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The covariance function of a sum of sinusoids is itself a sum of sinusoidal functions. Thus, the power spectral density function is a set of Dirac delta functions. The locations of these are at multiples of ''n''. Thus, on a power spectrum, deterministic processes such as gravitational loading manifest themselves as spikes. This can be seen from analysing generator torque.
For analysis of torque on a single blade, the spikes occur at <math>k'\Omega_0</math> where ''k''' is 1,2,3,.. <ref>Aerodynamics of Wind Turbines, Martin O. L. Hansen</ref>. This can be seen from taking the autocovariance of equation 1, and then taking the Fourier transform of this result.
==Stochastic processes ==
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