"The Laser Interferometer Space Antenna (LISA) is a joint NASA/ESA project which will use laser interferometry between drag-free proof masses to measure gravitational waves from many galactic and cosmological sources. The same interferometer technology is also the key to future multi-spacecraft missions such as multi-aperture telescope missions. These missions could include several spacecraft all separated by potentially 10s of km, flying in a fixed formation with sub-wavelength variations in their distances. These multi-aperture or distributed aperture telescopes will revolutionize the angular resolution in the infrared, optical, and even X-ray band. This proposal addresses two components which are both critical to these missions.

The first component introduces a new technique to stabilize the laser frequency to an optical reference cavity. Laser frequency noise will be the limiting factor for most of the distributed aperture telescope missions; in contrast, LISA can trade frequency noise against ranging precision. This new technique is based on heterodyne interferometry which is also used to measure changes in the distances between the spacecraft. Because of this similarity, this technology can easily be integrated into the payload. It requires the same photo detectors and digital signal processing systems that are used for the interferometry. It utilizes to a large degree existing components, reducing R&D time and cost for all interferometric space missions. We have already started initial proof of principle experiments and have reached already a performance remarkably close to the performance of the standard and long time-favored modulation/demodulation technique. Now we propose to study this technique in more detail, study the limiting noise sources experimentally and theoretically, and push it to the limitations of the reference cavity itself. The expected final fractional frequency noise should be better than 0.01ppt for measurement times of a 1000s. This