Active control of the geometrical parameters of large ring-lasers arrays is a challenging task, requiring the development of an accurate length monitoring system acting on the edges of these devices. The proposed solution is based on what was outlined by NASA JPL for its discontinued Space Interferometric Mission (SIM). It will be made up of a network of compact laser heterodyne interferometers, working together to keep the geometrical frame constantly well known (and fixed) to nanometer accuracy. We took the first steps towards the realization of this device by focusing on its fundamental element, i.e. the compact heterodyne laser interferometer. A starting simplified prototype has been conceived and is now under development. Its preliminary design will let us to evaluate parts behavior, alignment issues and links between single components and overall gauge performances. Concerned distance is investigated by means of a reference and a measurement beam, obtained by spatial splitting of the laser source. The phase shift between two heterodyne beatings will provide a reliable signal for precise monitoring of a linear distance.