Microwave observations made at the third and fourth Stokes parameters can be used to determine the surface wind direction over oceans. However, due to their smaller amplitudes (less than 3 K at the third Stokes parameter and 1 K at the fourth Stokes parameter), the absolute calibration to these measurements becomes crucial. A new methodology is developed in this study to calibrate the Windsat third and fourth Stokes parameters through tropical rain forest measurements over the Amazon and central Africa. It is found that the Windsat fourth Stokes parameter at 18 GHz has biases of the order of 0.5 K, which could severely affect the wind vector retrievals.