Hi Kyuhwan,

The equation 1 is derivative of the crosstrack error. I understand this derivative is like the speed of the velocity in that direction. As the velocity is v, so the speed in the direction of crosstrack error is v*sin(𝜓-𝛿) , the denote 𝜓 and 𝛿 can be referred in that paper as well as in the fig.5 of my article. You can also refer to the Fig.2 of Stanley paper. Hope it can help you to understand it.