Just as there are several ways to skin a cat, so there are several ways into GPS-guided steering. A stand-alone lightbar or screen is the cheapest option, while at the other end of the scale are highly expensive autosteer set-ups. In between come a handful of steering-assist units.

What’s the difference? Lightbars and/or simple screens show you where to steer, but leave the graft to the driver. Easy to swap from tractor to sprayer to combine, a lightbar must be partnered with an alert, competent operator to deliver consistent results.

Autosteer is the other extreme. Not readily moved between machines, as it is physically plumbed into the vehicle’s steering, it generally includes a steering angle sensor and is potentially able to produce the most accurate result.

Steering-assist bridges the gap. These still-portable set-ups often appeal to buyers who have dipped a toe in the water with lightbar guidance, want something more, but can’t justify full-on autosteer. It is steering-assist we concentrate on here.

Four systems are offered in the UK. Three work by twiddling the steering wheel using an electric motor, the fourth sends hydraulic tendrils into the steering circuit.

Brands are Agrocom (Outback e-Drive), AutoFarm (OnTrac), John Deere (GreenStar Universal AutoTrac) and Trimble (EZ-Steer). No unit can be used alone, each works as an add-on to an existing system, or can be bought with one as a package. Prices vary a lot (see table, top right).

Ways, means…

The Deere, AutoFarm and Trimble systems were used on a 120hp John Deere 6420S; the Outback came pre-fitted to a 175hp Renault Ares 826. Testing happened over two days in an open, slightly undulating grass field of about 10ha (25 acres).

A common 130m A-B line formed the basis of five 24m passes. No implement was used. Please note that accuracy may change when a machine works under load.

Farmers tend not to pay when they can get something for nothing. So we asked manufacturers to supply equipment able to handle free EGNOS or SF1 correction. Driving pass-to-pass, these can deliver the sub-meter accuracy needed for spraying, spreading, cultivation and harvest, perhaps allowing less-skilled labour to be used.

Tractor position in the field was pinpointed with a Trimble AgGPS 214 RTK receiver, taking signal from RTK base station located about 1.5km from the site. Position information was logged every 0.75m by a Trimble Recon unit in the cab, then analysed later with mapping software by an independent company.

…and the end result

All systems delivered 20cm-50cm pass-to-pass accuracy, often averaging within 20cm of the target line. Performance depended on how well the unit was set up and how the driver approached the next pass line. There were also substantial operational differences.

Deere’s Universal AutoTrac was the most consistent sharpshooter, Trimble’s EZ-Steer gave the best blend of performance, swappability and cost.

All achieved better results than most human pilots would with a simple lightbar, simultaneously lowering operator stress and freeing time to attend to an implement or to craft a rollup.

Important note: The test site and RTK/logging equipment were provided by AS Communications, which also supplied the EZ-Steer system for test. But no company personnel were on site or involved in the test in any way, beyond providing measuring equipment.

Click here to read our individual reviews of four current GPS-assisted steering systems