Heterodyne interferometers have been commercially available for many years. In addition, many versions have been built at JPL for various projects. This activity is aimed at improving the accuracy of such interferometers from the 1 - 30 nanometer level to the picometer level for use in the proposed OSI and SONATA missions as metrology gauges. In the null-gauge configuration, we obtained a precision of 0.6 picometers at time scales of 2,500 seconds. In the relative-gauge configuration, we obtained an accuracy of 3.5 picometers rms in vacuum at time scales of few minutes. As absolute gauge with an accuracy of 10 microns over a distance of 10 meters in under construction.