Laser Guide Stars -LGS- has been widely proposed to extend the domain of stable operation for adaptive optics systems in large telescopes. As LGS are formed in a finite distance, bistatic schemes with two or more additional telescopes has been noted as the most efficient configuration from the point of view of full aperture tip-tilt correction. In these schemes it is crucial to know the height of the LGS. In this sense, an adequate characterization of the messopheric sodium layer is needed. Based on the experience acquired in a bistatic instrumental setup to characterize the sodium layer over the Teide Observatory (Canary Islands), we describe the general height equations and the accuracy required.