Path to Precision

Accuracy: on your level

06 Dec 2021

​To start mapping some seriously valuable information just takes some sensors and GNSS— simply plug the two together on a vehicle and that’s it, right? Well, not exactly. The best things in life don't always come easy, and that means there’s a bit more to understand when it comes to GNSS. But never fear! Learning what you need to know to get GNSS working for you doesn't require a PhD.

Firstly, a receiver is the key to getting GNSS onboard your vehicle. That receiver can be provided by a machinery manufacturer, either factory fitted or as a dealer-installed accessory, or from a third-party. Once you have that receiver, a few more decisions need to be made and these decisions are based on your precision farming strategy. The main decision is around which accuracy will go for—or in other words, what error correction method will you use?

When it comes to accuracies there are two groups for each type:

Group 1 - short term accuracy, also known as pass-to-pass accuracy. This is the accuracy of your position within a 15-minute interval
Group 2 - long term accuracy, which is also known as absolute accuracy or repeatable accuracy. Absolute accuracy is the accuracy of your position year after year

For example, Case IH's AFS 1 accuracy signal has a pass-to-pass accuracy of 15 cm and an absolute accuracy of approx. 50cm. This means if you were to plant a row of seed, and drive over the same row within 15 minutes, you should expect to be within 15 cm of the original position. However, drive to the same position a year later and you could be +/- 50 cm off.

This is important to understand because this type of accuracy has an impact on the precision solution you want to use. Planting rows of sugarcane with auto guidance? Want to harvest that crop using auto guidance too? Well, in this scenario, you'd want to ensure you are using a high repeatable accuracy. Otherwise come harvest time, you'll line up to the planted rows with an auto guidance enabled harvester and find a mismatch in position!

On the flip side of that... if you just want to measure yield or track location of a machine, the lower accuracies would be fine because they don't require the position to be centimetre perfect every year.​ 

Right, so let's get down to the good stuff: what are your accuracy options?

1. Autonomous – autonomous is essentially a filter algorithm that comes built into your receiver as standard and is free to use. Today's autonomous accuracies are getting better and better with pass-to-pass around 20cm and absolute at 'sub-meter'.

2. Satellite Based Corrections – satellite corrections are additional signals sent to the receiver, computed by global reference stations, that correct the errors found in basic GNSS signals. These corrections require a subscription from the receiver provider with accuracies that range from 15cm - 2.5cm for pass-to-pass and 50cm - 2.5cm for absolute. That’s pretty accurate! Of course the more you pay, the better the accuracy. Side note: If you're lucky enough to have a regional SBAS (Satellite Based Augmentation System)   correction in your area then these are free to use, country-sponsored correction signals. Examples are WAAS in the US or EGNOS in the EU.

3. Ground Based Corrections – the holy grail in terms of accuracy in agriculture today, providing the highest accuracy achievable. Ground based error corrections are exactly that—errors corrected on the ground from localised reference stations or base stations. The reason they are so accurate is because they are closer to your position than satellite-based reference stations, meaning their error correction is better suited to your position! 'RTK' (Real-Time Kinematic) is the common term we use when talking about ground-based corrections. This accuracy can be obtained in two ways:

a) You own the base station: you have bought all the hardware for your own RTK correction. Once all hardware and one-time unlocks have been purchased there is no on-going cost. However, the cost of entry is relatively high

b) Third party RTK network: someone else has bought the equipment and setup the RTK correction. In this case all you need is the machine hardware and one-time unlocks, and you pay for subscriptions to access the RTK correction

The accuracy of RTK? Less than 2cm for both pass-to-pass and absolute. Incredible!

Remember to take into account what you want to achieve with your precision solutions and choose an accuracy that will help you reach your goals!

Your local Case IH AFS dealer can provide further details, but the information in this article will hopefully make you feel more comfortable with the terminology, and the technology.