With more and more tractors, combines and sprayers arriving on farm with in-built satellite-guided steering systems, the retrofit kits that have steadily gained favour over the past 10 years are being replaced and are finally finding their way on to the second-hand market.

But buying a used steering set-up is a potential minefield and requires some careful consideration if you’re to avoid ending up with a high-tech turkey.

See also: How precision farming is changing UK agriculture

There are certain key questions to ask before parting with any hard-earned cash:

What jobs do you want the steering system to do both now and in the future?

There are widely varying degrees of accuracy when it comes to GPS-assisted steering kits and that dictates the types of jobs you can reliably do with them.

Generally, manufacturers will guarantee a minimum accuracy of about +/-40-50cm – ie the steering can vary 40-50cm either side of the guidance line.

This is probably fine for jobs such as cultivations and harvesting but a bit more steering precision is going to be required for drilling, spraying and fertiliser spreading.

For that you’ll need a more advanced set-up that will run at sub-20cm accuracy – some can narrow it right down to 2cm.

Typically, bolt-on steering kits (without wheel-angle sensors on the front axle) can’t react as fast as a system integrated directly into the machine’s steering circuit, so for slow-speed applications such as vegetable bed-forming, they’re not the best option.

Aside from keeping a tractor, combine or sprayer on the straight and narrow, these GPS systems can also provide and record data for other tasks such as yield mapping, variable-rate fertiliser spreading and sprayer boom section shut-off. How well they communicate with implement control boxes is a big issue and will need checking.

What are the various accuracy levels and which will I need?

Enter the murky world of precision farming for the first time and you’ll be hit with a snowstorm of convoluted technical terminology and acronyms. Put simply, there are three broad accuracy levels for GPS guidance systems:

Egnos – a free digital correction signal – accuracy +/-40-50cm

Paid-for subscription correction signal (for example, Omnistar) – accuracy +/-2.5-5cm – £500-1,000/year

RTK – terrestrial base-stations or the mobile phone networks provide a fixed correction signal – repeatable accuracy of +/-2.5cm

• Eliminates satellite drift and allows machines to return to the same line time after time – £10,000-20,000 per base station or £500-2,000/year subscription/share ownership.

While this broad outline applies to the main key players – Trimble, TopCon and AgLeader – John Deere does things its own way:

Starfire 1 – free digital correction signal – accuracy +/-23-27cm

• Starfire 2 – paid-for subscription correction signal – accuracy +/-5-8cm – £510/year
• RTK – mobile network or base-station corrected signal – accuracy +/-2.5cm – roughly £10,000 per base station or £650/year subscription

How accurate are these systems in reality?

Generally, GPS suppliers underquote a system’s accuracy and quite often they will be at least as precise, if not more so, than officially quoted.

There are exceptions. Some RTK providers claim 1cm accuracy, which can be the case as long as the machine is working within 1km of the base station providing the correction signal.

However, for every kilometre travelled away from the base station, 1mm of accuracy is lost. For example, a tractor 15km from the antenna will be running at 2.5cm accuracy. With mobile RTK this error is removed, as long as you have a signal.

Is the receiver Omnistar enabled?

If the system can only receive an Egnos signal, it’s only going to provide the most basic guidance and isn’t really suitable for steering. Most can be upgraded to take a correction signal (for example, Omnistar) – at a cost.

Does the system use Glonass/GNSS as well as GPS?

Although it might sound like you’re signing up to the next communist revolution, knowing whether the second-hand steering system you’re looking at can talk to the Glonass/GNSS satellite network is critical in ensuring a reliable signal.

Glonass is the Russian version of GPS (which started life with the US military). Over the past 15 years, a number of the satellites in the US system have fallen out of service, with the result that the correction signal can suffer from shading and signal drop-outs.

In contrast, the Russians have blasted scores of new satellites into orbit in recent years, hugely boosting the reliability of Glonass. For that reason, it’s important to have a receiver capable of getting both GPS and Glonass signals.

GPS-only receivers are not necessarily less accurate but can be more susceptible to shading and loss of signal. Glonass-compatible systems see nearly twice as many satellites in the sky, after all.

Does the system have terrain/slope compensation?

Gyroscopes within the receiver significantly improve accuracy when working on banked ground. This is essential for any auto-steering set-up.

Can I do variable rate fertiliser applications or drilling?

Generally it’s not the GPS set-up that is the limiting factor for what other jobs it can be used for – more the display’s ability to communicate with implement control boxes.

Isobus in-cab computers capable of dealing with a wide range of tackle and tractors make communication easier but typically it’s the display’s age that is the limiting factor in who it can talk to.

Can old GPS systems be updated?

Manufacturers roll out software updates for their systems on a regular basis and it’s worth finding out when that last occurred. However, after a while they become obsolete and upgrades are no longer available.

That said, most recent Glonass-enabled systems can run at any one of the three different accuracy levels. However, while some providers allow the licences to move across when kits are sold on, others register it to the original buyer so you must purchase new activation codes for correction signals.

Where’s the best place to look for second-hand steering set-ups?

These are complex pieces of kit and so it’s worth buying from a licensed supplier if you want back-up.

Most machinery dealerships will hold a GPS franchise but there are also specialist suppliers who have infinite expertise in this mind-boggling area.

However, if you do end up buying privately then it’s probably best to try to see the system working before you decide to buy. Also ask friends and neighbours already using that particular system in your area how they get on.

Buying through a dealer also means you’ll get training to help you set up and use the system properly. However, with a little workshop willingness, it is possible to do DIY installations with the simpler set-ups (such as Trimble’s EZ Steer, pictured above).

What to pay?

Second-hand steering systems come up less often than you might think as operators that have been using the technology for a while really recognise the benefits and will often retain their old kits moving them down the fleet on to rolling tractors, etc.

Consequently there is no established rule-of-thumb for pricing. Below are a few examples of set-ups that have recently changed hands:

TopCon 150 controller with receiver and electronic steering wheel

• Glonass and RTK enabled with 2.5cm repeatable accuracy
• £5,000-6,000 (X30 colour touchscreen adds £1,500-2,000)

Trimble EZ Steer 500 with receiver and electronic steering wheel motor

• Glonass and Omnistar HP enabled 2.5-5cm accuracy
• £2,500-3,000 (£3,500-4,000 for CFX 750 for variable rate)

John Deere Universal AutoTrac steering with ITC receiver and 1800 controller

• Starfire 1 – +/-27cm accuracy – not Glonass-enabled
• £3,000-4,000

John Deere Universal AutoTrac steering with 3000 receiver and 2600 controller

• RTK – +/-2.5cm repeatable accuracy
• £6,500-8,500

AgLeader OnTrac controller, receiver and electronic steering wheel motor

• Glonass and RTK ready – 2.5cm repeatable accuracy
• £4,500-5,000

Correction signals

Don’t forget there is generally a £500-1,000 subscription to pay each year in addition for either tier-two accuracy or RTK repeatable signals. And your own base station will cost at least £10,000 if there’s no RTK signal available either via the mobile network or a designated RTK network.

Wiring

There can also be significant cost in the wiring required to get these various set-ups up and running. Some harnesses can stretch into the thousands of pounds.

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Thanks to GPS specialists Chris Limb at LH Agro (TopCon), Jeff Richings at Farols (John Deere), Richard Tattershall at AS Communications (Trimble), Derek Johnstone at Precise Solutions (AgLeader) and Ian Warman at Fyfield Farm)