Standard free-to-access GPS might provide sufficient accuracy for a task like cultivation or rolling, but for jobs that require more precise working, RTK provides a correction signal for GPS navigation that, in most cases, can position a vehicle with an accuracy of +/-2cm.
The only trouble is that, while the main positioning signal is beamed down from a series of satellites, the RTK correction signal has to be sourced from one or more earth-bound aerials.
Though an individual farmer can buy his own RTK system, it makes sense to create a network that several users can subscribe to and share the costs involved.
One of the first companies to offer farmers a subscription network was Claas, which successfully trialled a system in the summer of 2009.
But before looking at available network systems, a quick refresher on what RTK is all about. RTK is a process where GPS signal corrections are transmitted in real time from a reference receiver to receivers fitted to tractors or combines, which then activate the steering system.
Positioned at a known position, the RTK unit provides a point of comparison from where the GPS signal thinks you are and where you actually are.
The initial source of positional information is provided by the GPS system which comprises a network of satellites orbiting the planet. These satellites transmit signals down to earth where receivers use triangulation to calculate the position of the user.
For really spot-on work it’s the RTK system which provides the goods and, thanks to the establishment of RTK networks, it is a system which is now increasingly available for growers.
This resulted in the company taking the decision to install 11 masts to provide RTK coverage across Norfolk, Suffolk, Essex, Lincolnshire and part of Leicestershire – in all, a total of 2.5m hectares. A second phase saw the installation of masts to cover a large part of Kent and increase coverage to more than 3m hectares and there are also plans to extend coverage to counties in the south-west.
According to Claas precision farming product manager Edward Miller, setting up an RTK network is a case of “chicken and egg”.
“You can’t have subscribers until there is a network to subscribe to, and it’s very difficult to justify the high investment in putting up masts if there are no subscribers,” he says. He adds that, at this stage of proceedings, Claas has yet to start making any money on its RTK network business.
“We have every confidence we will start breaking even within a few years and then move into profitability,” says Mr Miller. “An increasing number of farmers are now using tractors and combines with automatic steering, which could use the system.”
Each mast represents an investment of £15,000 and the initial plan was to place masts at Claas dealer premises with the aim of providing a minimum of five subscribers for each mast. To subscribe, the cost a year is £800 for one unit, £1250 for two and £1500 for three or more units.
“It was a sound idea, but one we soon discovered was not possible if the maximum amount of coverage from each mast was to be exploited – most of the dealer premises were simply in the wrong place,” he explains.
Extensive surveys were made to discover the best alternative positions for the masts. Depending on topography, a mast will provide RTK coverage over a 20-30km radius although accuracy degrades with distance. At 20km accuracy is +/-4cm and at 30km it is +/-6cm.
Claas, however, is not the only company offering an RTK network in the eastern counties. While there are a number of individual machinery dealers offering coverage for their customers, one of the more recent additions is a farmer-run system based in Cambridgeshire that aims to provide coverage for up to 160,000ha with the potential for more than 2000 farmers.
RTK Farming says it currently has six base stations in operation to provide RTK coverage for 70,000ha in four counties.
A second phase will see a further six base stations established which will bring the area covered up to 160,000ha.
According to founder member Edward Banks, the fact that the RTK system has been designed with generous overlaps means that users avoid having those annoying parts of fields where a single signal cannot reach.
“Farmers who set up their own RTK transmitters very often discover that, when asked to take on an extra area of land, they are out of contact with the signal,” he says. “Subscribing to RTK Farming usually means there is a correction signal within reach.”