An Introduction to Machine Autosteer

GPS (Global Positioning System )
North American worldwide radio-navigation system formed from 24 satellites whose position is, effectively, fixed in space. By measuring the time for a signal broadcast from 3 or more of these satellites to reach you it is possible to work out your position to within 15m or so.

GLONASS
The Russian version of GPS. The nearer to the North Pole a machine is working the fewer satellites are visible due to the angles involved. This can be an issue at certain times of the day and when there are service issues with satellites. In the UK there are occasions when the ability to use both GLONASS and GPS satellites offers a reliability and accuracy benefit. Together all the satellites systems are known as the GNSS – Global Navigation Satellite System.

DGPS (Differential GPS)
Several systems have been developed to improve the accuracy of standard GPS. The most common – DGPS – uses signals broadcast from another networks of satellites. These networks are specific to different regions of the world. The European network is known as EGNOS (European Geostationary Navigation Overlay Service). The US version is known as WAAS (Wide Area Augmentation System).

These systems are used by most economical lightbar and simple driver guidance systems. Typical accuracy is about 1m but some systems are capable of <50 cm and can, effectively, guide to less 20cm. pass to pass

Pass To Pass Accuracy / Repeatability
This phrase refers to the accuracy of guidance systems between consecutive implement passes. When accuracy is quoted in terms of ‘pass to pass’ the implication is that the position is not repeatable ie should you go back to the same point, according to your gps guidance, the next year, the next day or even after your lunch you will not be in the same exact position due to errors in correction of the satellite signal. All gps correction signals, with the exception of RTK, experience drift to some degree. Repeatability is essential for all forms of working which require multiple passes in the same place e,g. weeding, controlled traffic farming.

OmniStar / John Deere Starfire
These systems use a similar technology to Differential GPS but employ improved receivers and higher accuracy satellite corrections than EGNOS/WAAS. It is possible to get accuracy of 10cm pass to pass, but these systems still suffer from position drift and lack repeatability.

RTK (Real Time Kinematics)
Originally developed for the surveying industry, RTK involves analysis of the structure of the gps signal as well as the data content. By combining this analysis with some complex maths and a signal from a fixed base station, the accuracy of GPS RTK can be improved to an impressive 2cm with the important benefit of repeatability. Because of the complexities of calculating the position it is important that the correction signal is maintained at all times if true sub-2cm accuracy is to be achieved.

Base Station / Network RTK
There are two methods in which the tractor (the ‘rover’) can receive the RTK correction signal. A physical base station can be installed in approximate line-of-site of the working vehicle, this could be a mobile unit mounted on a tripod or could be permanently mounted on a high building. In order to achieve repeatability the base must always be in or returned to the exact same position. The base station can provide a useful signal to any working vehicle within a 15km radius provided something approximating to line-of-site can be achieved. The ability to share the signal makes it possible for base station owner to lease the signal to other operators in the area. Absolute accuracy deteriorates by approximately 1 mm for every km the rover receiver moves away from the fixed base station.
An alternative to using a fixed base station is to use network RTK. Rather than use a radio link to transmit from a local base this method uses a country wide network of bases and the mobile phone system to broadcast a calculated correction signal. The advantages are that the signal can be received over a wide area, rather than just in the locality of a base, and difficult terrain can be covered which would have presented problems to radio transmission from line-of-sight base provided that mobile phone reception is available in all the working area. The disadvantages are that the system carries a yearly subscription charge for data transmission and correction signal, and it may be necessary to have a subscription to two mobile networks in order to achieve good mobile signal over the entire worked area.

Implement steer
The ability to position the working implement in a precise and repeatable manner. For many applications, such as row crop work and controlled traffic farming, this will be in addition to the ability to position and steer the tractor. Where repeatability is required the ability to steer and position the tractor is not sufficient as slight changes in terrain and/or working depth will move the point of work of the implement away from the intended position despite the correct positioning of the tractor. This will only become obvious when an attempt is made to return to the work at a later date for a further operation.