LISA data acquisition

The method for measuring changes in the lengths of the three arms for the LISA gravitational-wave detector is described. Accurate phase measurements are required on signals with Doppler shifts of about I MHz for the two main arms of the interferometer, and up to 15 MHz for the third arm. This is accomplished by shifting the signal frequencies down to near 100 kHz, filtering and then timing zero crossings of the signals. The basic methods for correcting for the laser phase noise and for phase noise in the ultra-stable oscillators used to derive various radio frequencies are described briefly. The corrections are made on the spacecraft, before the data are telemetred to the ground.