This function defines a chirp signal in the form of an external function. A chirp signal is a sinusoid with a frequency that changes continuously over a certain band and over a certain time period :

The "instantaneous frequency" in this signal is obtained by differentiating the argument with respect to time t:

And we see that it increases from to . When applying the signal to a system it gives a good control over the exited frequency band, it is therefore often used for system identification.

The following c-function is used in the model:

double Chirp(double w1, double w2, double A, double M, double time)
{
  double y;
  y = A * cos(w1 * time + (w2 - w1) * time * time / (2 * M));
  return y;
}

For a step by step tutorial see Using External Code.