agriculture 101," an Ohio State University researcher offers growers more information on what to consider before purchasing GPS receivers.

By Nathan Watermeier

Several important features must be considered when purchasing suitable GPS (Global Positioning System) receivers and supporting hardware and software for your operation. The GPS receiver is one of the critical pieces to tie all the data collection, navigation, and application maps together to make the necessary operation or decision within a field. There are several different GPS receivers on the market that serve different purposes. The following are some suggested recommendations when planning to get started with GPS.

Consider the cost and accuracy of the GPS receiver. The costs can range from $100 to $50,000. The greater the accuracy, the greater the cost -- this is usually the general rule-of-thumb on prices for GPS receivers. For agricultural applications, receivers can be sold separately or in most cases they have been packaged with electronic controllers such as variable rate drives, yield monitors, guidance systems, and field data collection systems.

• A low-cost receiver without differential correction is useful for road navigation, hiking, and fishing and usually provides greater error (average 30 feet) for most field activities in agriculture. It can be used for simple documentation purposes to get you in the right vicinity. Within the past several years, low-cost GPS receivers with WAAS differential correction have entered the market to provide suitable reception for several field data collection and operational activities. These receivers provide differential correction and are suited for collecting field boundaries, crop scouting, soil sampling, and general crop documentation. The low-cost WAAS receivers generally run between $130 and $500 and indicate an accuracy range up to 10 feet. Some of these receivers have a higher hertz rate (5 Hz) that collect points of position much faster than the majority of low-cost receivers at 1 Hz. Higher hertz rated receivers should be considered for faster moving data collection operations for uses with yield monitoring and variable rate applications for seeding, lime, and chemicals.
• Receivers that typically cost between $1,000 and $3,500 are generally the most acceptable for variable rate technologies and guidance systems. These receivers can get you well within a few feet of accuracy. Some can handle dual frequencies allowing differential correction signals such as WAAS, Coast Guard, or satellite subscription service. Commercial service providers should consider these receivers for reliability and accuracy needs.
• When you look at very expensive receivers expect to pay up to $40,000 depending on the accessories you add to your system. You are getting into 1- to 2-inch accuracy. Producers who invest in this technology use it for precision guidance or auto-piloting a tractor, reducing driver fatigue, controlled traffic to reduce compaction, accurate fertilizer placement, tile and surface drainage applications, determining accurate field elevation, better equipment efficiency for nighttime field work and, in some cases, less reliance on foam markers. They usually require some base station nearby to further correct its position. This technology essentially allows the tractor to guide itself from pass-to-pass through the field.

Additional Pointers

• Take inventory of your operation. Consider your cash flow and budget. Consider the ability to utilize one or two GPS receivers to move around to different pieces of equipment for different operations.
• When selecting GPS receivers, make sure it is compatible with your intended hardware. Make sure the receiver provides differential corrections with either a Wide Area Augmentation System (WAAS), Coast Guard, or satellite subscription service. In some cases, investing in more expensive receivers that have dual frequencies can handle multiple signal channels and can go a long ways for reliability and guaranteed GPS service.
• It is important that the unit can communicate GPS data strings. This will require the use of standard NMEA 0183 strings. Additional cabling and connectors may be needed to connect GPS receivers with external antennas and connections to other computers and controllers. Additional external antennas or mounting of a receiver outside of a tractor cab or vehicle windshield can improve the accuracy of the device.
• Battery life is essential to power GPS units and controllers. Additional back-up battery such as an external rechargeable battery and the ability to power unit through a cigarette or AC adapter is essential for long field usage. Take into consideration manufacturer guidelines when hooking it up to 12v battery charged systems. This may result in damage to your device if you do not have the correct power adaptor.
• Talk with others who have invested in this technology. Inquire about ease of use and current return on investments. Try to borrow equipment or work with your local sales representative to learn more or try for yourself how this equipment works.

Watermeier is program director and Geospatial Extension specialist at The Ohio State University Extension; watermeier.2@osu.edu.