Nacelle direction offsets to North¶
The nacelle north offset is the error between the nacelle direction a turbine reports in SCADA and its true direction relative to geographic North. Tipspeed measures it from high-resolution satellite images, with a patent-pending method.
The resulting true nacelle direction is required for an absolute estimate of yaw misalignment.
Typical use: SCADA nacelle directions can carry unknown offsets, often after controller software resets. Correcting them is required before any absolute yaw or wake analysis.
Turbine projection model¶
The method is based on a 3D turbine and terrain model that predicts the geometry of the ground shadow for any sun and satellite position. The model includes the nacelle, blades and tower, parameterised by rotor diameter, hub height, tilt angle and cone angle. It projects these components according to:
- the sun and satellite positions;
- the terrain slope;
- the nacelle direction;
- the rotor position.
Feature identification on satellite images¶
The positions of the tower, blade and nacelle, and of their shadows, are identified on high-resolution satellite images. The true nacelle direction is estimated by minimising the error between the predicted and the observed features. A goodness-of-fit metric discards images whose marked positions deviate significantly from the expected positions. Operational experience shows that at least three images per turbine are required for good accuracy. The method is described in detail in (Tipspeed, 2025a).
Offset to North¶
The offset to North is the value that minimises the deviation between the true nacelle directions and the SCADA nacelle directions.