Kp index, RTK and GNSS for spray drone operators

Spray drones depend on satellite positioning to fly accurate, evenly spaced passes. Most agricultural drones use RTK (real-time kinematic) corrections to reach centimetre-level accuracy. Two things outside the operator's control can degrade it: geomagnetic activity, measured by the Kp index, and the number and geometry of the satellites in view.

What the Kp index is

The Kp index measures global geomagnetic activity on a scale from 0 (quiet) to 9 (extreme), in three-hour intervals. It is derived from magnetometer stations around the world; the official index is produced by GFZ Potsdam, and forecasts are published by space weather services such as NOAA's Space Weather Prediction Center.

On the NOAA scale, a Kp of 5 corresponds to a G1 (minor) geomagnetic storm, rising to G5 (extreme) at Kp 9. Values of 4 are often described as active conditions.

Why it matters for RTK

Geomagnetic storms disturb the ionosphere, the layer of the upper atmosphere that GNSS signals pass through. Disturbances can add errors to the signal, cause rapid fluctuations known as scintillation, and make it harder for the receiver to keep an RTK fix. Effects are usually strongest at high latitudes, and ionospheric irregularities also affect equatorial regions. For a spray drone, the result can be a lost fix, drifting lines, gaps and overlaps between passes, or a drone that will not start an automatic mission.

Which Kp values to watch

There is no universal cut-off, because the effect depends on your location, your receiver and the corrections you use. As a practical approach:

• Low values (0–3): normal conditions.
• Around 4: be alert. Watch the RTK status and pass accuracy.
• 5 and above: storm conditions. Expect degraded accuracy and consider postponing precision work.

If the RTK fix keeps dropping to float or single mode, stop and investigate before continuing — whatever the Kp says.

Visible satellites and geometry

Accuracy also depends on how many satellites the receiver can see and how they are spread across the sky. Using several constellations — GPS, GLONASS, Galileo and BeiDou — increases the number in view. Tree lines, buildings, hills and deep valleys block low satellites, and receivers usually ignore satellites below an elevation mask angle to avoid poor-quality signals. Fewer satellites, or satellites bunched in one part of the sky, weaken the solution.

Other checks before a precision job

• Confirm the RTK base station or network connection and that the drone reports a fixed solution.
• Check the mobile data or radio link that carries the corrections.
• Keep the take-off point away from large metal structures and power lines.
• Watch for multipath near buildings, silos and water tanks.

For the wider weather picture for drones, see drone spraying weather.

Further reading

• Planetary K-index — NOAA Space Weather Prediction Center
• Space Weather and GPS Systems — NOAA Space Weather Prediction Center
• Kp index — GFZ Potsdam

In PulveCast

For aerial equipment, PulveCast shows the Kp index from GFZ Potsdam against a default limit of 4, the number of visible GPS, GLONASS, Galileo and BeiDou satellites against a minimum of 9 (15° elevation mask), and an RTK signal estimate; all are adjustable.

More guides

• Delta T for spraying: what it is and why 2 to 8 °C
• Delta T chart: how to read it
• Thermal inversion and spraying
• Spray drift: causes and how to reduce it
• Wind speed for spraying
• The best time of day to spray
• Humidity and dew point for spraying
• Rain and spraying: rainfast periods and forecasts
• Temperature and spraying
• Droplet size and nozzle selection
• Weather for drone spraying
• Pre-spray checklist