In Synthetic Aperture Radar (SAR) interferometry from a satellite, the altimetric information is obtained from the phase difference of two focused complex images gathered by the same sensor in two passes along parallel or crossing orbits. The altimetric resolution of such a system improves when the satellites displacement is increased in the cross-track direction. The maximum allowed displacement, limited by speckle noise, increases with the spatial resolution of the SAR image. Excluding the additive noise, we show that the achievable vertical resolution is better than the slant range resolution times the cosine of the off-nadir angle for about 99 per cent of the image points. As an example, an altimetric map of the Panamint Valley area is calculated using repeated passes of the Seasat satellite. The effect of the additive noise is visible when the cross-track distance of the two orbits is low.