Precision farming: Sharing an RTK base station
Tractor guidance systems are now being increasingly accepted by growers as a means of maximising output from wide implements. However, for those who need the highest degree of positional accuracy – for drilling or vegetable growers, for example, an RTK base station offering accuracies down to + or – 2cm is generally required.
Such keen accuracy does not come cheaply (£16,000 for the full set-up is typical) – according to John Deere’s tractor guidance specialist Mark James, a group of neighbouring farmers could club together to set up a single base station.
“An RTK base station can handle distances of 10km or more, provided there are no hills or other serious obstructions such as tall buildings or trees,” he explains. “In areas such as the Fens where it is flat, it would probably work well, but definitely not in, say, the hills and valleys of north Wales.”
If the terrain is suitable and the farms are within a 10km distance of a suitable broadcasting base, sharing an RTK system starts to make sense since it can save individual growers several thousands of pounds.
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| An RTK base station can offer accuracies down to + or -2cm. |
“One of the biggest influences on how well it will work is the height of the antenna used by the RTK unit, which has a limited power output of just 0.5W – unless you are willing to buy a licence for a higher powered radio,” says Mr James. “It’s all about line of sight and the higher the radio antenna, the better it will usually perform.”
“Each area needs to be surveyed to discover the best possible site for the radio antenna,” he says. “It is very unlikely that all of an area covered by a group of farms will be able to receive a signal.”
For a base station to be set up, the antenna should be placed in the best possible position at an optimum height. It shouldn’t be moved once it has been switched on and, instead, should be left to be programmed with positioning data from passing satellites for a 24-hour period.
This is an “absolute base” that has an extremely accurate position logged in its system and is able to provide a correction signal for the vehicle-based receivers working in the surrounding fields.
Mr James points out that RTK accuracy degrades with distance. At 20km, for example, accuracy is +/-4cm and, at 30km, +/-6m. The most important thing is that the receiver and the RTK unit use the same satellites for their information – you can’t have the RTK unit listening to information broadcast by one set of satellites and expect it transmit accurate correction details to a receiver linked to a different set.
If extra distance beyond 10km is required or if an outlying field is not in view, a repeater can be used to convey the RTK correction signal beyond the 10km range but the system is limited to one repeater – it is not possible to link up a chain of repeaters at 10km intervals due to the interaction of the signal waves and the distortion that occurs as a result.
It is possible to provide a quick survey base which can be set up on its tripod on the edge of a field and can be used after just five minutes of being switched on. While the pass-to-pass accuracy is the same, the degree of repeatability is not, however, as good as the absolute base due to the limited survey time.
The placing of this base should also be chosen with some care so it does not receive a reflected signal (known as multipathing) from a nearby metal roof, lake or even a parked vehicle.
If multipathing occurs the receiver becomes confused – it has received more than one time-coded message from the same satellite. So it assumes there is an error with the satellite and decides not to use it until the situation has been resolved – that is, when the satellite has moved out of the reflective position (usually after several minutes).
“Given the right circumstances, a shared RTK system is a viable and money-saving proposition but the project needs careful research. Users must ask themselves the question whether or not their farming operations actually need that degree of positional accuracy,” concludes Mr James.
For more information on precision farming, visit our dedicated page, which also gives you details on the Precision Farming Event 2009, to be held at the East of England Showground on 4 March.
How accurate do you want to be? |
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Raw GPS navigation is generally assumed to be accurate to within about 10m with cheaper receivers – errors can be from a number of sources including ionospheric disturbances and satellite clock drift. The actual degree of accuracy depends on the type of antenna used, the sophistication of the electronics and the positional calculations in the GPS receiver. A really good receiver could be accurate to between 1 to 3m just with raw GPS. Greater accuracy can be achieved by using Differential GPS (DGPS) which use a series of beacons with a known, highly accurate position that receive GPS signals and then transmit corrected information to receivers. Accuracy with DGPS is down to about 1m which, while being more than sufficient for a car’s navigation, is still a tad wayward for accurate field work. Having said that, John Deere’s StarFire receiver uses an advanced system to correct for ionospheric disturbances by making comparisons between two signals broadcast on different frequencies by the GPS satellites (L1 and L2), plus the company’s StarFire1 correction signal and has an accuracy of +/- 30cm or 10cm for over 90% of the time. A new system – StarFire2 – introduced in 2004 is even more accurate and is able to achieve accuracies of +/-10cm or 5cm more than 93% of the time. This is claimed to be suitable for operations such as spraying, harvesting or mowing. But for those jobs that demand repeat accuracy (controlled traffic farming, vegetable production, or bed placement) RTK can cost up to £16,000 to fully install which is significantly more expensive than StarFire2, even when you consider the annual payment of €600 to be made for the service. |
