Rational curves on a smooth Hermitian surface

In characteristic $p>0$ and for $q$ a power of $p$, we compute the number of nonplanar rational curves of arbitrary degrees on a smooth Hermitian surface of degree $q+1$ under the assumption that the curves have a parametrization given by polynomials with at most $4$ terms. It is shown that a smooth Hermitian cubic surface contains infinitely many rational curves of degree $3$ and $6$. On the other hand, for all other cases the numbers of curves are finite and they are exactly determined. Further such rational curves are given explicitly up to projective isomorphism and their smoothness are checked.