Absolute method for an optical measurement of the earth gravitational axis: application to Watt balance

We show how a Fabry Perot interferometer could be used to determine the gravitational axis of the earth g with an high sensitivity. Our method lie g axis to an optical beat frequency value between two lasers. The principle is described and a budget error is presented. The sensitivity of the sensor is estimated to be below of 85 nrad/MHz. This method could be used as a high sensitivity and absolute tilt sensor or even as a seismometer. This sensor is dedicated to the french watt balance project as described in this paper.