GNSS in Precision Agriculture

Precision agriculture relies on accurate, repeatable GNSS positioning to reduce input costs, minimize environmental impact, and improve yield consistency. By knowing exactly where every piece of equipment is in the field — within centimetres — farmers can eliminate overlap in seeding and spraying, apply inputs at variable rates matched to soil conditions, and build season-over-season maps that drive continuous improvement.

The economic case is straightforward: a 10% reduction in overlap on a 1,000-hectare operation saves significant fuel, seed, and chemical costs every season. When combined with variable-rate application guided by prescription maps, the savings compound further while improving crop uniformity.

Modern agricultural GNSS solutions range from sub-metre SBAS guidance (adequate for broad-acre spraying) to centimetre-level RTK (required for controlled-traffic farming and precision planting). The choice depends on the operation's accuracy requirements, crop value, and correction infrastructure availability.

Auto-Steer Guidance

Automatic steering is the most widely adopted precision agriculture technology, and it depends entirely on GNSS positioning. The system receives GNSS coordinates at 5-20 Hz, computes the cross-track error from the planned guidance line, and sends steering corrections to the tractor's hydraulic steering system or electric motor.

Accuracy Levels

  • Sub-metre (SBAS/DGPS) — ±15-30 cm pass-to-pass accuracy. Adequate for broad-acre spraying and fertilizer spreading where some overlap is acceptable. Uses free SBAS corrections (WAAS in North America).
  • Decimetre (Atlas H30/H10) — ±3-10 cm pass-to-pass accuracy. Suitable for most seeding, spraying, and harvesting operations. Delivered via satellite (no base station required), making it ideal for remote farms.
  • Centimetre (RTK) — ±2-3 cm pass-to-pass accuracy. Required for controlled-traffic farming (CTF), precision planting, strip-till, and operations where every centimetre of overlap or gap affects yield or input cost.

Pass-to-Pass vs. Absolute Accuracy

For auto-steer, pass-to-pass accuracy (the consistency between adjacent swaths within a single session) matters more than absolute position accuracy. A receiver with 2 cm pass-to-pass accuracy may have 1-2 cm of absolute offset from the true WGS84 coordinate, but that doesn't affect guidance quality because adjacent passes are equally offset.

However, year-to-year repeatability matters for controlled-traffic farming, where the same wheel tracks must be followed season after season. This requires RTK or PPP corrections referenced to a stable geodetic datum, so the tractor returns to the same absolute position each year.

AB Lines & Curved Guidance

Most auto-steer systems support:

  • AB straight lines — The operator marks an A point and a B point; the system generates parallel straight lines at the implement width.
  • A+ heading — The operator marks a single point and enters a heading; parallel lines are generated at that azimuth.
  • Curved guidance — The operator drives the first pass along a curved headland or contour; the system generates parallel offsets from that curved path. This is essential for contour farming, terraced fields, and irregularly shaped paddocks.

Variable-Rate Application

Variable-rate application (VRA) adjusts the rate of seed, fertilizer, herbicide, or other inputs in real time based on a prescription map and the implement's GNSS position. The GNSS receiver tells the rate controller where the implement is in the field, and the rate controller adjusts the output accordingly.

How It Works

  1. Prescription map creation — Agronomists create a spatially-variable rate map based on soil sampling, yield maps, satellite imagery, or sensor data. The map divides the field into management zones, each with a target application rate.
  2. Map loading — The prescription map is loaded into the tractor's guidance display or rate controller in a standard format (shapefile, ISOXML, or proprietary format).
  3. Real-time rate adjustment — As the tractor moves through the field, the GNSS position is matched to the prescription map, and the rate controller adjusts the implement output to match the target rate for that zone.

Accuracy Requirements

VRA typically requires ±1-3 metre positional accuracy to match the implement to the correct management zone. The prescription map zones are usually 10-50 metres across, so sub-metre GNSS is more than adequate for rate switching. The challenge is latency: the rate controller must anticipate zone transitions based on the tractor's speed and the GNSS update rate to change rates at the correct field position.

Section Control

Section control automatically turns individual sections of a sprayer boom, planter row units, or spreader segments on and off based on GNSS position and a coverage map. This eliminates overlap at headlands, point rows, and around obstacles — areas where manual operation typically results in 5-15% overlap.

Economic Impact

On a 36-metre sprayer boom with section control, a typical 1,000-hectare operation reduces chemical overlap by 5-10%, saving tens of thousands of dollars per season. For planters, section control eliminates double-planting on headlands, which reduces seed costs and improves stand uniformity in overlap zones.

Technical Requirements

Section control requires:

  • Sub-metre GNSS accuracy — Adequate for most section control applications. RTK is preferred for row-crop planters where ±2 cm accuracy prevents skips and doubles.
  • Coverage map tracking — The system logs which sections applied where, building a real-time coverage map that drives the on/off decisions.
  • Implement geometry — The system must know the exact position and width of each section relative to the GNSS antenna on the tractor. This requires careful calibration of implement offset and boom geometry.

Field Boundary & Yield Mapping

Boundary Mapping

GNSS enables accurate field boundary mapping for area calculation, input planning, and regulatory compliance. The operator drives the field perimeter with a GNSS receiver logging position at 1-5 Hz, creating a polygon that represents the field boundary. Sub-metre accuracy is adequate for area-based calculations; centimetre-level accuracy is needed when boundaries are used for legal or government reporting purposes.

Yield Mapping

Combine-mounted GNSS receivers log position alongside yield sensor data (grain mass flow, moisture) at 1-second intervals throughout harvest. The resulting yield map shows spatial variation across the field at 3-5 metre resolution, enabling:

  • Identification of problem areas — Low-yield zones may indicate drainage issues, compaction, nutrient deficiency, or pest pressure.
  • Prescription map creation — Multi-year yield maps drive variable-rate fertilizer and seeding prescriptions.
  • ROI analysis — Comparing yield maps with input maps quantifies the return on variable-rate investments zone by zone.

Correction Services for Agriculture

Agricultural GNSS users choose from several correction tiers:

Free SBAS (WAAS)

WAAS provides free, satellite-delivered corrections across North America, delivering ±30-60 cm horizontal accuracy. This is sufficient for broad-acre spraying guidance but inadequate for precision planting or controlled-traffic farming.

Hemisphere Atlas PPP

Atlas corrections are delivered via L-band satellite, providing decimetre to centimetre accuracy depending on the subscription tier:

  • Atlas Basic — ±30 cm accuracy, suitable for general guidance and spraying.
  • Atlas H30 — ±8 cm accuracy, suitable for most auto-steer and section control applications.
  • Atlas H10 — ±3 cm accuracy, approaching RTK performance without a base station.

Atlas is particularly valuable for large farming operations where the cost and logistics of maintaining base stations across multiple farms would be prohibitive.

RTK (Base Station or Network)

RTK provides ±2 cm accuracy for controlled-traffic farming, precision planting, and strip-till. Corrections come from either a farm-owned base station (transmitting via UHF radio) or a regional network RTK service (via cellular/NTRIP).

A farm base station with a SATEL radio modem can cover a radius of 10-30 km depending on terrain and antenna height, serving multiple tractors simultaneously.

Agriculture Software

  • LandMetrix — OEM agriculture solution toolkit providing auto-steer integration, guidance line management, section control, and variable-rate application interfaces for tractor OEMs and aftermarket guidance platforms.

Receivers

  • R632 GNSS Receiver — Multi-frequency receiver with RTK and Atlas PPP support. Flexible correction source selection allows farmers to use RTK near the base station and switch to Atlas when operating on remote fields.

OEM Boards

  • Phantom 20/34 OEM Boards — Compact multi-frequency boards for integration into aftermarket auto-steer systems and precision agriculture displays. Small form factor with low power consumption.

Smart Antennas

  • S631 Smart Antenna — Roof-mounted smart antenna combining receiver and antenna for simple tractor integration. Bluetooth connectivity for tablet-based guidance.

Antennas

  • A45 Antenna — Multi-frequency survey and agriculture antenna with excellent multipath rejection and phase centre stability for consistent RTK performance across seasons.

Integration Considerations

For agriculture applications, key selection criteria include:

  • Atlas PPP support — Essential for farms without cellular coverage or RTK base station infrastructure. Provides correction-source independence.
  • Pass-to-pass repeatability — More important than absolute accuracy for auto-steer quality. Specify and test this metric rather than relying on absolute accuracy specifications alone.
  • Multi-constellation tracking — GPS + GLONASS + Galileo + BeiDou provides maximum satellite availability, especially at high latitudes and in partially tree-lined fields.
  • CAN bus interface — Required for integration with ISOBUS-compatible implements and rate controllers. Standard J1939 CAN messaging enables plug-and-play integration with most modern farm equipment.
  • Correction source flexibility — The ability to switch between RTK, Atlas, and SBAS based on availability and accuracy requirements prevents downtime when one correction source is unavailable.