Precision Agriculture Sensors: GNSS Guidance, Yield Mapping, and Field Calibration

Precision agriculture in Poland has expanded substantially over the past decade, with GNSS-guided tractors and combine yield monitors now common on mid-to-large cereal farms. However, the value of these systems depends almost entirely on correct installation and calibration — uncalibrated yield monitors, for example, can report errors of 10–15% compared to weighed-out grain totals, making field variability maps unreliable as a basis for variable-rate applications.

This article covers the practical aspects of three main sensor categories used in Polish cereal production: GNSS guidance systems, combine-mounted yield and moisture sensors, and soil electrical conductivity (EC) sensors used for zone mapping.

GNSS Guidance Systems: Signal Levels and Correction Sources

GNSS guidance for agricultural equipment in Poland uses the GPS (US), GLONASS (Russia), and increasingly Galileo (EU) satellite constellations. The base accuracy of uncorrected GPS — approximately 3–5 metres — is insufficient for pass-to-pass guidance in row crops or for tramline establishment in cereals. All practical guidance applications require some form of differential correction.

Correction signal types available in Poland

The Polish ASG-EUPOS network (Aktywna Sieć Geodezyjna EUPOS) provides RTK NTRIP correction streams through the GUGiK (Head Office of Geodesy and Cartography). Access requires registration and an annual subscription. Coverage is generally reliable across agricultural regions, with base stations spaced at 60–80 km intervals. In areas of poor mobile data connectivity, a local RTK base station may be a more reliable option.

Receiver mounting and antenna placement

GNSS antenna placement is as important as signal quality. The antenna should be mounted at the highest unobstructed point on the machine — typically the cab roof — and must have a clear sky view of at least 10° above the horizon in all directions. Metal surfaces within 30 cm of the antenna create multipath reflections that degrade accuracy. On tractors with front-mounted loaders, the loader arm can occlude satellite signals during headland turns — account for this when choosing antenna location.

Combine Yield Monitors: Calibration Methodology

Yield monitors measure grain flow by detecting the impact of grain on a mass-flow sensor mounted in the clean grain elevator. The sensor output — typically expressed as a frequency or voltage change — must be converted to a mass flow rate by a calibration factor specific to the crop, moisture level, and machine configuration.

Pre-calibration checks

Before running calibration loads, confirm that the clean grain elevator is running at the correct speed and that there are no partial blockages in the elevator paddle clearances. An elevator running at 80% of rated speed will produce consistently low yield readings regardless of calibration. Also verify that the grain moisture sensor is clean and making proper contact with the grain flow — a dirty moisture sensor is one of the most common causes of inaccurate yield maps.

Calibration load procedure

Run a minimum of three calibration loads of at least 3,000 kg each, weighed on a certified scale (a grain lorry weigh ticket from an accredited grain merchant is acceptable). For each load: record the monitor's reported mass, weigh the actual load, and calculate the ratio. The average of three ratios becomes the calibration factor. In Polish wheat conditions at 14–16% moisture, factors below 0.90 or above 1.10 indicate an installation or sensor issue that should be investigated before field mapping begins.

Moisture sensor calibration

Dielectric moisture sensors used on combines provide real-time grain moisture readings as the crop flows through the elevator. Calibrate the moisture sensor separately by comparing its reading against a bench-top grain moisture meter (Dickey-John GAC, Kett, or equivalent) on samples drawn from each calibration load. A maximum moisture error of ±0.5% is achievable with a properly calibrated sensor; larger errors indicate a sensor that requires cleaning or replacement.

Soil EC Mapping and Zone Management

Soil electrical conductivity (EC) mapping has gained adoption in Polish cereal farming as a way to identify within-field soil texture variation that correlates with yield potential and fertilizer response. EC sensors mounted on sled-pulled frames or on dedicated EC mapping machines measure the soil's resistance to an electrical current — sandy soils have higher resistance (lower EC), clay-rich soils have lower resistance (higher EC).

Sensor configurations

The most widely used EC measurement method in Poland is the Veris 3100 sled system, which measures EC at two depths simultaneously (0–30 cm shallow and 0–90 cm deep). The dual-depth measurement provides information on topsoil texture as well as subsoil clay content, the latter being particularly relevant for water-holding capacity in the light soils that dominate areas of Mazovia, Greater Poland, and Silesia.

EC survey parameters

For reliable zone maps, EC surveys should be conducted at a field resolution of 6–10 m transect spacing (corresponding to 80–120 measurements per hectare). Surveys should be conducted when soil moisture is uniform across the field — typically in autumn after harvest, or in early spring before soil warming creates large temperature gradients that alter EC readings independently of soil texture. Survey speed should be kept below 12 km/h to avoid dragging the sensor array.

Interpreting EC maps

EC data is interpolated into zone maps using kriging or inverse-distance weighting. The number of management zones appropriate for a field depends on the EC range: fields with a shallow EC variation of less than 10 mS/m across their area often do not benefit from more than two zones. Fields with EC ranges of 30–80 mS/m (common on edge-of-valley glacial soils in central Poland) typically support three to five meaningful management zones for nitrogen, phosphorus, or seeding rate variation.

Variable-Rate Application Controllers

GNSS-linked variable-rate controllers adjust application rates as equipment traverses zone boundaries defined by prescription maps. Controller accuracy depends on both the GPS position fix quality and on the hydraulic response time of the application system.

Section control for sprayers and spreaders

Automatic boom section control — which switches individual boom sections on and off to avoid overlapping at field boundaries and already-sprayed areas — requires a GPS fix of at least 50 cm accuracy for reliable operation. At section widths of 3–5 m (typical for Polish-scale equipment), a 50 cm position error represents 10–17% of section width, which is within acceptable overlap limits. Use of RTK correction for section control on irregular fields is advisable to minimise input waste.