# matlab code question with fractional fourier tranform

I want to estimate the parameters of a chirp signal using the fractional fourier transform. Following the paper estimating chirp parameters, the chirp rate $$\mu$$ can be obtained with (minus) the cotangent of the rotated angle $$\theta$$, $$\mu=-\cot{\theta}$$. The simulation code I wrote is

   fs=256;
t = 0:1/fs:1023/fs;
y = exp(-j*24*pi*t.^2);
XF = [];
for alpha = 0:0.001:1
XF = [XF,frft(y,alpha)] ;
end;

figure;
surf((abs(XF)).^2);
colormap('jet'); view(0,90);

The figure peaks at $$\alpha = 0.772$$. As a result, the rotated angle is $$\theta=0.772*\pi/2$$ and the estimated chirp rate is $$\cot\theta=0.3743$$. Obviously, this is not the desired result.