An RTK receiver on a tractor, sprayer or farm robot is only as good as the corrections it gets. The receiver measures satellites to within millimetres of carrier phase, but without corrections it can only place itself to within a metre or two. The correction source decides three things: how accurate your lines are, whether they repeat next season, and what happens in the corner of the field with no mobile signal.
Disclosure: I work at Eview GNSS, which makes Septentrio-based RTK receivers, including base stations.
There are three ways to get corrections onto a farm machine. Here's how they compare.
Quick comparison
| Network RTK (NTRIP) | Your own base station | PPP / satellite service | |
|---|---|---|---|
| Typical accuracy | 1–3 cm | 1–3 cm | 3–20 cm (service-dependent) |
| Time to accuracy | Seconds | Seconds | Several minutes to tens of minutes |
| Needs at the machine | Mobile data + NTRIP client | Radio receiver or mobile data | Satellite or internet delivery |
| Coverage limit | Distance to network station; mobile coverage | Radio range or your own caster | Near-global |
| Repeatable year to year | Yes, if the network datum is stable | Yes, if the base coordinates are fixed and surveyed | Usually, within the service accuracy |
| Ongoing work | Subscription admin (some networks are free) | Look after the base hardware | Subscription admin |
1. Network RTK (NTRIP)
A network of permanent reference stations streams corrections over the internet. The machine's receiver or guidance display logs into a caster, picks a mountpoint (a single station, or a virtual one built for your position), and receives RTCM messages over mobile data.
It works well when:
- you have reliable mobile coverage across every field, not just at the yard;
- the nearest station is within about 20–30 km (with single-station mountpoints, error grows by roughly 1 ppm of the distance);
- you'd rather not install or maintain hardware.
What to check:
- Your display or receiver has an NTRIP client, or you have a modem that can feed corrections into the receiver over serial.
- The message format. RTCM 3.x MSM (MSM4 or MSM7) is standard now; older networks may still send legacy messages that some receivers handle badly.
- What the receiver does when mobile data drops. Most hold an RTK fix for a short time and then fall back to float or standalone.
2. Your own base station
A fixed GNSS receiver at the yard or on a mast sends corrections to your machines over UHF radio, or over the internet through your own NTRIP caster.
It works well when:
- coverage is patchy, or you farm far from the nearest network station;
- you want no correction subscription;
- you want full control over the coordinates your lines are stored in.
Get these right:
- Rigid mount and clear sky. A pole or concrete pillar, not a tripod, with a view of the sky down to about 10–15°, away from metal sheds and grain bins. A base with multipath passes the error to every machine.
- Proper base coordinates. "Survey-in" (averaging for a few minutes) only puts the base within about a metre. Your lines work today but shift the next time you set up. For repeatable lines, log several hours of raw data and post-process it (PPP or against a reference network), then enter those coordinates permanently.
- Range. Keep machines within about 10–20 km for consistent centimetre results; a UHF radio usually reaches a few km over open ground, more with a tall antenna or a repeater.
- Bandwidth. If you use a serial radio, check that its air data rate can carry your RTCM stream. MSM4 with two or three constellations is a safe start.
3. PPP and satellite correction services
Precise Point Positioning uses precise orbit and clock corrections, delivered by satellite (L-band) or over the internet, instead of a nearby base.
It works well when:
- fields are large and remote, with no mobile coverage and no practical base site;
- 5–20 cm is enough for the job (tillage, broadacre spraying, mapping).
Limits: convergence takes minutes, longer after a long signal outage, and the best PPP-RTK services are only available in some regions. Check the convergence time and accuracy for your area, not the headline figure.
Match the correction to the job
| Job | Accuracy needed | Usually fine |
|---|---|---|
| Tillage, broadacre spraying, scouting maps | 10–30 cm | PPP, or RTK |
| Seeding, planting, strip-till | 2–5 cm, repeatable | RTK (network or own base) |
| Controlled-traffic farming, bed forming, row-crop cultivation | 2–3 cm, repeatable every season | RTK with fixed, surveyed coordinates |
| Field robots and autonomous machines | 2–3 cm + heading | RTK, often with dual-antenna heading |
Repeatability is the key word. If your seeding lines need to match your cultivation lines next spring, the coordinates behind the corrections must not change. That means either a network with a stable datum, or your own base on surveyed coordinates.
Troubleshooting from the cab
| Symptom | Likely cause |
|---|---|
| Stays in float near the base | Base antenna multipath or obstruction, or missing constellations in the message set |
| Fix drops in one corner of the field | Mobile dead zone (network RTK) or radio shadow (own base) |
| Lines shifted a few cm or a metre since last season | Base moved, or base coordinates re-averaged; or a datum change on the network |
| Fix comes and goes when driving | Correction latency or dropouts; check the radio air rate or modem signal |
| Slow to reach accuracy each morning | PPP convergence; consider RTK if the job needs it at the field edge |
Summary
- Good mobile coverage, near a network station: network RTK is the simplest.
- Patchy coverage, or you want no subscription and full control: your own base, set up properly.
- Very large remote fields where 10 cm is enough: PPP.
Many farms end up with a base station at the yard for the main fields and network RTK or PPP as a backup elsewhere.
If you're retrofitting an older tractor, I've written up the full kit (receiver and antenna, heading, corrections, display and steering) here: Tractor RTK retrofit kit guide. Our own base option is the HB62 GNSS base receiver.
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