The accuracy of the GNSS or GPS is affected by various factors including the quality of your system, atmospheric conditions, and operator errors. 

Typically, the accuracy of autonomous GPS (that is, with no correction) is about +/- 10 metres on the ground. With correction from a base station, this can be reduced to +/- 2cm.

There are several forms of correction signals, and the most commonly used ones in agriculture are: RTK, nRTK and PPP. These are explained in the following sections.

Space Based Augmentation systems (SBAS)

SBAS (space based augmentation systems) provide corrections to a GPS/GNSS receiver in order to improve the accuracy of the navigation solution. SBAS corrections originate from a network of reference stations and provide the signal directly into the receiver from space. The receivers in these reference station networks can estimate errors in the GPS because, unlike the general population of GPS receivers, they have an accurate knowledge of their position. 

SBAS uses the known errors to correct GPS location in real time. As a result, the accuracy of GNSS/GPS is greatly improved, down to about +/- 10 centimetres. Examples of SBAS include John Deere SF2™ and OmniSTAR® HP (which guarantees 10cm accuracy), Trimble Centrepoint RTX®, and also the Australian Governments’ SouthPAN system. Users need to purchase a subscription to the correction stream for a fixed period and need to have a compatible receiver to receive the subscription. SouthPAN is free, however requires specialised hardware to access it at this stage.

One of the issues that may be encountered with this type of technology is longer convergence times – that is when you start the receiver first up. It may take up to 40 minutes to obtain a highly accurate correction signal.

Real-Time Kinematic (RTK)

RTK is used to enhance the precision of satellite-based positioning systems by using a ground-based single reference point (base station) to provide real-time corrections and enable centimetre-level accuracy. It requires a local base station to be set up within a few kilometres (but typically no more than 20km to ensure high accuracy) from the rover (tractor/sprayer) and is usually positioned at a high point in the landscape to ensure maximum coverage and is then calibrated. The John Deere StarFire™ RTK or Trimble RTK, for example, is reported to provide repeatable, sub +/- 2.5cm accuracy. 

The base station transmits the corrections in real time (less than once per second) to the rover units using UHF and VHF radios, typically in the 450 to 900-megahertz range. In some cases, depending on the type and power of the radio in the base station, you may need to licence it. As a general rule you cannot communicate between different manufacturers’ base stations/rovers. 

If you have your own base station, then there will be no ongoing subscription costs.

Find out more

For more information about nRTK, go to: www.alldayrtk.com.au/

Network Real-Time Kinematic (nRTK)

nRTK is a system that also offers centimetre-level accuracy in real time but without the need for an operator’s reference or base station, as it uses GPS observations gathered from a network of Continuously Operating Reference Stations (CORS). The coverage of any nRTK GNSS service is only limited by the number of available CORS and the quality of the wireless data link used to transmit the correction to the users (i.e. mobile phone networks capable of allowing data, such as Telstra NextG). 

nRTK requires payment of a subscription as well as the data costs. Some examples include AlldayRTK and SmartNetAus, which have established a network of CORS bases around Australia. SmartNetAus typically achieves a reported RTK accuracy of 1 to 2cm horizontally and 2 to 3cm vertically.