\[h_{\phi}^{2}=\left(\frac{\partial x}{\partial\phi}\right)^{2}+\left(\frac{%
\partial y}{\partial\phi}\right)^{2}+\left(\frac{\partial z}{\partial\phi}%
\right)^{2}=c^{2}(\xi^{2}-1)(1-\eta^{2}).\]