If you’re looking at GPS guidance or autosteer for your tractor, one of the first questions you’ll come across is:

How accurate do I actually need to be?

You might hear terms such as EGNOS, SBAS, RTX, RTK and VRS, along with claims of meter, decimeter, or centimeter accuracy.

But what does all of that mean in practical farming terms?

And do you really need an RTK subscription just to spread fertiliser or lime?

The answer depends very much on what you are using the GPS system for.

What determines GPS accuracy?

A GPS or GNSS receiver calculates the tractor’s position using signals from satellites.

However, satellite positioning isn’t perfect. There can be errors caused by atmospheric conditions, satellite geometry, signal interference and other factors.

A correction service provides additional information that helps the receiver improve its position.

As PTx Trimble explains, correction services can range from free satellite-based corrections through to high-accuracy subscription services. The appropriate option depends on the application and the level of accuracy required.

This is why two tractors can both have GPS but produce very different levels of accuracy.

What is EGNOS?

EGNOS stands for European Geostationary Navigation Overlay Service.

It is Europe’s Satellite-Based Augmentation System (SBAS).

Rather than requiring a paid subscription or a local base station, EGNOS corrections are broadcast via satellite.

That makes EGNOS a relatively simple and accessible way of improving on uncorrected GNSS positioning.

It’s important to understand that EGNOS and SBAS aren’t really two competing accuracy levels.

EGNOS is the European SBAS system, just as WAAS is used in North America.

PTx Trimble receivers can use SBAS corrections including EGNOS.

Is EGNOS free?

Yes.

There is no RTK-style correction subscription required to receive EGNOS.

That makes it an attractive option for farmers who want basic GPS guidance without adding an ongoing correction-service cost.

However, the trade-off is accuracy and repeatability.

How accurate is SBAS/EGNOS?

SBAS is generally a sub-metre correction solution, rather than a centimetre-level system.

For example, Trimble’s published NAV-500 specifications show approximately 20 cm pass-to-pass accuracy with SBAS, although actual performance can vary depending on the receiver, environment and operating conditions.

For many broad-acre farming jobs, that can be perfectly usable.

You aren’t necessarily trying to put a tractor within 2 cm of the previous pass when spreading fertiliser across a large field.

The question is whether the level of accuracy is appropriate for the job.

What is RTK?

RTK stands for Real-Time Kinematic.

It is a much higher-accuracy positioning technology that uses correction data from a known reference station or network of reference stations.

RTK can provide centimetre-level positioning.

Trimble’s published specifications for RTK show horizontal accuracy around the centimetre level, while network RTK services such as VRS use internet connectivity to deliver corrections from a network of reference stations.

This is why RTK is commonly used where extremely accurate and repeatable passes are required.

For example:

  • High-value row crops
  • Bed forming
  • Drilling
  • Controlled traffic systems
  • Drainage
  • Land forming
  • Repeatable operations year after year
  • Applications where implement position needs to be extremely precise

But that doesn’t mean RTK is automatically the right choice for every farmer.

Do I need a paid SIM card for RTK autosteer in Ireland?

Not necessarily — but if you are using network RTK delivered over the mobile network, you need a way for the tractor’s receiver to access the internet.

This is an important distinction.

A network RTK service such as VRS delivers correction data to the tractor through an internet connection. Trimble describes VRS Now as a network RTK service delivered over the internet.

That internet connection can be provided through the equipment’s cellular connectivity, depending on the system and setup.

So when someone says:

“Do I need a SIM for RTK?”

the real answer is:

You need a communication link for the RTK correction data if you’re using an internet-delivered RTK service. How that connection is provided depends on the system.

It isn’t necessarily as simple as buying a normal phone SIM and putting it into the display.

Your dealer will configure the appropriate connectivity and correction service for the guidance system being installed.

What if I don’t have good mobile coverage?

This is an important consideration for Irish farms.

If network RTK relies on a cellular connection, the tractor needs a suitable data connection to receive the corrections.

There are alternative approaches, including:

  • Local RTK base stations
  • Radio-delivered corrections
  • Satellite-delivered correction services

There are also technologies designed to provide a backup when an RTK correction stream is interrupted.

For example, Trimble xFill can provide satellite-delivered, centimetre-level backup when RTK or VRS corrections are temporarily lost. On compatible agricultural systems, Trimble states that xFill can provide up to 20 minutes of included backup coverage.

That can be particularly useful where mobile coverage isn’t perfect.

Do I need RTK to spread fertiliser?

For many farms, no.

This is where it is important not to assume that the highest possible accuracy is automatically the best solution.

If your main requirement is basic manual guidance for fertiliser or lime spreading, an SBAS/EGNOS-level system may be sufficient for the job.

PTx Trimble itself notes that free correction services can be suitable for broad-acre applications, while higher-accuracy correction services become more valuable for applications requiring greater precision.

Imagine a fertiliser spreader covering a wide bout.

The aim is generally to maintain consistent passes and avoid significant skips or overlaps.

A guidance screen showing your position relative to the previous pass can already make this considerably easier than relying entirely on visual markers.

You may not need centimetre-level RTK accuracy to achieve a useful improvement.

What about autosteer for fertiliser spreading?

This is where the decision becomes slightly different.

Manual guidance and autosteer aren’t the same thing.

With manual guidance, the GPS system tells you where to drive and you make the steering corrections.

With autosteer, the steering system automatically keeps the tractor on the guidance line.

For basic spreading, manual guidance with an appropriate correction source may be perfectly adequate.

If you’re doing long days, using wide implements or want to reduce operator fatigue and maintain very consistent passes, autosteer can provide another level of convenience and consistency.

PTx Trimble’s EZ-Pilot Pro is designed as a high-accuracy automated steering system and can be used across a wide range of farming operations.

When does RTK start to make more sense?

RTK becomes more attractive when repeatability and very high accuracy are important.

For example, consider two different jobs.

Spreading fertiliser

You’re working across a large field with a 24-metre spreader.

Your main objectives are:

  • Maintain consistent passes
  • Reduce overlaps
  • Avoid skips
  • Make the operator’s job easier

An entry-level guidance correction service may provide sufficient accuracy for this application.

Drilling or precision bed work

Now imagine you’re drilling a crop where you need extremely consistent rows or returning to the same position later in the season.

The consequences of small positioning errors can become much more significant.

In that situation, the investment in a higher-accuracy correction service such as RTK can make considerably more sense.

What are the main GPS accuracy options?

A simple way to think about the different levels is:

Correction

Typical accuracy

Subscription

Suitable applications
Uncorrected GNSS Around metre-level No Basic positioning
EGNOS / SBAS Sub-metre / decimetre-level No Basic guidance, broad-acre work
RTX / other enhanced corrections Decimetre to centimetre-level depending on service Yes More accurate guidance and steering
RTK / VRS Centimetre-level Yes High-accuracy and repeatable operations

The actual accuracy achieved depends on the receiver, correction service, satellite conditions, environment and the particular system being used. Trimble specifically cautions that published accuracy figures can vary according to these factors.

Is a more expensive correction service always better?

Not necessarily.

The right question isn’t:

“What is the most accurate GPS system I can buy?”

It’s:

“What level of accuracy does my farming operation actually need?”

If you’re primarily spreading fertiliser and lime, a basic guidance system may provide all the accuracy you need.

If you’re drilling, planting, carrying out drainage work or need the tractor to return to exactly the same position year after year, investing in RTK-level accuracy may be worthwhile.

The cost of the correction service should therefore be considered alongside the jobs the tractor is going to carry out.

What about RTK subscriptions in Ireland?

There are a number of RTK correction services available to Irish farmers.

Network RTK providers use reference stations around the country and deliver correction data to the tractor through an internet connection.

For example, Irish network providers currently offer NTRIP-based RTK correction services designed for precision agriculture, with centimetre-level positioning available through subscription packages.

The important thing is that RTK is not one single subscription.

Different systems can use different correction services, and some precision guidance systems can also use satellite-delivered correction services rather than cellular network RTK.

When buying a new system, ask your dealer:

What correction service does this system use, what does it cost annually, and what happens if I lose mobile coverage?

That will give you a much clearer picture of the ongoing cost than simply looking at the price of the autosteer hardware.

Choosing the right accuracy for your tractor

At Vantage Ireland, we supply PTx Trimble guidance and steering systems that can work with a range of correction options.

Our EZ-Pilot Pro system can be paired with the appropriate GNSS receiver and correction service for the work the tractor is carrying out.

The system can therefore be configured around the farm’s requirements rather than simply selecting the most expensive correction signal available.

So, what accuracy do you actually need?

There isn’t one answer for every farm.

For basic manual guidance and jobs such as fertiliser and lime spreading, EGNOS/SBAS-level guidance can be a practical starting point.

For higher-accuracy automated steering and applications where repeatability is important, RTK or another high-accuracy correction service can provide a significant step up.

And for farmers who want to introduce autosteer, the correction service is only one part of the system. The receiver, display, steering system, antenna installation and correction signal all need to work together.

Thinking about GPS or autosteer?

Vantage Ireland supplies and installs PTx Trimble GPS guidance and steering systems, including the EZ-Pilot Pro.

We can help you choose a system based on what you actually want to do with the tractor, whether that’s basic guidance for spreading, automated steering for long days or centimetre-level repeatability for precision operations.

Talk to Vantage Ireland about the right guidance and correction solution for your farm.

Find out more about EZ-Pilot Pro →

Accuracy figures are indicative and can vary with receiver, correction service, satellite availability, terrain, atmospheric conditions and other factors. Always confirm the specifications and subscription requirements for the specific system being considered.