One of the most important dynamical processes in the tropical stratosphere is the quasi-biennial oscillation (QBO) of the zonal wind. Still, the QBO is not well represented in weather and climate models. To improve the representation of the QBO in the models, a better understanding of the driving of the QBO by atmospheric waves is required. In particular, the contribution of gravity waves is highly uncertain because of the small horizontal scales involved, and there is still no direct estimation based on global observations. We derive gravity wave momentum fluxes from temperature observations of the satellite instruments HIRDLS and SABER. Momentum flux spectra observed show that particularly gravity waves with intrinsic phase speeds