LISA’s distance measuring system is a continuous interferometric laser ranging scheme, similar to systems used for radar-tracking of spacecraft. But for LISA, the direct reflection of laser light, such as in a normal Michelson interferometer, is not feasible due to the large distance of million km between the spacecraft: Diffraction expands the laser beam so much that for each Watt of laserpower sent, only about 250 pW are received. Direct reflection would thus result in an attenuation factor of about 6.25 x 10-20, yielding about one photon in every three days.