Accuracy

There are many terms used to describe GPS/GNSS accuracy. Below are common types of accuracy and their meaning.

Mechanical or field accuracy

This refers to the ability of the auto-steer or assisted-steer to react to the GPS information and maintain the appropriate path in the field. Many factors will affect this including ‘play’ in the steering components, the agility of the spray rig, suspension of the cab, wheel ruts, wheel centres and tyre pressure, which all influence steering stability.

GPS accuracy

This refers to the inherent accuracy of the GPS receiver and signal type that you are using. GPS accuracy is classified into three levels of accuracy: 50 per cent, 90 per cent and 95 per cent. This is based on the number of readings that a stationary receiver makes that are within the specified accuracy limits during a specified timeframe. 

Most manufacturers with guidance products on the market today have a pass-to-pass accuracy listed using the 95 per cent level, but there are still a few manufacturers that list their accuracy based on the 50 per cent level. Be careful not to confuse these accuracy statements with ‘pass-to-pass’ and ‘repeatable’ (described below), which are related to the time period in which the testing has been conducted.

Pass-to-pass accuracy

Manufacturers typically report pass-to-pass accuracy in product literature. This value represents the short-term (<15 minute) relative accuracy of a GPS receiver, but does not necessarily reflect long-term accuracy, which includes GPS drift.

GPS accuracy relies heavily on satellite configuration, which is constantly changing as the satellites orbit the Earth. However, it can be assumed that the satellite configuration will not change considerably within a 15-minute period. If a different satellite configuration is used to determine GPS receiver position, the reported position will almost certainly not correspond to that obtained previously. This ‘shift’ of reported GPS receiver position over time is termed GPS drift.

Year-to-year repeatable accuracy

This is the measure of repeatable accuracy, which means that you can drive the same rows a day, week, month or year later. Thus +/- 2.5cm year-to-year accuracy means you can drive the same rows next year within 2.5cm of this year’s rows, typically 95 per cent of the time.