How does RTK GPS work? The technology behind centimeter-accurate agriculture

What is RTK GPS?

RTK stands for Real Time Kinematic. It is a technology that significantly improves the accuracy of standard GPS signals. Whereas a standard GPS receiver typically has a margin of error of 1 to 5 meters, RTK GPS can determine a position with an accuracy of 2 to 3 centimeters.

This high level of precision makes it possible to steer agricultural machinery automatically, repeat tasks along the exact same guidance lines, and prevent overlap or missed strips.

Why is regular GPS not accurate enough?

GPS satellites constantly transmit signals to receivers on earth. Based on these signals, a GPS receiver calculates its position.

However, accuracy is affected by several factors:

• Atmospheric disturbances;

• Satellite orbits that are not fully exact;

• Signal reflections off buildings or objects;

• Minor discrepancies in the satellite's clock.

This creates a margin of error that reduces the accuracy of the receiver. For car navigation this is perfectly fine, but for agricultural applications this deviation is far too large. This is why RTK GPS uses an extra reference point to correct these errors almost entirely.

How does RTK GPS work?

Step 1: A base station determines its own exact location

An RTK base station is located in a single fixed spot. Since the position of this station is known exactly, there is no doubt about its position.

Step 2: Corrections are calculated

The base station then performs a single measurement of its position. It then compares this measured position with the actual position known to it. The difference between the two is the deviation present in the signal.

Step 3: Corrections are transmitted

These corrections are sent in real-time to the GPS receiver on the tractor. This can be done via a direct radio connection or via the internet (SIM card). Today, practically only the SIM card is used to receive the corrections.  

In the past, this was done via a direct radio signal from a mast. However, this often resulted in the radio signal being disrupted by buildings, trees, bridges, or other obstacles. Because of this, the switch to the internet was quickly made after just a few years.

Step 4: The machine corrects its position

The tractor's GPS receiver then processes these corrections into its final position determination by correcting its own measured position with the generated correction signal. The tractor's deviation is reduced to just a few centimeters.

Difference between radio or internet:

Radio signal:

In the past, all corrections were sent to the tractor via a radio signal. This was the cheapest and simplest solution at the time. Mobile internet coverage was also not yet sufficient in the early years. However, the disadvantage of this direct radio signal was that buildings, trees, or other obstacles quickly caused signal interference.

SIM card/internet:

As mobile internet became cheaper over the years, signals were no longer sent directly via a radio signal, but over the internet. The base station sent its signal to a server on the internet. The tractor was then equipped with a SIM card with mobile internet. This tractor retrieved the data from this server, thus still receiving the corrections without being affected by obstacles. 

RTK Network

As an additional benefit, this allowed the tractor to send data back to this server. Because the tractor sent back data about its own position, the server could determine which station was closest to the tractor. The tractor then automatically received the forwarded signal from the nearest base station. As a result, today you can drive uninterrupted from Groningen to Maastricht on RTK GPS without having to switch masts. 

RTK vs. regular GPS

The inaccuracy of a GPS antenna without RTK is therefore not necessarily due to the technology of the antenna itself. The problem is that the signal is disrupted by the atmosphere along the way, making the exact position harder to determine without corrections from a base station.

In space, where these disturbances do not occur, the same antenna could in principle achieve an accuracy of less than a millimeter.