GPS guidance and auto-steer: which accuracy level do you actually need?

Three levels of kit

Guidance systems come in three broad forms. A lightbar or screen-only system shows the line and how far you are off it, and you steer. An assisted-steering kit adds a motor on the steering column or a replacement wheel that turns the wheel for you. A fully integrated auto-steer system works through the tractor's own steering hydraulics and is usually factory-fitted or dealer-installed from the manufacturer's own range.

All three use the same satellite receiver and a correction signal. The steering hardware affects how smoothly the tractor holds the line, particularly on slopes and at speed; the correction signal sets how accurate the line can be.

Correction signals and what the numbers mean

Manufacturers quote accuracy as pass-to-pass (how close each bout is to the last within a working session) or as repeatable (how close you are to a line saved in a previous season). Trimble's explanation of the terms makes the distinction clear: a signal can be good pass-to-pass yet drift over months, which is fine for spreading but not for returning to the same tramlines or strip-tilled rows next year.

Using Trimble's published figures as an example of a typical ladder:

• Free satellite-based augmentation (SBAS, such as EGNOS in Europe or WAAS in North America): horizontal accuracy typically more than one metre.
• Entry-level paid services (Trimble ViewPoint RTX and RangePoint RTX): down to about 30 cm, or 15 cm pass-to-pass for RangePoint.
• Premium satellite corrections (Trimble CenterPoint RTX): down to about 2.5 cm, with fast convergence in covered regions.
• John Deere's StarFire 7500 receiver with its SF-RTK signal is quoted at up to 2.5 cm pass-to-pass with long-term repeatability.
• Local RTK, from your own base station or a subscription network, is the usual choice when centimetre-level lines must be found again year after year.

Match the accuracy to the job

The job determines the accuracy you need, not the other way round. Fertiliser spreading, spraying on established tramlines and grassland work are forgiving: a sub-metre or 15-30 cm signal already removes most of the guesswork and fatigue. Drilling, bed forming, strip-till, inter-row hoeing and controlled traffic farming need centimetre-level, repeatable positioning, because the next machine has to find the same rows.

A good rule of thumb is to buy for the most demanding job you will actually do in the next three to five years, and to check that the receiver can be unlocked to a higher signal later without replacing hardware.

Working out whether it pays

The saving from guidance is the overlap you stop paying for, plus less operator fatigue and the ability to work longer or in poor light. Iowa State University Extension measured overlap using RTK logging on working farms and found averages of 3.3% during planting and 7% during spring tillage. Your figure may be higher or lower, so measure it: most displays can log coverage and show overlap for a few fields before you commit to a full system.

A worked example shows the method. If you spray 300 ha five times a season and your as-applied maps show 5% overlap, you are treating an extra 15 ha on every pass, or 75 ha of product a year. Multiply that by your own cost per hectare of product, and add fuel and time, to get the ceiling on what reduced overlap is worth. Section control on the sprayer or drill usually delivers more of the saving than steering alone, because headlands and point rows are where most double-application happens.

Remember the running costs: annual correction subscriptions, network fees for RTK, display unlocks and any mobile data. Iowa State's figures date from 2012 and US corn prices, so treat them as a method, not a forecast.

Questions to ask the dealer

Guidance systems are sold as a package of hardware, software and subscriptions, and the running costs and compatibility matter as much as the purchase price. Put these questions to every dealer and get the answers in writing, so quotes can be compared on the same basis over five years rather than on the first invoice.

• Which correction levels does the receiver support, and what does each cost per year?
• Is the receiver locked to one brand's display, or will it work with other makes on the farm?
• What is the quoted pull-in (convergence) time, and what happens if the signal drops mid-field?
• Is terrain compensation included for slopes?
• Can guidance lines, boundaries and coverage data be exported in a standard format such as ISOXML or shapefile?
• Who installs and calibrates the system, and is calibration repeated when the kit moves to another tractor?
• What is the upgrade path to RTK, and is there local network or base-station coverage where you farm?
• What local support is available in spring and harvest, and how quickly can a failed receiver or display be swapped?

Common first-season mistakes

Most early disappointment comes from set-up rather than the signal. Measure and enter implement width and offsets carefully, including the difference between working width and physical width. Recalibrate after changing tyres or moving the receiver. Set up field boundaries and headland lines before the busy season, not on the first drilling day. And keep a paper or offline copy of key lines, because a lost subscription or corrupted card in spring is expensive.

Guidance also changes how operators work. Plan a short training session for everyone who will use it; our guide to closing the farm skills gap covers that. Once lines and coverage maps exist, think about who owns and can access that data, which is the subject of our farm data ownership guide. Accurate steering also underpins the section control and spot spraying covered in our precision spraying guide, and the FarmFleets machine search lists guidance-ready options by tractor model.

Sources

Sources checked 2026-09-25

• Trimble Agriculture (PTx): Precision agriculture and repeatable accuracy — checked 2026-09-25
• John Deere: StarFire 7500 receiver with SF-RTK signal — checked 2026-09-25
• Iowa State University Extension: Precision ag technology savings — checked 2026-09-25

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