Surfaces with $p_g=q=1$, $K^2=7$ and non-birational bicanonical mpas

Let $S$ be a minimal surface of general type with $p_g=q=1,K_S^2=7$. We prove that the degree of the bicanonical map is 1 or 2. Furthermore, if the degree is 2, we describe $S$ by a double cover.