eLISA's distance measuring system is a continuous interferometric laser ranging scheme, similar to systems used for radar-tracking of spacecraft.
For eLISA, the direct reflection of laser light, such as in a normal Michelson interferometer, is not feasible due to the large distance of 1 Mio 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. Therefore, lasers at the end of each arm operate in a “transponder” mode: A laser beam is sent out from the central spacecraft to an end spacecraft. The laser in the end spacecraft is then phase-locked to the incoming beam thus returning a high-power phase replica. The returned beam is received by the central spacecraft and its phase in turn compared with the phase of the local laser. A similar scheme is employed for the second arm.
In addition, the phases of the two lasers serving the two arms are compared in the central spacecraft.
The set of phase measurements together with some auxiliary modulation then allows us to determine optical path difference changes, laser frequency noise, and clock noise. The amplitude spectral density of the displacement noise characterises the performance of the measurement system, the second basic function of the science instrument.
A unique feature of eLISA interferometry is the virtual elimination of the effects of laser frequency noise, which would otherwise couple to the science signal through the sizable armlength difference. Stabilization to a reference cavity, as built into the payload, is not enough to suppress it completely. The remaining noise is removed by “Time-Delay Interferometry” (TDI), which synthesizes a virtual balanced armlength interferometer in postprocessing. This requires knowledge of the absolute armlengths to roughly 1 m accuracy, measured with an auxiliary ranging phase modulation of the laser beam. A second modulation is used to measure and remove noise caused by timing jitter of the ADC sampling clocks in the phasemeters.