In short: RTK GPS farming gives 2cm pass-to-pass accuracy only while the receiver keeps getting a correction signal, and on most UK farms that signal now arrives over a mobile SIM. When the tractor crosses a field with no coverage, the receiver drops from RTK fix to float within seconds, and the choice between a farm base station, a network subscription, satellite corrections or a farm-wide private network decides how often that happens.
Key Takeaways
- RTK is only as good as its correction link — The receiver needs a fresh correction message every second or so, and once the correction age passes a few seconds it falls from 2cm fix to float, which can mean 20 to 50cm of error.
- Mobile coverage decides where guidance fails — Most network RTK subscriptions reach the tractor over a single operator's SIM, so a valley, a wood edge or a hedge line with no signal becomes a strip of the field where auto-steer cannot hold the line.
- A farm-owned base plus a private network removes the dependency — A surveyed base station on a building roof, an NTRIP caster on the farm server and private 4G or 5G across the fields give every tractor, including the contractor's, the same correction stream without relying on a public mast.
In a nutshell
What is RTK GPS in farming, and why does it need a signal?
RTK GPS in farming is satellite positioning corrected in real time by a fixed reference receiver, and it needs a constant data link between that reference and the tractor. A standalone GNSS receiver on a cab roof is accurate to a metre or two. A reference station at a known, surveyed position sees the same satellite errors, works out the difference between where the satellites say it is and where it really is, and sends that correction to the moving receiver. With a steady stream of corrections, the tractor knows its position to about 2cm and can return to the same line next season.
That repeatability is why RTK sits under almost everything in modern arable precision farming: auto-steer on the drill, tramlines that the sprayer follows all season, controlled traffic farming where machines stay on permanent wheelings, inter-row hoeing in sugar beet and vegetables, and section control that shuts off boom sections on headlands. A 2cm error is invisible. A 30cm error takes out a row of lettuce.
The correction data itself is small, usually a few kilobits per second in RTCM 3 format. The difficulty is continuity. Receivers expect a new correction message roughly every second, and most displays show a "correction age" counter for that reason. Once that age runs past a manufacturer's threshold, typically somewhere between 5 and 30 seconds, the receiver downgrades from RTK fix to float and the position error opens up to tens of centimetres.
Why does RTK GPS lose fix in the field?
RTK GPS loses fix when the correction link drops, and on UK farms that link is now usually a mobile connection. There are three common ways corrections reach a tractor, and each fails differently.
Firstly, a farm or dealer base station can broadcast corrections over UHF radio. Radio range is typically quoted at 5 to 15km, but it depends on line of sight between the base antenna and the tractor, so a ridge, a block of woodland or a steel grain store in the way can cut the signal inside a kilometre. Most UHF RTK radios in the UK also need an Ofcom business radio licence for the base.
Secondly, and increasingly common, the tractor can pull corrections from a network RTK service over the internet using NTRIP, the protocol developed by Germany's federal mapping agency, BKG, for this job. In Great Britain those services draw on reference stations such as Ordnance Survey's OS Net, which has more than 100 stations, and are resold by manufacturers and dealers under their own brands. The tractor connects through a modem with a SIM, which means the correction signal is only as good as that SIM's mobile coverage in that field.
Thirdly, satellite-delivered corrections such as Trimble's CenterPoint RTX or John Deere's SF3 come down from a geostationary satellite, with no mobile or radio link on the farm. They avoid the coverage problem but trade away some accuracy, with pass-to-pass figures quoted at around 2.5 to 3cm, and can take from a minute to half an hour to converge after a restart depending on the service and region.
The mobile route is where most of the complaints we hear come from. The Shared Rural Network programme aims for 4G from at least one operator across 95% of the UK landmass, and coverage maps look reassuring. However, the SIM in the tractor modem is on one network, not "at least one", and landmass figures describe outdoor coverage at a point rather than an unbroken connection along every pass of a 40 hectare field. A dip in the valley bottom, a gap along a wooded boundary or a busy cell at harvest time is enough to push the correction age past the threshold. The operator sees a brief data gap; the tractor sees a drill line that wanders.
Should a farm use a base station, a network subscription or satellite corrections?
Each option has a sensible place, and the right choice depends on the shape of the holding, the mix of machinery and how much the farm relies on repeatable lines.
A farm-owned base station with a UHF radio suits a compact, reasonably flat holding with one brand of kit. There is no annual subscription beyond the radio licence, and nothing depends on a mobile operator. The trade-offs are (i) line of sight, which rules it out on rolling or wooded ground, (ii) the capital cost of the base receiver, radio and a surveyed mounting, and (iii) compatibility, since contractors and neighbours on a different radio protocol or frequency often cannot use it.
A network RTK subscription over mobile removes the base station entirely and works anywhere the SIM has signal. Annual fees per receiver typically run from a few hundred to over a thousand pounds depending on the provider and the brand. It suits farms with good, consistent coverage from one operator, and it becomes an irritant on farms where coverage is patchy, because the dropouts follow the fields rather than the calendar.
Satellite corrections suit large, remote holdings where neither radio nor mobile is dependable, and they are often used as a fallback. Some manufacturers bridge short radio or mobile dropouts by switching to satellite-delivered corrections for a limited window, typically minutes, which helps with a single wooded headland but not with a whole field in a valley.
Farms that run tramlines year after year, or controlled traffic, or inter-row weeding, tend to find that none of these gives complete confidence on their own. That is the gap a farm-wide network fills.
What does a lost RTK fix cost on a UK farm?
A lost fix costs accuracy on the passes where it happens, and the cost depends on the job being done at the time. On a sprayer with section control, float accuracy means boom sections overlap or leave strips on the return pass, which puts extra product on some ground and none on the rest. On a drill, a wandering line shows up all season, because the sprayer and fertiliser spreader will follow the tramlines it laid.
The sharper costs appear in systems built around 2cm repeatability. Controlled traffic farming keeps all wheels on permanent lanes, typically confining compaction to around 15 to 20% of the field. A tractor that drifts 30cm off those lanes compacts new ground, the damage the system exists to avoid. Camera and RTK guided hoes in sugar beet, onions or brassicas work inside a margin of a few centimetres either side of the row, so a float period during hoeing damages the crop directly.
There is also an operator cost that rarely appears in the farm accounts. A driver who knows the fix drops on the far side of a particular field disengages auto-steer early and steers by hand, which is slower, more tiring over a 14 hour drilling day, and less accurate than the kit the farm has paid for.
Can a farm run its own RTK corrections over a private network?
Yes, and it combines the independence of a farm base station with the reach and compatibility of network RTK. The arrangement has three parts. A GNSS reference receiver sits on a building roof at a surveyed position. An NTRIP caster, a small piece of software on a farm server or router, publishes that receiver's corrections as a standard RTCM stream. A private 4G or 5G network, built for the holding, carries the stream to every tractor modem, so each machine connects to the farm's own caster rather than to a public service over a public mast.
This set-up has three practical advantages. Firstly, coverage is designed around the fields rather than inherited from a mast five kilometres away, so the valley bottom and the far side of the wood can be covered deliberately. Secondly, NTRIP and RTCM are open standards, so a mixed fleet can use the same stream: the farm's Deere, a Fendt bought second hand, and the contractor's combine at harvest, subject to each receiver having its RTK capability unlocked. Thirdly, the same network carries the other traffic a modern farm generates, including telemetry from the machines, yield and application maps back to the office, cameras in the yard and lone worker devices for someone checking stock at night.
There are honest limits. RTK accuracy degrades with distance from the base, roughly 1cm for every 10km, so a single base suits holdings within a radius of about 10 to 20km; larger or scattered estates need more than one. A private network costs more up front than a subscription, and it makes most sense where coverage is poor and the network will carry more than corrections. Some manufacturer receivers are also tied to that manufacturer's own correction service by licensing, which is worth checking with the dealer before planning around a farm caster.
In the Llanthony Valley in the Brecon Beacons, we built a private network for a community where the public networks had given up on steep valleys and dense woodland. That is the same terrain that makes RTK dropouts worst, and the reason we think a farm-owned network is the durable fix for correction signals on difficult ground.
Where should a farm start with RTK GPS problems?
Start with the data the tractor already collects. Most guidance displays log fix status and correction age, and a season of those logs plotted against field boundaries shows where and how often the fix drops. Farms are often surprised to find that the problem is concentrated in three or four fields rather than spread across the holding.
Then check which network the RTK modem's SIM uses, and whether that operator covers the problem fields at cab height. A walk with a phone on the same network, showing signal in dBm rather than bars, answers that in an afternoon. If a different operator covers those fields, a SIM change may be all that is needed. If no operator covers them reliably, the choice is between satellite corrections as a fallback and a farm-owned network that carries corrections and everything else.
Ask the contractor too. A contractor who drills or harvests for several farms in the area will know which fields lose fix, and whether their own kit could use a farm correction stream. We are happy to look at a season's guidance logs alongside a coverage survey and set out what a private network across the holding would cover and cost.