What options do you have for GPS/GNSS corrections on your farm? | Agriculture GNSS correction services explained

Like many farmers today, you may rely on GNSS technology to help you farm more accurately and efficiently. Or you may be looking to increase your existing accuracy to better the performance of your farming operation. You understand that GNSS technology helps you operate seamlessly, improve input costs and gain higher yields – all because of your equipment’s precise position in the field. But how? 

This impressive performance is all made possible by GNSS correction services, which provide positioning information essential for achieving and maintaining accuracy in today’s world of precision farming. And with the continuous expansion of services and technology offerings, understanding how corrections work can help you make informed decisions to take your operation to the next level. So let’s learn. 

Correction services mitigate positioning errors 

Simply, GNSS positioning relies on satellites that continuously broadcast positioning and timing data to a GNSS receiver. When a receiver acquires this information from at least four satellites, it calculates its distance from each to determine its own position on the Earth.  

But the long journey that the signal takes from satellite to receiver does not come without challenges. Effects throughout the journey cause delays, distortions and inaccurate positioning. These effects can cause errors like: 

• Clock errors – inaccuracies in the satellite’s atomic clock caused by drift  
• Orbit errors – variations in the predicted orbit in which the satellite is travelling 
• Atmospheric errors – distortions and delays caused by ions, humidity, temperature, and atmospheric pressure as the signal travels through the Earth’s atmospheric layers (specifically ionosphere and troposphere) 

Left unchecked, errors can cause horizontal accuracy to be off by over 7 metres (22 feet). That’s a huge distance when it comes to certainty in your farming operations. So how can these errors be accounted for to ensure your equipment receives accurate positioning information?  

Correction services resolve sources of errors in the raw GNSS data, so your receiver can compute a more accurate position. GNSS receivers cannot correct for errors themselves. They rely on external data and error models from reference stations or fixed position receivers to estimate the error and send the corrected location to your receiver.  

Correction services deliver positioning accuracy  

Every farm is different. Every farmer operates differently. That’s why there are a number of available correction service types that provide various degrees of accuracy and delivery methods.  

Real-time kinematic (RTK) 

RTK resolves sources of errors by comparing the data received between a fixed base station on a known-coordinate and in-field receivers. This method assumes the base station and agriculture equipment detect the same errors as they are located close together, so the common GNSS errors can be eliminated. RTK corrections provide highly precise corrections but limit the user’s range as they must remain relatively close to the base stations. The convergence time – the time it takes for the in-field equipment receiver to compute an RTK position solution once it receives the RTK corrections – is usually within one minute. 

• Accuracy Level: 1-2 cm (but accuracy degrades the further the equipment moves from the base station) 
• Convergence Time:  1 minute 
• Range: 10-30 kilometres from the base station 
• Delivery: radio or cellular connection 
• Requirements: local fixed base station or subscription access to a local network RTK service. Requires a radio or cellular modem with a data plan on the equipment to receive corrections 

Precise point positioning (PPP) 

The PPP method uses a network of reference stations to model satellite clock and orbit errors. Some PPP solutions also model regional atmospheric error sources. This data is broadcast to the in-field receiver removing these errors in its real-time positioning solution. PPP is available worldwide and users are not limited to staying within a base station’s range. Positioning accuracy and convergence times can vary based on the service used.   

• Accuracy Level: ranges from 2.5 cm – 50 cm depending on the selected correction source 
• Convergence Time: ranges from <1 min to 30 mins depending on the selected correction source  
• Delivery: satellite or cellular connection 
• Requirements: dual frequency GNSS receiver in the coverage area and subscription to correction services 

RTK assistance 

RTK assistance is for RTK and hybrid PPP & RTK users. RTK is used as the primary correction source until there is an interruption in the signal due to radio or cellular reception losses. Instead of operating without RTK corrections and having the accuracy degrade, your receiver can leverage PPP corrections to maintain centimetre-level accuracy until the RTK corrections return. This method, called bridging,  ensures you stay up and running with a high-accuracy solution rather than having to wait for the RTK corrections to return. There are different levels depending on the length of outages experienced. 

• Bridging Time: 20 min to unlimited  
• Delivery: satellite  
• Requirements: dual frequency GNSS receiver and bridging subscription to correction services 

Satellite-based augmentation systems (SBAS)

SBAS compares data from networks of stationary GNSS ground stations against the incoming satellite data. The corrections are then sent to geostationary satellites to be broadcast to receivers. Coverage varies by the regional system (WAAS for North America or EGNOS for Europe, for example), which are usually managed by government organisations. Accuracy levels are not as precise as paid services and are not available in all regions.  

• Accuracy Level: 60 cm 
• Convergence Time: ~5 min 
• Delivery: satellite 
• Requirements: single frequency GPS receiver in SBAS-supported region 

Choose the right correction service for your needs 

Choosing a correction service largely depends on your farming applications’ requirements. Hardware investment and maintenance, service costs, fleet size, terrain and crop type are all considerations when exploring your options.  

You may be new to precision farming and need an entry-level option for positioning. There’s a correction service for that. You may have a high-value row crop where accuracy is of the utmost importance in order to maximize crop health and reduce crop damage. There’s a correction service for that. You may want the highest level of accuracy but lack the funds for additional base station hardware. There’s a correction service for that. Or maybe you want to balance accuracy levels with costs. The good news is that no matter what you farm or how you farm, there’s a suitable solution for your needs. 

Contact your dealer to learn how you can benefit from TerraStar Correction Services in agriculture and to help you select the best corrections for your farming operation.   

Additional resources 

Learn more about reducing GNSS & GPS errors in Hexagon | NovAtel’s How to reduce GNSS & GPS errors – Intro to GNSS Episode 4 – Reducing GNSS Errors video on YouTube